Chapter 2 Acids Bases and Salts Class 10 Notes (Science)
Chapter 2 Acids Bases and Salts Class 10 Notes (Science)
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NCERT Solutions for Science
Chapter 2 Acids Bases and Salts Class 10 Notes, Question/Answer, Activity & Projects
Chapter 2 Acids Bases and Salts
QUESTION
Q 1. You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?
Answer 1:
1. Initial Test with Red Litmus Paper
Begin by dipping a strip of red litmus paper into each test tube. Observe the reactions:
- In one of the test tubes, the red litmus paper will turn blue. This change indicates that the solution is basic.
- In the other two test tubes, the red litmus paper will remain red, suggesting they contain either an acidic solution or distilled water.
2. Further Testing
After identifying the basic solution, pour a small amount from this test tube to leave only a little liquid inside. Then, pour the contents of one of the remaining test tubes into the one with the basic solution and observe:
- If the litmus paper turns red again: This indicates that the added solution is acidic, as acids neutralize bases. The remaining test tube, therefore, contains distilled water.
- If the litmus paper stays blue: This shows that the added liquid is distilled water, as it has no effect on the basic solution. The last test tube contains the acidic solution.
Following these steps will allow you to accurately identify the contents of each test tube.
Activity 2.1
- Collect the following solutions from the science laboratory– Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Nitric acid (HNO₃), Acetic acid (CH₃COOH), Sodium hydroxide (NaOH), Calcium hydroxide (Ca(OH)₂), Potassium hydroxide (KOH), Magnesium hydroxide (Mg(OH)₂) and Ammonium hydroxide (NH₄OH).
- Put a drop of each of the above solutions on a watch-glass one by one and test with a drop of the indicators shown in Table 2.1.
- What change in colour did you observe with red litmus, blue litmus, phenolphthalein and methyl orange solutions for each of the solutions taken?
- Tabulate your observations in Table 2.1.
Q 1. Tabulate your observation in Table 2.1.
| Sample Solution | Red Litmus | Blue Litmus | Phenolphthalein Solution | Methyl Orange Solution |
Ans 1: Here’s the table with the sample solutions and their respective reactions with red litmus, blue litmus, phenolphthalein, and methyl orange:
| Sample Solution | Red Litmus | Blue Litmus | Phenolphthalein | Methyl Orange |
| HCl | No change | Turns red | Colourless | Red |
| H₂SO₄ | No change | Turns red | Colourless | Red |
| HNO₃ | No change | Turns red | Colourless | Red |
| CH₃COOH | No change | Turns red | Colourless | Red |
| NaOH | Turns blue | No change | Pink | Yellow |
| Ca(OH)₂ | Turns blue | No change | Pink | Yellow |
| KOH | Turns blue | No change | Pink | Yellow |
| Mg(OH)₂ | Turns blue | No change | Pink | Yellow |
| NH₂OH | Turns blue | No change | Pink | Yellow |
Activity 2.2
Experiment
- Take some finely chopped onions in a plastic bag along with some strips of clean cloth. Tie up the bag tightly and leave overnight in the fridge. The cloth strips can now be used to test for acids and bases.
- Take two of these cloth strips and check their odour.
- Keep them on a clean surface and put a few drops of dilute HCl solution on one strip and a few drops of dilute NaOH solution on the other.
- Rinse both cloth strips with water and again check their odour.
- Note your observations.
- Now take some dilute vanilla essence and clove oil and check their odour.
- Take some dilute HCl solution in one test tube and dilute NaOH solution in another. Add a few drops of dilute vanilla essence to both test tubes and shake well. Check the odour once again and record changes in odour, if any.
- Similarly, test the change in the odour of clove oil with dilute HCl and dilute NaOH solutions and record your observations.
Ans 1: [Do it yourself with the help of teacher]
[note: Sample of this experiment]
This experiment explores the concept of odor changes in the presence of acidic and basic solutions, which can help detect the nature of substances. Here’s a detailed step-by-step outline with expected observations:
Materials
- Finely chopped onions
- Clean cloth strips
- Plastic bag
- Dilute HCl (hydrochloric acid) solution
- Dilute NaOH (sodium hydroxide) solution
- Dilute vanilla essence
- Clove oil
- Water
- Two test tubes
(Procedure and Observations)
1. Preparation of Onion Indicator Cloth Strips
- Place finely chopped onions with clean cloth strips in a plastic bag.
- Seal the bag tightly and leave it in the fridge overnight.
- The cloth strips will absorb the odor of the onions, which will act as an indicator.
2. Testing Onion-Soaked Cloth Strips with Acids and Bases
- Take two cloth strips and check their initial onion odor. This odor will be used as a baseline.
- Place one strip on a clean surface, add a few drops of dilute HCl to it, and add a few drops of dilute NaOH to the other strip.
- Rinse both strips with water and check their odors again.
(Expected Observations:)
- With Dilute HCl: The onion odor may be neutralized or reduced because acidic conditions can alter the volatile compounds responsible for the smell.
- With Dilute NaOH: The onion odor may become more intense due to the alkaline condition, which can release more volatile sulfur compounds, enhancing the smell.
3. Testing Vanilla Essence with Acids and Bases
- Check the initial odor of dilute vanilla essence.
- Add a few drops of vanilla essence to each of the two test tubes, one containing dilute HCl and the other containing dilute NaOH. Shake well.
- Check the odor in both test tubes.
(Expected Observations:)
- With Dilute HCl: The vanilla odor may remain relatively stable as acidic conditions often do not alter esters, which are responsible for the vanilla aroma.
- With Dilute NaOH: The vanilla odor might diminish or disappear because bases can break down ester compounds, altering the smell.
4. Testing Clove Oil with Acids and Bases
- Check the initial odor of clove oil.
- Add a few drops of clove oil to the test tubes with dilute HCl and dilute NaOH, shaking well.
- Observe any changes in odor.
Expected Observations:
- With Dilute HCl: The clove odor may remain unchanged because clove oil contains compounds stable in acidic conditions.
- With Dilute NaOH: The clove odor might weaken or change, as basic conditions can affect the eugenol in clove oil, altering its scent.
Summary of Observations
This experiment illustrates how acids and bases interact differently with natural substances and odors.
Activity 2.3
Experiment
CAUTION: This activity needs the teacher’s assistance.
- Set the apparatus as shown in Fig. 2.1.
- Take about 5 mL of dilute Sulphuric acid in a test tube and add a few pieces of zinc granules to it.
- What do you observe on the surface of zinc granules?
- Pass the gas being evolved through the soap solution.
- Why are bubbles formed in the soap solution?
- Take a burning candle near a gas filled bubble.
- What do you observe?
- Repeat this Activity with some more acids like HCl, HN and C
- Are the observations in all the cases the same or different?
Ans 1: [Do it yourself with the help of teacher]
This activity demonstrates a reaction between an acid and a metal (zinc) to produce hydrogen gas, along with testing hydrogen’s properties. Here’s a breakdown of what each step accomplishes and what students can observe:
1. Setting up the Apparatus: This involves arranging a test tube to collect the gas produced when zinc reacts with an acid. The setup likely includes a delivery tube to channel the gas into the soap solution, where bubbles will form.
2. Adding Dilute Sulphuric Acid to Zinc: When dilute Sulphuric acid (H₂SO₄) is added to zinc granules (Zn), a chemical reaction occurs:
Zn + H₂SO₄ → ZnSO₄ + H₂↑
Observation: Students will notice bubbles forming on the surface of zinc. This indicates the release of hydrogen gas (H₂).
3. Passing Gas through Soap Solution: When the hydrogen gas passes through the soap solution, it creates bubbles.
- Why bubbles form: The soap traps the hydrogen gas, forming bubbles, because hydrogen gas is less dense than water and rises, making the soap solution foam.
4. Testing with a Burning Candle: Bringing a burning candle near a soap bubble filled with hydrogen gas will result in a pop sound or a small explosion.
- Observation: Hydrogen is a flammable gas, and it combusts rapidly upon contact with fire, producing the popping sound.
5. Repeating with Other Acids (HCl, HNO₃, CH₃COOH):
- Comparing Observations: In general, hydrochloric acid (HCl) will also react with zinc to produce hydrogen gas similarly to Sulphuric acid.
- Nitric acid (HNO₃), however, behaves differently. It usually does not release hydrogen gas with zinc due to its oxidizing nature, producing nitrogen oxides instead.
- Acetic acid (CH₃COOH), being a weak acid, will react with zinc to produce hydrogen, but the reaction will be slower and less vigorous compared to HCl or H₂SO₄.
Observations may differ based on the acid’s reactivity and strength, allowing students to understand how different acids interact with metals.
Activity 2.4
Experiment
- Place a few pieces of granulated zinc metal in a test tube.
- Add 2 mL of sodium hydroxide solution and warm the contents of the test tube.
- Repeat the rest of the steps as in Activity 2.3 and record your observations
The reaction that takes place can be written as follows.
2NaOH(aq)+Zn(s)→NZn(s)+(g) (Sodium zincate)
You find again that hydrogen is formed in the reaction. However, such reactions are not possible with all metals.
Ans 1: [Do it yourself with the help of teacher]
In Activity 2.4, you are conducting reactions involving sodium carbonate (Na₂CO₃) and sodium bicarbonate (NaHCO₃) with hydrochloric acid (HCl), followed by the passage of carbon dioxide (CO₂) through lime water. Additionally, you are reacting granulated zinc with sodium hydroxide (NaOH) to produce sodium zincate (Na₂ZnO₂) and hydrogen gas (H₂).
Breakdown of Reactions:
1.Test Tube A (Sodium carbonate and hydrochloric acid):
Na₂CO₃ (s) + 2HCl (aq) → 2NaCl (aq) + H₂O (l) + CO₂ (g)
Observation:
- The gas evolved is carbon dioxide (CO₂), which is colorless and odorless.
- When CO₂ is passed through lime water (Ca(OH)₂), a white precipitate of calcium carbonate (CaCO₃) forms:
Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O (l)
- The lime water turns milky due to the formation of calcium carbonate.
2. Test Tube B (Sodium bicarbonate and hydrochloric acid):
NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
Observation:
- Carbon dioxide gas is also evolved here, which can be confirmed by passing it through lime water (similar to Test Tube A), causing a white precipitate of calcium carbonate (CaCO₃).
3. Reaction with Zinc and Sodium Hydroxide:
When you add granulated zinc to sodium hydroxide solution and heat the mixture, the following reaction occurs:
2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g)
Observation:
- Hydrogen gas (H₂) is released, which is colorless and odorless.
- This reaction produces sodium zincate (Na₂ZnO₂), which is a solid.
Note: Not all metals react with sodium hydroxide. For example, zinc is amphoteric and reacts with both acids and bases, but metals like iron or copper do not react similarly with NaOH.
This experiment demonstrates the evolution of gases, precipitation reactions, and the properties of different metals in reaction with bases.
Activity 2.5
- Take two test tubes, label them as A and B.
- Take about 0.5 g of sodium carbonate (NC) in test tube A and about 0.5 g of sodium hydrogen carbonate (NaHC) in test tube B.
- Add about 2 mL of dilute HCl to both the test tubes.
- What do you observe?
- Pass the gas produced in each case through lime water (calcium hydroxide solution) as shown in Fig. 2.2 and record your observations.
Figure 2.2: Passing carbon dioxide gas through calcium hydroxide
Q 1. What do you observe? and record your observations.
Ans 1: Observation for Test Tube A (Na₂CO₃ + HCl):
- When you add dilute HCl to sodium carbonate in test tube A, it reacts with the sodium carbonate and produces carbon dioxide (CO₂) gas, water, and sodium chloride (NaCl).
- Reaction:
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑
- You’ll see effervescence (bubbling) as the CO₂ gas is released.
Observation for Test Tube B (NaHCO₃ + HCl):
- When you add dilute HCl to sodium hydrogen carbonate in test tube B, it also releases carbon dioxide gas, along with water and sodium chloride.
- Reaction:
NaHCO₃ + HCl → NaCl + H₂O + CO₂ ↑
- Here too, effervescence occurs as CO₂ gas is produced.
Passing CO₂ through Lime Water (Ca(OH)₂ Solution):
- Now, if you pass the CO₂ gas produced from each test tube into lime water (a solution of calcium hydroxide), you’ll observe that the lime water turns milky. This happens because carbon dioxide reacts with calcium hydroxide to form calcium carbonate (CaCO₃), which is insoluble in water and gives a milky appearance.
- Reaction:
Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O
- This milky appearance confirms the presence of CO₂ gas.
Activity 2.6
Q 1. What is the colour of the solution?
Ans 1: The color of the solution is pink.
Q 2. Is there any colour change for the reaction mixture?
Ans 2: Yes, the mixture gradually turns colorless.
Q 3. Why did the colour of phenolphthalein change after the addition of an acid?
Ans 3: When acid is added, it neutralizes the effect of the base, causing phenolphthalein to lose its color.
Q 4. Does the pink colour of phenolphthalein reappear?
Ans 4: Yes, the pink color of phenolphthalein can reappear
Q 5. Why do you think this happened?
Ans 5: The solution turns basic again due to the addition of NaOH.
Activity 2.7
Q 1. What has happened to the copper oxide?
Ans 1: When dilute hydrochloric acid is added to copper oxide, it reacts with the copper oxide (CuO) to form copper chloride (CuCl₂) and water. The copper oxide, which is black, dissolves in the acid, and the solution typically turns a blue-green color due to the formation of copper (II) chloride.
QUESTIONS
Q 1. Why should curd and sour substances not be kept in brass and copper vessels?
Ans 1: Curd and sour foods contain acids, which can react with metals like copper and brass. This reaction produces salts and hydrogen gas, leading to the corrosion of the metal. Therefore, it’s best to avoid storing such substances in these types of containers.
Q 2. Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Ans 2: When an acid reacts with a metal, hydrogen gas is typically released. For example, consider the reaction between zinc and sulfuric acid.
To demonstrate this, set up the experiment as shown in the diagram (Fig. 2.1). Begin by placing around 5 ml of dilute sulfuric acid (H₂SO₄) into a test tube. Then, add a few granules of zinc to the acid. Attach a delivery tube to the test tube to direct the gas into a soap solution. As the gas is produced, it passes through the solution, forming soap bubbles. To test for the gas, bring a lit candle near the bubbles. The gas inside the bubbles ignites with a characteristic “pop” sound. This confirms that the gas produced in the reaction is hydrogen.
Q 3. Metal compound a reacts with dilute hydro- chloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.
Ans 3: The metal compound involved in this reaction is likely calcium carbonate (CaCO₃), as it reacts with dilute hydrochloric acid (HCl) to produce effervescence (carbon dioxide gas), which can extinguish a burning candle.
The balanced chemical equation for this reaction is:
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
In this reaction:
- Calcium carbonate (CaCO₃) reacts with hydrochloric acid (HCl).
- Calcium chloride (CaCl₂) and water (H₂O) are formed along with carbon dioxide gas (CO₂).
- The effervescence is due to the release of CO₂, which extinguishes a burning candle as it is a non-supporter of combustion.
Activity 2.8
Q 1. What do you observe?
Ans 1: the bulb glows brightly in solutions of HCl and sulfuric acid because they dissociate into ions and conduct electricity. The bulb does not glow in glucose and alcohol solutions because they do not dissociate into ions and do not conduct electricity.
Q 2. Repeat the experiment separately with glucose and alcohol solutions. What do you observe now?
Ans 2: When the experiment is repeated separately with glucose and alcohol solutions, the bulb will not glow in both cases. This is because glucose and alcohol are non-electrolytes, meaning they do not dissociate into ions and therefore do not conduct electricity.
Q 3. Does the bulb glow in all cases?
Ans 3: No, the bulb does not glow in all cases. It glows when HCl and sulfuric acid solutions are used, but it does not glow with glucose and alcohol solutions.
Activity 2.9
- Take about 1g solid NaCl in a clean and dry test tube and set up the apparatus as shown in Fig. 2.4.
- Add some concentrated Sulphuric acid to the test tube.
- What do you observe? Is there a gas coming out of the delivery tube?
- Test the gas evolved successively with dry and wet blue litmus paper.
- In which case does the litmus paper change colour?
- On the basis of the above Activity, what do you infer about the acidic character of:
(i) dry HCl gas
(ii) HCl solution?
Figure 2.4 Preparation of HCl gas
Q 1. What do you observe? Is there a gas coming out of the delivery tube?
Ans 1: Yes, there is gas coming out of the delivery tube. The gas is hydrogen chloride (HCl) gas, which is produced when concentrated sulfuric acid reacts with sodium chloride (NaCl).
Q 2. In which case does the litmus paper change colour?
Ans 2: The litmus paper changes color in the case of wet blue litmus paper when it is exposed to hydrogen chloride (HCl) gas. The gas dissolves in the water on the litmus paper, forming hydrochloric acid, which causes the blue litmus paper to turn red.
Q 3. On the basis of the above activity, what do you infer about the acidic character of:
(i) dry HCl gas
(ii) HCl solution?
Ans 3: Based on the activity described, the inferences about the acidic character of:
(i) Dry HCl gas:
- Dry hydrogen chloride gas does not show an immediate reaction with dry litmus paper. This is because the gas itself does not have enough moisture to dissolve and exhibit its acidic properties directly. However, when dissolved in water, HCl exhibits acidic behavior, but in its dry state, it does not turn the blue litmus paper red.
(ii) HCl solution:
- When hydrogen chloride gas dissolves in water, it forms hydrochloric acid (HCl solution), which is acidic in nature. This is demonstrated by the wet blue litmus paper turning red upon exposure to the HCl solution, indicating its acidic properties. Thus, HCl solution clearly shows an acidic character, as it reacts with water to form hydrochloric acid.
Activity 2.10
Q 1. Is there a change in temperature?
Ans 1: Yes, there is a change in temperature. Both the dissolution of concentrated sulfuric acid (H₂SO₄) and sodium hydroxide (NaOH) pellets in water result in an increase in temperature. This is because both reactions are exothermic, releasing heat as the substances dissolve in water.
Q 2. Is this an exothermic or endothermic process?
Ans 2: The process described in the passage is exothermic. Both the dissolution of concentrated sulfuric acid (H₂SO₄) and sodium hydroxide (NaOH) pellets in water release heat, which makes the solution feel hot to the touch. This release of heat is characteristic of exothermic reactions.
Q 3. Repeat the above Activity with sodium hydroxide pellets and record your observations.
Ans 3: When repeating the activity with sodium hydroxide (NaOH) pellets, here are the observations:
- Change in temperature: As the sodium hydroxide pellets dissolve in water, the beaker will feel hot to the touch. This is due to the exothermic reaction where heat is released as the NaOH dissolves and reacts with water.
- Exothermic or Endothermic: This process is exothermic, meaning heat is released when sodium hydroxide dissolves in water, causing an increase in temperature.
So, similar to concentrated sulfuric acid, sodium hydroxide also undergoes an exothermic dissolution process.
QUESTIONS.
Q 1. Why do HC1, HNO3, etc. show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Ans 1: HCl and HNO₃ are strong acids that dissociate completely in water, releasing H⁺ ions, which give the solution its acidic properties. Alcohol and glucose, however, do not dissociate into H⁺ ions in water, so they do not exhibit acidic behavior.
Q 2. Why does an aqueous solution of an acid conduct electricity?
Ans 2: An aqueous acid solution conducts electricity because the acid dissociates into ions when dissolved in water. For example, HCl splits into H⁺ and Cl⁻ ions, which carry electrical charge and allow current to flow.
Q3. Why does dry HCl gas not change the colour of the dry litmus paper?
Ans 3: Dry HCl gas doesn’t change the color of dry litmus paper because there is no water present to allow the gas to dissolve and ionize into H⁺ and Cl⁻ ions. Without the presence of water, HCl cannot act as an acid and thus doesn’t affect the litmus paper.
Q 4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Ans 4: When diluting an acid, it’s recommended to add acid to water, not water to acid, to prevent a violent reaction. Adding water to concentrated acid can cause the acid to heat rapidly, potentially splashing and causing harm. By adding acid to water, the heat is absorbed more gradually, reducing the risk of dangerous splashing.
Q 5. How is the concentration of hydronium ions (H2O*) affected when a solution of an acid is diluted?
Ans 5: When an acid solution is diluted, the concentration of hydronium ions (H₃O⁺) decreases. This happens because the acid molecules become more spread out, reducing the number of hydronium ions per unit of solution.
Q 6. How is the concentration of hydroxide ions (OH) affected when excess base is dissolved in a solution of sodium hydroxide?
Ans 6: When excess base is added to a sodium hydroxide (NaOH) solution, the concentration of hydroxide ions (OH⁻) increases. This is because NaOH dissociates into sodium ions (Na⁺) and hydroxide ions (OH⁻) in water, and adding more base introduces additional OH⁻ ions, making the solution more basic.
Activity 2.11
- Test the pH values of solutions given in Table 2.2.
S.no. | Solution | Colour of pH Paper | Approximate pH Value | Nature of Substance |
1 | Saliva (before meal) | |||
2 | Saliva (after meal) | |||
3 | Lemon juice | |||
4 | Colourless aerated drink | |||
5 | Carrot juice | |||
6 | Coffee | |||
7 | Tomato juice | |||
8 | Tap water | |||
9 | 1M NaOH | |||
10 | 1M HCl |
- Record your observations.
- What is the nature of each substance on the basis of your observations?
Q 1. Record your observations.
| S. No. | Solution | pH Paper Colour | Approximate pH Value | Nature of Substance |
| 1 | Saliva (before meal) | Neutral/Light Yellow | 6.5 – 7.4 | Slightly acidic/neutral |
| 2 | Saliva (after meal) | Yellow/Light Orange | 5.5 – 6.5 | Slightly acidic |
| 3 | Lemon juice | Red | 2.0 – 3.0 | Acidic |
| 4 | Colourless aerated drink | Light Orange/Yellow | 3.0 – 4.0 | Acidic |
| 5 | Carrot juice | Light Orange | 5.5 – 6.5 | Slightly acidic/neutral |
| 6 | Coffee | Yellow/Orange | 5.0 – 5.5 | Acidic |
| 7 | Tomato juice | Yellow/Orange | 4.0 – 4.5 | Acidic |
| 8 | Tap water | Green | 7.0 | Neutral |
| 9 | 1M NaOH | Blue | 13.0 – 14.0 | Basic |
| 10 | 1M HCl | Red | 0.0 – 1.0 | Acidic |
Ans 1: Here is a table with approximate pH values for the substances you listed, along with the nature of each substance (acidic, neutral, or basic):
S. No. | Solution | pH Paper Colour | Approximate pH Value | Nature of Substance |
1 | Saliva (before meal) | Neutral/Light Yellow | 6.5 – 7.4 | Slightly acidic/neutral |
2 | Saliva (after meal) | Yellow/Light Orange | 5.5 – 6.5 | Slightly acidic |
3 | Lemon juice | Red | 2.0 – 3.0 | Acidic |
4 | Colourless aerated drink | Light Orange/Yellow | 3.0 – 4.0 | Acidic |
5 | Carrot juice | Light Orange | 5.5 – 6.5 | Slightly acidic/neutral |
6 | Coffee | Yellow/Orange | 5.0 – 5.5 | Acidic |
7 | Tomato juice | Yellow/Orange | 4.0 – 4.5 | Acidic |
8 | Tap water | Green | 7.0 | Neutral |
9 | 1M NaOH | Blue | 13.0 – 14.0 | Basic |
10 | 1M HCl | Red | 0.0 – 1.0 | Acidic |
These values are approximate and may vary depending on factors like concentration and individual variations.
Q 2. What is the nature of each substance on the basis of your observations?
Ans 2: Nature of Each Substance:
- Saliva (before meal): Neutral, slightly acidic to neutral in nature, depending on the individual.
- Saliva (after meal): Slightly acidic due to food residues and digestive processes.
- Lemon juice: Acidic, due to citric acid.
- Colourless aerated drink: Acidic, due to carbonation (carbonic acid).
- Carrot juice: Neutral to weakly acidic in nature.
- Coffee: Acidic, due to its organic acids.
- Tomato juice: Acidic, due to citric and malic acids.
- Tap water: Neutral, usually has a pH of around 7.
- 1M NaOH: Strongly alkaline, basic in nature.
- 1M HCl: Strongly acidic, highly acidic in nature.
Activity 2.12
- Put about 2 g soil in a test tube and add 5 mL water to it.
- Shake the contents of the test tube.
- Filter the contents and collect the filtrate in a test tube.
- Check the pH of this filtrate with the help of universal indicator paper.
- What can you conclude about the ideal soil pH for the growth of plants in your region?
Q 1. Check the pH of this filtrate with the help of universal indicator paper.
Ans 1: To check the pH of the filtrate, follow these steps:
- Prepare a soil-water mixture by adding 2 g of soil to 5 mL of water in a test tube and shaking gently.
- Filter the mixture to separate the liquid (filtrate).
- Dip universal indicator paper into the filtrate for a few seconds.
- Compare the color of the paper with the pH scale to determine if the soil is acidic, neutral, or alkaline.
This will help assess whether the soil is suitable for plant growth based on its pH level.
Q 2. What can you conclude about the ideal soil pH for the growth of plants in your region?
Ans 2: The ideal soil pH for plant growth in your region is typically between 6.0 and 7.5, which is neutral to slightly acidic. This pH range supports most plants, ensuring optimal nutrient availability.
QUESTIONS
Q 1. You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?
Ans 1: Solution A has a higher concentration of H+ ions, making it acidic, whereas Solution B has a lower concentration of H+ ions, making it basic.
Q 2. What effect does the concentration of H+(aq) ions have on the nature of the solution?
Ans 2: The acidity of a solution increases as the concentration of H+ ions rise. Higher H+ ion concentrations make the solution more acidic.
Q 3. Do basic solutions also have H+(aq) ions? If yes, then why are these basic?
Ans 3: Yes, basic solutions do contain H⁺ ions, but these ions come from the dissociation of water. Despite the presence of H⁺ ions, basic solutions have a higher concentration of hydroxide ions (OH⁻) compared to H⁺ ions, which is why they are considered basic. The excess OH⁻ ions give the solution its basic properties.
Q 4. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydro-xide) or chalk (calcium carbonate)?
Ans 4: A farmer would use quick lime, slaked lime, or chalk to treat the soil if it has become too acidic. These substances act as bases that help neutralize the acidity, restoring the soil’s pH balance for better crop growth.
Activity 2.13
Q 1. Write the chemical formulae of the salts given below:
Potassium sulphate, sodium sulphate, calcium sulphate, magnesium sulphate, copper sulphate, sodium chloride, sodium nitrate, sodium carbonate and ammonium chloride.
Ans 1: Here is a table with the chemical formulae of the given salts:
| Salt | Chemical Formula |
| Potassium sulphate | K₂SO₄ |
| Sodium sulphate | Na₂SO₄ |
| Calcium sulphate | CaSO₄ |
| Magnesium sulphate | MgSO₄ |
| Copper sulphate | CuSO₄ |
| Sodium chloride | NaCl |
| Sodium nitrate | NaNO₃ |
| Sodium carbonate | Na₂CO₃ |
| Ammonium chloride | NH₄Cl |
Q 2. Identify the acids and bases from which the above salts may be obtained.
Ans 2: Here is the table based on the given information for the acids, bases, and salts:
| Salt | Acid | Base |
| Potassium sulphate | H₂SO₄ | KOH |
| Sodium sulphate | H₂SO₄ | NaOH |
| Calcium sulphate | H₂SO₄ | Ca(OH)₂ |
| Magnesium sulphate | H₂SO₄ | Mg(OH)₂ |
| Copper sulphate | H₂SO₄ | Cu(OH)₂ |
| Sodium chloride | HCl | NaOH |
| Sodium nitrate | HNO₃ | NaOH |
| Sodium carbonate | H₂CO₃ | NaOH |
| Ammonium chloride | HCl | NH₄OH |
Q 3. How many families can you identify among the salts given in this activity?
Ans 3: The salts can be categorized into the following families:
1. Sulfate Salts:
- Potassium sulfate (K₂SO₄)
- Sodium sulfate (Na₂SO₄)
- Calcium sulfate (CaSO₄)
- Magnesium sulfate (MgSO₄)
- Copper sulfate (CuSO₄)
2. Chloride Salts:
- Sodium chloride (NaCl)
- Ammonium chloride (NH₄Cl)
3. Sodium Salts:
- Sodium sulfate (Na₂SO₄)
- Sodium chloride (NaCl)
- Sodium carbonate (Na₂CO₃)
- Sodium nitrate (NaNO₃)
Activity 2.14
Q 1. Report your observations in Table 2.4.
Ans 1: Table 2.4: Observations of Salt Solubility, pH, and Used Solutions
| Salt | Ph range | Acid used | Base used | Water Solubility |
| AlCl₃ | 4.0 – 4.4 | HCl | Al(OH)₃ | ✔ |
| ZnSO₄ | 4.0 – 5.2 | H₂SO₄ | ZnO | ✔ |
| CuSO₄ | 3.7 – 4.5 | H₂SO₄ | CuO | ✔ |
| CH₃COONa | 8.9 | CH₃COOH | NaOH | ✔ |
| Na₂CO₃ | 9.6 | H₂CO₃ | NaOH | ✔ |
| NaHCO₃ | 9.5 | H₂SO₃ | NaOH | ✔ |
| NaCl | 6.7 – 7.3 | HCl | NaOH | ✔ |
| KNO₃ | 7.0 | HN₃ | KOH | ✔ |
Activity 2.15
Q 1. What is the colour of the copper sulphate after heating?
Ans 1: It turns white.
Q 2. Do you notice water droplets in the boiling tube? Where have these come from?
Ans 2: Yes, the water droplets form on the surface of the boiling tube. They are produced as the CuS crystals release moisture during the heating process.
Q 3. What do you observe? Is the blue colour of copper sulphate restored?
Ans 3: When water is added, the blue color of the copper sulfate crystals reappears.
QUESTIONS
Q 1. What is the common name of the compound CaOCl₂?
Ans 1: The compound CaOCl₂ is commonly known as bleaching powder.
Q 2. Name the substance which on treatment with chlorine yields bleaching powder.
Ans 2: The substance is dry slaked lime, also known as calcium hydroxide, with the chemical formula Ca(OH)₂. When it reacts with chlorine, it forms bleaching powder.
Q 3. Name the sodium compound which is used for softening hard water.
Ans 3: Washing soda, also known as sodium carbonate (Na₂CO₃), is used to soften hard water.
Q 4. What will happen if a solution of sodium hydro carbonate is heated? Give the equation of the reaction involved.
Ans 4: When a solution of sodium bicarbonate (also known as sodium hydrogen carbonate) is heated, it undergoes a chemical decomposition. The heat causes the sodium bicarbonate to break down into sodium carbonate, carbon dioxide gas, and water.
The reaction is represented by the following equation:
2NaHCO₃ → (heat) Na₂CO₃ + CO₂ + H₂O
In simple terms, heating sodium bicarbonate causes it to release carbon dioxide and water while forming sodium carbonate. This is a typical thermal decomposition reaction.
Q 5. Write an equation to show the reaction between Plaster of Paris and water.
Ans 5: When Plaster of Paris (calcium sulfate hemihydrate) reacts with water, it forms calcium sulfate dihydrate, commonly known as gypsum. The chemical equation for this reaction is:
CaSO₄·½H₂O + H₂O → CaSO₄·2H₂O
This reaction is exothermic, meaning it releases heat as it sets.
EXERCISES
Q 1. A solution turns red litmus blue, its pH is likely to be
(a) 1
(b) 4
(c) 5
(d) 10
Ans 1: (d) 10
Q 2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
(a) NaCl
(b) HCl
(c) LiCl
(d) KCl
Answer: (b) HCl
Explanation:
Eggshells are made of calcium carbonate (CaCO₃). When hydrochloric acid (HCl) reacts with calcium carbonate, it releases carbon dioxide (CO₂) gas, which turns lime water milky due to the formation of calcium carbonate again.
Chemical Equation:
CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + CO₂ (g) + H₂O (l)
The CO₂ gas turns lime water milky, confirming the presence of an acid in the solution.
Q 3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be
(a) 4 mL
(b) 8 mL
(c) 12 mL
(d) 16 mL
Ans: (d) 16 mL
Explanation:
The reaction between sodium hydroxide (NaOH) and hydrochloric acid (HCl) is a neutralisation reaction, given by:
NaOH + HCl → NaCl + H₂O
From the question:
10 mL NaOH solution is neutralised by 8 mL HCl solution.
This implies the ratio is:
NaOH : HCl = 10 : 8 = 5 : 4
Now, if we take 20 mL of NaOH (which is double the original volume), the required HCl will also be double:
8 mL × 2 = 16 mL
So, 16 mL of HCl will be needed to completely neutralise 20 mL of NaOH.
Q 4. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Ans 4: (c) Antacid
Q 5. Write word equations and then balanced equations for the reaction taking place when:
(a) dilute Sulphuric acid reacts with zinc granules.
(b) dilute hydrochloric acid reacts with magnesium ribbon.
(c) dilute sulphuric acid reacts with aluminum powder.
(d) dilute hydrochloric acid reacts with iron filings.
Ans 5: Here are the word equations and the corresponding balanced chemical equations for the reactions:
Ans (a) Dilute Sulphuric Acid reacts with Zinc Granules
Word Equation: Dilute Sulphuric acid + Zinc → Zinc Sulphate + Hydrogen gas
Balanced Chemical Equation:
H₂SO₄ (aq) + Zn (s) → ZnSO₄ (aq) + H₂ (g)
Ans (b) Dilute Hydrochloric Acid reacts with Magnesium Ribbon
Word Equation: Dilute Hydrochloric acid + Magnesium → Magnesium Chloride + Hydrogen gas
Balanced Chemical Equation:
2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)
Ans (c) Dilute Sulphuric Acid reacts with Aluminum Powder
Word Equation: Dilute Sulphuric acid + Aluminum → Aluminum Sulphate + Hydrogen gas
Balanced Chemical Equation:
3H₂SO₄ (aq) + 2Al (s) → Al₂(SO₄)₃ (aq) + 3H₂ (g)
Ans (d) Dilute Hydrochloric Acid reacts with Iron Filings
Word Equation: Dilute Hydrochloric acid + Iron → Iron Chloride + Hydrogen gas
Balanced Chemical Equation:
6HCl(aq) + 2Fe(s) → 2FeCl₃(aq) + 3H₂(g)
Chapter 2 Acids Bases and Salts Class 10 Notes, Question/Answer, Activity & Projects
Updated Solution 2024-2025
Q 6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Ans 6: Activity to Prove Alcohols and Glucose are Not Acids:
1. Materials Needed:
- Alcohol (e.g., ethanol)
- Glucose solution
- Hydrochloric acid (HCl)
- Sodium hydroxide (NaOH) solution
- Litmus paper (red and blue)
2. Procedure:
- Add a few drops of alcohol to a test tube and observe its reaction with litmus paper. The litmus paper will remain unchanged, indicating it is neutral.
- Similarly, dissolve glucose in water and test with litmus paper. It will also show no change, proving that glucose is neutral.
- In contrast, add a few drops of hydrochloric acid to a separate test tube, and the litmus paper will turn red, confirming it is acidic.
- Finally, add sodium hydroxide solution to the alcohol and glucose solutions. Neither will react or show any signs of neutralization, while the acid will react with NaOH to form water and a salt.
3. Conclusion: Alcohol and glucose do not exhibit acidic properties as they do not alter the color of litmus paper or react with bases, unlike acids.
Fig. 2.2. Acid solution in water conducts electricity.
Q 7. Why does distilled water not conduct electricity, whereas rain water does?
Ans 7: Distilled water lacks the ions needed to conduct electricity because it does not contain dissolved substances that can break into charged particles. In contrast, rainwater contains dissolved carbon dioxide (CO₂), which reacts with water to form carbonic acid (H₂CO₃).
The reaction is:
CO₂ + H₂O → H₂CO₃
This carbonic acid then dissociates into ions:
H₂CO₃ → (H₃O⁺) + HCO₃⁻
These free-moving ions—hydronium (H₃O⁺) and bicarbonate (HCO₃⁻)—are responsible for the electrical conductivity of rainwater.
Q 8. Why do acids not show acidic behaviour in the absence of water?
Ans 8: Acids require water to dissociate into hydrogen ions (H⁺) and anions. Without water, this dissociation does not occur, and as a result, acids do not display their typical acidic properties.
Q 9. Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is:
(a) neutral?
(b) strongly alkaline?
(c) strongly acidic?
(d) weakly acidic?
(e) weakly alkaline?
Arrange the pH in increasing order of hydrogen-ion concentration.
Ans 9: (a) D, (b) C, (c) B, (d) A, (e) E.
Here are the answers based on the pH values:
- (a) Neutral: pH = 7, which corresponds to solution D.
- (b) Strongly alkaline: pH = 11, which corresponds to solution C.
- (c) Strongly acidic: pH = 1, which corresponds to solution B.
- (d) Weakly acidic: pH = 4, which corresponds to solution A.
- (e) Weakly alkaline: pH = 9, which corresponds to solution E.
Q 10. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCI) is added to test tube A, while acetic acid (CH₃COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?
Ans 10: The fizzing will be more vigorous in test tube A, where hydrochloric acid (HCl) is used. This is because hydrochloric acid is a strong acid, meaning it dissociates completely in water, releasing more hydrogen ions (H⁺). The increased concentration of hydrogen ions leads to a faster reaction with magnesium, resulting in more bubbles (fizzing). In contrast, acetic acid (CH₃COOH) is a weak acid and only partially dissociates, producing fewer hydrogen ions and causing less vigorous fizzing.
Q 11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Ans 11: When milk turns into curd, it becomes sour, which indicates a drop in pH. As the milk ferments, bacteria convert lactose into lactic acid, causing the pH to decrease and making the curd more acidic than the original milk.
Q 12. A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
Ans 12 (a): Baking soda, being an alkaline substance, increases the pH of the milk, making it slightly alkaline. This shift happens because baking soda neutralizes the natural acidity of the milk.
Ans (b): Curd formation requires an acidic environment to trigger the process of curdling. Since the addition of baking soda makes the milk alkaline, it interferes with the acidification process, causing it to take longer to set into curd.
Q 13. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Ans 13: Plaster of Paris has the ability to absorb moisture from the air, which causes it to react and turn into solid gypsum. To prevent this transformation, it is important to store Plaster of Paris in a moisture-proof container. If exposed to humidity, the material will lose its effectiveness and become unusable.
Q 14. What is a neutralisation reaction? Give two examples.
Ans 14: A neutralization reaction occurs when an acid reacts with a base to form a salt and water. This type of chemical reaction typically balances out the acidic and basic properties of the substances involved.
Examples:
1. Sodium hydroxide (NaOH) reacts with hydrochloric acid (HCl) to form sodium chloride (NaCl) and water (H₂O):
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
2. Calcium hydroxide (Ca(OH)₂) reacts with sulfuric acid (H₂SO₄) to produce calcium sulfate (CaSO₄) and water (H₂O):
Ca(OH)₂(aq) + H₂SO₄(aq) → CaSO₄ + H₂O
Q 15. Give two important uses of washing soda and baking soda.
Ans 15: Uses of Washing Soda:
- Sodium carbonate, commonly known as washing soda, plays a key role in the glass, soap, and paper manufacturing industries.
- It is also essential in producing various sodium compounds, including borax.
Uses of Baking Soda:
- Sodium bicarbonate, or baking soda, is commonly found in antacids. Its alkaline nature helps neutralize excess stomach acid, offering relief from acidity.
- It is also an important component in soda-acid fire extinguishers, where it helps to put out fires.
Chapter 2 Acids Bases and Salts Class 10 Notes, Question/Answer, Activity & Projects
Updated Solution 2024-2025
This complete solution is prepared as per the latest syllabus of 2024-25. If you have any further queries, feel free to ask! 😊