Chemistry, Xth

Answers to 10th NCERT Mid-chapter Q/A Chapter 2 – Acid, Base, and Salt

In this section, we’ll explore the answers to the mid-chapter questions in Chapter 2 of the 10th-grade NCERT chemistry book, focusing on acids, bases, and salts. These questions help us understand the core concepts better and test our knowledge about these important chemical substances.

Part I

  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?
Ans: Step 1 - dip litmus in all the 3 tube, which makes litmus blue , that solution is basic.
Step 2- now separate basic solution, we have 2 solution acidic and distilled water.
Step 3- As our litmus is blue now, dip in both, which turns blue litmus red that is acidic solution other one is distilled water.

2. Why should curd and sour substances not be kept in brass and copper vessels?

Ans: As curd and sour substances contains acid, and brass and copper are metals.
We know that when acid reacts with metal it produces hydrogen gas and salts. So when we place curd and sour substance in metallic vessels, it reacts and produces toxic compound which can create food poisoning and can be harmful for health.

3. 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: Hydrogen gas(H2) is liberated when an acid reacts with metal. If we bring burning candle near to the test-tube, it will make pop sound. The "pop" sound is a characteristic feature of hydrogen gas combustion and is a clear indicator of its presence.

4. Metal compound A reacts with dilute hydrochloric 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: As here question says a metal A is reacting with HCl(aq) a gas evolve which stops burning the candle. Its happens only when carbon dioxide is released and we have studied that when an acid (here HCl) reacts with carbonates  or hydrogen carbonates it gives water and carbon dioxide with salt. Here we are using Calcium Carbonate because question has mentioned that Calcium chloride is produced. 
So the Reaction will be:
CaCO3 + HCl(aq) -> CaCl2 + H20 + CO2(g)
After Balancing it :
CaCO3 + 2HCl -> CaCl2 + H20 + CO2(g)

Part II

1. Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Ans: HCl (hydrochloric acid), HNO3 (nitric acid), and similar compounds show acidic properties in aqueous solutions due to their ability to release hydrogen ions (H+) when dissolved in water. 
Whereas Alcohol (C2H5OH) and glucose (C6H12O6) do not easily release H+ ions. Instead, these substances remain largely in their molecular form when dissolved in water, this is why they don't show acidic character in water.

2. Why does an aqueous solution of an acid conduct electricity?

Ans: When acid dissolves in water it release H+ ions which is called ionization or dissociation. 
HCl (aq) → H+ (aq) + Cl- (aq)
These Positively charged H+ ions carry electrons with them and hence it conduct electricity.

3. Why does dry HCl gas not change the colour of the dry litmus paper?

Ans: Because HCl do not release H+ ions without water or in absence of water. So without any ionization or dissociation it cannot be identified that its an acid or base.

4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Ans: Acid should be added to water because of safety reasons. This practice helps reduce the risk of occurring a  highly exothermic reaction, which can occur causes burns. That is why it is safe to add acid to water, so that it will create less amount of acid and heat.

5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

Ans: When you dilute an acidic solution, the concentration of H3O+ ions decreases because the same amount of acid is distributed in a larger volume of the solution. As a result, the solution becomes less acidic and has a higher pH value, indicating a lower concentration of hydronium ions.

6. How is the concentration of hydroxide ions (OH) affected when excess base is dissolved in a solution of sodium hydroxide?

Ans: When excess base is dissolved in a solution of sodium hydroxide (NaOH), the concentration of hydroxide ions (OH-) increases. This is because the base, which is added, releases hydroxide ions and adds more hydroxide ions to the solution. Hence, It increases the concentration of (OH-) ions.

Part III

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: As we know the solution which has pH value less than 7 is acidic and greater than 7 is basic, so we can say that solution A which has pH value 6 is acidic and it has more hydrogen ion concentration, where as solution B which has pH value 8 is basic solution.

2. What effect does the concentration of H+ (aq) ions have on the nature of the solution?

Ans: The concentration of H+ ions define the acidic nature of the solution. As H+ concentration increases, it becomes more acidic and as H+ concentration decreases, it becomes less acidic.

3. Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?

Ans: Yes, even basic solutions contain a small concentration of H+ ions, but the reason they are considered basic rather than acidic is because they have a higher concentration of hydroxide ions (OH-) than hydrogen ions (H+).

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 hydroxide) or chalk (calcium carbonate)?

Ans: A farmer might treat the soil of his fields with quicklime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate) to adjust the pH of the soil and improve its quality, when soil becomes more acidic.

Part IV

1. What is the common name of the compound CaOCl2?

Ans: The common name for the compound CaOCl2 is "bleaching powder". 

2. Name the substance which on treatment with chlorine yields bleaching powder.

Ans: Calcium Hydroxide (Ca(OH)2)

3. Name the sodium compound which is used for softening hard water.

Ans: Sodium Carbonate (Na2CO3), commonly known as "washing soda".

4. What will happen if a solution of sodium hydro-carbonate is heated? Give the equation of the reaction involved.

Ans: When a solution of sodium bicarbonate (Sodium Hydro-carbonate, NaHCO3) is heated, it undergoes a thermal decomposition reaction.

2NaHCO3(sodium bicarbonate) → Na2CO3(sodium carbonate) + H2O(water) + CO2(carbon dioxide)

5. Write an equation to show the reaction between Plaster of Paris and water.

Ans: Plaster of Paris combines with water to form gypsum by incorporating additional water molecules into its structure.
CaSO4·1/2H2O(Plaster of Paris) + 1.5H2O(water) → CaSO4·2H2O(Gypsum)