2. Acids, bases and salts

Acids, Bases, and Salts

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

We make three pieces of the litmus paper. Now, dip each piece one by one into into each test tube.

The solution of the tube who changed the colour of litmus paper is basic.

Now, pour the identified basic solution into othe two test tube. The solution of the test tube who changed colour is identified as acid.

The solution of the test tube did not change colour is identified as distilled water.

Understanding the chemical properties of acids and base

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

Answer
We know that curd and sour substances are acids whereas and brass and copper vessels are metals.
Metals react with acids and displaces hydrogen atoms as hydrogen gas.
Therefore, curd and sour substances should not be kept in brass and copper vessels.
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?
Answer
When an acid reacts with a metal hydrogen gas is usually liberated.

Take about 5 mL of dilute sulphuric acid in a test tube and add a few pieces of zinc granules to it.
Pass the gas being evolved through the soap solution.
Bubbles are formed in the soap solution.
Take a burning candle near a gas filled bubble.
The gas is started to burn by a pop sound and tested as hydrogen gas.
3. 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.
Answer
The suitable balanced chemical equation for the reaction is as follow.
CaCO₃ + 2HCl → CaCl₂ (aq) + H₂O (l) + CO₂ (g)

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?

Answer

The dissociation of HCL or HNO₃ to form hydrogen ions always occurs in the presence of water. Hydrogen ions (H⁺) combine with H₂O to form hydronium ions (H₃O⁺)

The reaction is as follows

HCL + water - H⁺ + Cl⁻

H⁺ + H₂O - H₃O⁺

Although aqueous solutions of glucose and alcohol contain hydrogen ions. Hence, they do not show acidic charactor.

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

Answer

Acids dissociate in aqueous solution to form ions. These ions are responsible for conduction of electricity.

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

Answer

Colour of the litmus paper is changed by the hydrogen ions. Dry HCL gas does not contain H⁺ ions. It is only in the aqueous solution that an acid dissociates to give ions. Since in this case, neither HCL is in the aqueous nor the litmus paper is wet.

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

Answer

The process of dissolving an acid or a base in water is a high exothermic one. Care must be taken while mixing concentrated nitric acid or sulpheric acid with water. The acid must always be added slowly to water with constant stirring. If water is added to concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to axcessive local heating.

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

Answer

When an acid is diluted. The concentration of hydronium ions (H₃O⁺) per unit volume decreases. This means that the strength of the acid decreases.

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

Answer

The concentration of hydroxide ions (OH⁻) would increase when excess base is dissolved in a solution of sodium hydroxide.