Acids Bases and Salts NCERT In Text Solution
Question 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 know that a basic solution turns red litmus to blue and an acidic solution turns the blue litmus to red.
- First of all, let us label three test tubes as A, B and C.
- Dip the red litmus paper in test tube A. If the red litmus paper turns to blue colour then test tube A contains a basic solution. Otherwise, it may have distilled water or acidic solution.
- Let us assume that the red litmus paper turned to blue when dipped in test tube A. Now, dip this blue litmus paper in test tube B. If the colour changes to red, then B contains acid. Otherwise, test tube B contains distilled water.
- Let us assume that the red litmus paper did not change colour on dipped in either A or B. Then it will surely turn to blue colour on being dipped in test tube C.
Question 2: Why should curd and sour substances not be kept in brass and copper vessels?
Answer: Curd and other sour substances react with brass and copper vessels and form hydrogen and other harmful substances due to presence of acid in them. That’s why curd and other sour substances should not be kept in brass and copper vessels.
Question 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?
Answer: When acid reacts with metal, it usually liberates hydrogen gas. This can be shown by following activity:
- Take some zinc granules in the test tube and pour sulphuric acid on them.
- Allow the gas to pass through the solution of soap.
- When gas starts coming out from the soap solution, bubbles are formed.
- Bring a burning candle near the bubbles of gas, it burns with a pop sound.
- Burning of gas with pop sound proves that it is hydrogen gas.
Reaction involved in this process can be written as follows:
Zn + H2SO4 ⇨ ZnSO4 + H2
Question 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.
Answer: When calcium carbonate reacts with hydrochloric acid, it produces carbon dioxide gas with effervescence. Carbon dioxide is used as fire extinguisher because it does not support combustion. Hence, it extinguishes a burning candle. Therefore, the metal compound A is calcium carbonate.
CaCO3 + 2HCl ⇨ CaCl2 + CO2 + H2O
Question 5: 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: We know that dissociation of hydronium ion is necessary for acidic behaviour. HCl, HNO3, etc. form hydronium ions in aqueous solutions while alcohol and glucose do not dissociate hydronium ion in the solution. Hence, HCl, HNO3, etc show acidic characters in aqueous solution while solutions of compounds like alcohol and glucose do not.
Question 6: Why does an aqueous solution of an acid conduct electricity?
Answer: Acid dissociates into ions in aqueous solution. Ions are necessary for conduction of electricity through a liquid. Due to this, aqueous solution of an acid conducts electricity.
Question 7: Why does dry HCl gas not change the colour of the dry litmus paper?
Answer: Dry HCl gas does not contain hydronium ions. So, it does not show acidic behaviour. Hence, it does not change the colour of dry litmus paper.
Question 8: While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Answer: When acid is mixed with water it evolves heat. The reaction that takes place in this process is exothermic. If water is mixed with acid, the mixture becomes warmer. Large amount of acid will result in evolution of more heat. This mixture may splash out and can result into accident due to burning. Therefore, it is recommended that acid should be added to water and not the other way round.