Acids Bases and Salts

In Text Solution

Part 1

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: The acidic and basic nature of the solution can be identified by using litmus paper. A basic solution turns red litmus to blue and an acidic solution turns the blue litmus to red. The solution which changes the red litmus to blue is basic solution. Now, between two solutions, which turn the litmus (which turned blue because of basic solution) to red, is acidic solution. The last one solution is distilled water.

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 liberates hydrogen gas usually. This can be shown by following activity:

acid metal reaction set up
  • 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 instead they contain hydrogen. 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.

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