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:
- dip the red litmus paper in all the test tubes, one by one. The solution which turns red litmus to blue will be a basic solution. The blue litmus paper formed here can now be used to test the acidic solution in later steps.
- Now dip the blue litmus paper obtained above in the remaining two test tubes, one after the other. The solution which turns the blue litmus paper to red will be the acidic solution.
- The solution which shows no reaction with any litmus paper will be neutral and hence it will be distilled water.
Question 2
Why should curd and sour substances not be kept in brass and copper vessels?
Answer:
Curd and sour substances should not be kept in brass and copper vessels because these food-stuffs contain acids which can react with the metal of the vessel to form poisonous metal compounds which are responsible for causing food poisoning and affect our health adversely.
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:
(i) Hydrogen (H2) gas is liberated when an acid reacts with a metal.
(ii) Illustration: arrange the apparatus as shown in the given figure. Take some zinc granules in the test tube. Add about 5 mL dilute hydrochloric acid slowly. Soon the reaction initiates between zinc and hydrochloric acid and hydrogen gas is evolved.
iii) Test for H2 gas:
H2 gas is insoluble in water. When passed through a soap solution, it gets trapped into the bubbles.
Bring a burning candle near the soap bubble filled with gas. The soap bubble bursts and hydrogen gas burns with a pop sound.
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
CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (l)
Metal compound A is Calcium Chloride.
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
HCl or HNO3 dissolves in water to form H+ or H3O+ ions which shows their acidic character. For example, let’s see the following reactions
HCl (aq) → H+ + Cl–
H+ + H2O → H3O+
When alcohols and glucose are mixed with water then they do not dissolve to form ions. Hence they do not show acidic character.
Question 6.
Why does an aqueous solution of acid conduct electricity?
Answer An aqueous solution of acid contains hydrogen (H+) or Hydronium (H3O+) ions which are responsible for conducting electricity.
Question 7. Why does dry HCl gas not change the colour of the dry litmus paper?
Answer dry HCl gas does not change the colour of the dry litmus paper because it does not have H+ ions.
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 The process of dissolving an acid or a base in water is highly exothermic in nature. If water is added to a concentrated acid, a huge amount of heat is generated which may cause the mixture to splash out and cause burns. Therefore, It should be kept in mind that acid should always be added to water slowly with constant stirring.
Question 9.
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. Therefore the strength of the acid decreases.
Question 10.
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 would increase in the solution when excess base is dissolved in a solution of sodium hydroxide.
Question 11.
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 these is acidic and which one is basic?
Answer On a pH scale any value less than 7 indicates an acidic solution, while any value greater than 7 indicates a basic solution. Therefore, the solution A is acidic and solution B is basic.
Question 12.
What effect does the concentration of H+ (aq) ions have on the nature of the solution?
Answer More the concentration of hydrogen ions in a solution, higher will be its acidic nature
Question 13.
Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?
Answer: Basic solutions do have H+ (aq) ions. But these are very less in number as compared to OH– ions that are responsible for their basic nature.
Question 14.
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)?
Answer If the soil is too acidic (having low pH) then it is treated with materials like quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate).