In-Text Activities Solved

1.7) During the electrolysis of water, we observe the formation of bubbles at both the electrodes. These bubbles displace water in the test tubes.

• Is the volume of the gas collected the same in both the test tubes?
• What happens to these gases when we bring a burning candle close to the mouth of each of the test tubes containing the gases?
• Which gas is present in each test tube?

Ans) (a)The volume of the gas collected in both the test tubes is not the same. The volume of one of the gases, namely, hydrogen, is twice the volume of the gas(oxygen) collected in the other test tube.

(b) When we bring a burning candle close to the mouth of one of the test tube, the gas in the test tube catches fire and burns with a pop sound indicating that the gas in the test tube is hydrogen whereas when a burning candle was introduced into the other test tube, the candle started burning brightly indicating that the test tube contained oxygen because the abundance of oxygen accelerates the combustion reaction. The higher the oxygen concentration is, the more vigorously the candle will burn.

(c)The gas collected at the anode is oxygen and the gas collected at the cathode is hydrogen.

1.8) What happens to the colour of silver chloride when it is exposed to sunlight?

Ans) White silver chloride turns grey in sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light.

1.9) Two test tubes are taken and labeled A and B. About 10 ml of copper sulphate solution was added to both the test tube. Two iron nails that were cleaned by rubbing with sand paper were immersed into test tube B and left undisturbed for 20 min. Compare the intensity of the blue colour of copper sulphate solutions in test tubes (A) and (B). Also, compare the colour of the iron nails dipped in the copper sulphate solution with the one kept aside.

Ans) Copper sulphate solution was initially blue in colour when the iron nails were immersed into it but gradually the colour of the copper sulphate solution in test tube changes. This happens due to the following displacement reaction. Iron being more reactive than copper, displaces copper from copper sulphate and forms a new product namely iron sulphate and copper metal.

The following chemical reaction takes place in this Activity:

The blue colour of the copper sulphate solution turns light greenish when the iron nail was dipped, whereas the colour of copper sulphate solution in the other test tube does not change. The greenish colour of the solution in the test tube shows that ferrous ions are present in the solution. This shows that iron is more reactive than copper as ferrous ions have displaced cupric ions from copper sulphate solution and form light greenish coloured ferrous sulphate solution.

There is a brown coating on the iron nail which was dipped in the copper sulphate solution. This shows that copper is deposited on the iron nail by displacing iron, whereas the iron nail placed in petri dish shows greyish colour of iron (remains unchanged).

1.10) What happens when 3 ml of sodium sulphate solution is mixed with 3 mL of barium chloride solution in a test tube?

Ans) When 3 ml of sodium sulphate solution is mixed with 3 mL of barium chloride solution in a test tube, a white precipitate of barium sulphate is formed. Any reaction that produces a precipitate can be called a precipitation reaction.

1.11) What do you observe when we heat a china dish containing copper powder?

Ans) The surface of copper powder becomes coated with black copper(II) oxide. This is because oxygen is added to copper and copper oxide is formed.