In-Text Activities Solved

1.1) When magnesium ribbon is burning, what do you observe?

Ans) Magnesium ribbon burns with a dazzling flame and watch powder is collected on the watch glass placed under it. The white powder is magnesium oxide formed by the reaction of magnesium and oxygen present in the air.

2Mg(s) + O₂(g) → 2MgO

1.2) what do you observe when potassium iodide solution is added to lead nitrate solution in a test tube?

Ans) White lead nitrate reacts with white potassium iodide to form yellow lead iodide which is insoluble in water.

Pb(NO₃)₂(aq) + 2KI(aq) → 2KNO₃(aq) + PbI₂(s)

1.3) What happens to zinc granules when dilute hydrochloric acid or sulphuric acid is added to it? Is there any change in its temperature?

Zinc being an active metal readily reacts with dil. hydrochloric acid at room temperature to form soluble zinc chloride and hydrogen. We can observe bubbles of hydrogen gas being formed around zinc granules. If we touch the conical flask, we can feel a sense of hotness as there is a rise in temperature. So, it’s a exothermic reaction.

Zn + 2HCl → ZnCl₂ + H₂

As zinc displaces hydrogen from HCl, this a displacement reaction. Hydrogen gas is usually prepared in the laboratory by this reaction.

When zinc granules are added to dilute sulphuric acid, there occurs a change in the state of zinc granules and they are converted to zinc sulphate(ZnSO₄) and hydrogen gas is evolved.

Zn + H₂SO₄ → ZnSO₄ + H₂

This reaction is exothermic, i.e., it involves release of heat and so the flask becomes hot and temperature of the system increases.

1.4) Is there any change in temperature when water is added to calcium oxide or quick lime in a beaker?

Ans) Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

In this reaction, calcium oxide and water combine to form a single product, calcium hydroxide. Such a reaction in which a single product is formed from two or more reactants is known as a combination reaction.

1.5) Do you observe any change in the colour of ferrous sulphate crystals when they are heated in the boiling tube over the flame of a burner or spirit lamp?

Ans) The green colour of ferrous sulphate crystals changes to brownish black oxide and smell of burning sulphur will be observed. Ferrous sulphate decomposes to form ferric oxide (Fe₂O₃), sulphur dioxide (SO₂) and sulphur trioxide (SO₃). So, the gas emitted smells like burning sulphur. In this reaction, the single reactant FeSO₄ decomposes to form three different products. So, the reaction is a decomposition reaction.

1.6) What do you observe when a boiling test tube containing lead nitrate powder is heated it over a flame?

Ans) On heating lead nitrate decomposes with a crackling sound along with the evolution of brown fumes of nitrogen dioxide(NO₂) which has an irritating smell. The gas turns blue Litmus red; hence, it is acidic in nature. A yellow solid residue of lead oxide (PbO) remains in the test tube.