e) Hydrogen and water ​

2.25 Describe the reactions of dilute hydrochloric and dilute sulfuric acids with magnesium, aluminium, zinc and iron ​

Magnesium ​

With dilute hydrochloric acid ​

The reaction is as follows:

$$ Mg(s) + HCl(aq) \rightarrow MgCl2(aq) + H2(g) $$

When magnesium reacts with dilute hydrochloric acid, it forms magnesium chloride and hydrogen gas.

With dilute sulphuric acid ​

The reaction is as follows:

$$ Mg(s) + H2SO4(aq) \rightarrow MgSO4(aq) + H2(g) $$

When magnesium reacts with dilute sulphuric acid, it forms magnesium sulphate and hydrogen gas.

Aluminium ​

With dilute hydrochloric acid ​

The reaction is as follows:

$$ Al(s) + HCl(aq) \rightarrow AlCl3(aq) + H2(g) $$

When aluminium reacts with dilute hydrochloric acid, it forms aluminium trichloride and hydrogen gas.

With dilute sulphuric acid ​

The reaction is as follows:

$$ Al(s) + H2SO4(aq) \rightarrow Al2(SO4)3(aq) + H2(g) $$

When aluminium reacts with dilute sulphuric acid, it forms aluminium sulphate and hydrogen gas.

Iron ​

With dilute hydrochloric acid ​

The reaction is as follows:

$$ Fe(s) + HCl(aq) \rightarrow FeCl2(aq) + H2(g) $$

When Iron reacts with hydrochloric acid, it forms Iron(II) chloride and hydrogen gas.

With dilute sulphuric acid ​

The reaction is as follows:

$$ Fe(s) + H2SO4(aq) \rightarrow FeSO4(aq) + H2(g) $$

When iron reacts with sulphuric acid, it forms iron sulphate and hydrogen gas.

2.26 Describe the combustion of hydrogen ​

The reaction is as follows:

$$ 2H2(g) + O2(g) \rightarrow 2H2O(g) $$

Hydrogen gas burns in air or oxygen to form water (steam).

When hydrogen is pure, hydrogen burns quietly. However, when it is mixed with air or oxygen first, an explosion will occur if a spark or flame is applied.

Testing hydrogen ​

When a lighted splint is held at the mouth of the test tube that contains hydrogen gas, it will make a popping sound.

2.27 Describe the use of anhydrous copper(II) sulfate in the chemical test for water ​

Water turns white anhydrous copper(II) sulphate blue. Anhydrous copper(II) sulphate lacks water of crystallisation. Dropping water in it replaces the water of crystallisation and therefore colour changes.

The reaction is as follows:

$$ CuSO4(s) + 5H2O(l) \rightarrow CuSO4 \cdot 5H2O(s) $$

2.28 Describe a physical test to show whether water is pure ​

If water exactly freezes at 0°C and boils at exactly 100°C at 1 atmospheric pressure, then the water is pure.