(a) Explain the term chemical weathering of rocks. [2 marks] (b) Discuss four processes of chemical weathering of rocks [10 marks] (c) State four ways in wh...

(a) Chemical weathering of rocks

Chemical weathering is the breaking down and decay of rocks through chemical reactions that change the mineral composition of the rock, so that the rock is altered into new substances and eventually broken into fine soil particles. It is aided by water, oxygen, carbon dioxide and warmth.

(b) Four processes of chemical weathering

1. Solution: water dissolves soluble minerals in the rock (for example rock salt and limestone), removing them and weakening the rock.
2. Carbonation: rain water dissolves carbon dioxide to form weak carbonic acid, which reacts with limestone (calcium carbonate) to form soluble calcium bicarbonate that is washed away.
3. Hydrolysis: water reacts chemically with rock minerals such as feldspar, changing them into soft clay minerals.
4. Oxidation: oxygen in air and water reacts with minerals containing iron, forming reddish-brown iron oxides (rusting) that make the rock crumble. (Hydration, the absorption of water by minerals causing them to swell and flake, is also acceptable.)

(c) Four ways soil temperature is important for crop growth

1. It affects the germination of seeds; suitable warmth is needed before seeds can sprout.
2. It controls the rate of absorption of water and nutrients by plant roots.
3. It affects the activity of soil micro-organisms that decompose organic matter and release nutrients.
4. It influences the rate of chemical reactions, nutrient availability and general growth of the crop (very high or very low temperatures slow or stop growth).