Chemical weathering is where there is a chemical reaction for example with a plagioclase feldspar formed at a very high temperature like 1000 degrees centigrade exposed to low temperature atmospheric conditons, so that that mineral becomes unstable and must change to a mineral stable under those new conditions like kaolin or sericite. A plagioclase can have a composition as a sodium aluminum silicate or calcium aluminum silicate or a combination of the two and then with the addition of water and lower termperatures can convert to a more stable aluminum silicate hydroxide by leaching of calcium and sodium for example and the addition of OH from water. Physical weathering is something which affects the physical integrity of the rock such as freezing and thawing resulting in cracks which then allow water to percolate through the rock (through the cracks) and thus facilitate chemical weathering. This process was used by the underground miners from the 16th century who before gunpowder, lit fires at the rock face and then threw cold water on the rock to create cracks to allow the rock to be dug more easily.. Also, in a cliff face, the rock tends to slough and roll down the slope and thus gets broken down to smaller chunk sizes in the process, thus increasing its's surface area and making it more susceptible to chemical weathering so the two work together to convert rock ultimately to a soil.
Water quality is a great concern for sub Sub Saharan Africa's rural population. It is basically a region where water is very scarce and finding the right quality of water for drinking and cooking purposes is very hard for the rural people of this region. I hope the answer helps you.
