Before using a substance, we need to separate harmful or non-useful substances that may be mixed with it. Sometimes, we separate even useful components if we need to use them separately. For example:
1. We used to separate slightly larger sized impurities like the pieces of dirt, stone, and husk from wheat, rice or pulses by handpicking method.
2. Rice or pulses are usually washed before cooking. When we add water to these, the impurities like dust and soil particles get separated.