Salt and water are useful substances. Therefore, you should drink as much salt water you want.

Some mixtures can be separated into useful substances.

Handpicking is the only process that can be used to separate mixtures.

Distilled water is a product of separating mixtures.

Separating the components of a mixture will make it useless 

Acquiring pure substances present in air is beneficial.

Air is a pure gas and cannot be separated into substances.

Crude oil is a mixture that can be separated into useful fuel.

Filtration and distillation produce potable water.

Separating mixtures is harmful to the society.