Answer: Because it can be a challenge to dissolve all of the reagents needed for a reaction into the same solution so they can react with one another in a timely manner.
This is important for life as well, since it allows substrates like food to come into contact with enzymes and be converted to energy.
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Understanding the chemistry of solutions is important because most chemical reactions, both in the laboratory and in nature, take place in solutions.
Chemical reactions are very much faster in solution than as a solid. In solution substrates will be bouncing around making it much easier for them to meet and react.
