Which one of the following statements BEST describes a chemical reaction? Answers:
A new substance is formed and the process can usually be undone.
A new substance is formed and the process can usually NOT be undone.
A new substance is NOT formed and the process can usually be undone.
A new substance is NOT formed and the process can usually NOT be undone.

The correct statement is (A new substance is formed and the process can usually NOT be undone.)

Explanation:

A chemical reaction is simply defined as the reaction between two or more elements in which a new substance is formed and the process can usually not be undone. Different types of chemical reaction includes:

-- combination reaction: this occurs when two or more reactants form a product. For example: In the burning of coal, It combines with oxygen to produce carbon dioxide. Also in the burning of wood, carbon dioxide is given off and ashes are formed. Because new substance is being formed, they often can't be undone. The ashes formed can't be changed back into wood. Other types of chemical reaction are listed below.

-- Decomposition reaction

-- Single displacement reaction

-- Double displacement reaction

-- combustion reaction

-- Redox reaction

For the product of a chemical reaction to be undone (reversed), it has to undergo another chemical process different from the one that produced it.

- lead + phosphoric acid = hydrogen + lead(ii) phosphate