A chemical equation is a way of describing how
  A. 
a mixture of substances are separated into two or more pure substances that do not react with each other.
  B. 
pure substances are combined into a mixture that has the same properties as the original substances.
  C. 
pure substances change to produce one or more pure substances with different properties from the original substances.
  D. 
a mixture of substances change to produce one or more mixtures with the same properties as the original substances.