Final answer:
A mixture is a substance with a variable composition that can be altered by simple physical means. It is made up of two or more pure substances that retain their individual identities and can be separated by physical methods like distillation and crystallization.
Step-by-step explanation:
A substance with a variable composition that can be altered by simple physical means, such as by evaporating one of the components, is known as a mixture.
Mixtures are made up of two or more pure substances that retain their individual identities.
They can be separated by physical methods, such as distillation and crystallization, which rely on differences in physical properties.