What is the difference between a pure substance and a mixture? how are the atoms different?

Pure substances cannot be separated into any other kinds of matter while a mixture is a combination of two or more pure substances

A pure substance has constant physical and chemical properties while mixtures have varying physical and chemical properties

The atoms are different by prime difference lies in the fact that pure substances cannot by physically separated while mixtures can be (physical methods)

d. temperate deciduous forest

the answers would be

the first one and the second one