What is the difference between a compound and a mixture?

A mixture can be separated by physical means into its various components. A compound requires a chemical change to separate it into its components. For example, salt and sawdust can be separated by dissolving the salt in water, filtering, then evaporating the water. The salt will be left from the evaporation step and the sawdust will be collected in the filtration step. Water, on the other hand, is composed of 2 atoms of H and 1 atom of O (per molecule of H2O) and it can not be separated except by chemical means. Note also, that the sawdust and salt maintain their individual properties while in the mixture, while H and O are gases that are not similar to water when they are combined.