Finally, we must mention homogeneous mixtures, mixtures of more than one state of matter or mixtures of substances in the same state. These homogeneous mixtures are usually called solutions. We often think of solutions as being solids dissolved in a liquid. For example, we all know that sugar can easily be dissolved in water. When the solution contains as much sugar as it can hold, it is said to be saturated. Any sugar added beyond that point simply remains as solid--it does not enter the solution. The concentration of such solutions is frequently expressed as molarity (M): moles of solute (the sugar) per liter of solution. Sometimes it is more convenient to express the concentration as molality (m): moles of solute per kilogram of solvent.
We can also make solutions by bubbling gases into liquids, mixing liquids with liquids, mixing gases with gases, and even mixing gases with solids.
Think of examples of solutions of gases in liquids, liquids in liquids, and gases in gases.
Soda water contains gaseous carbon dioxide dissolved in water. Gasoline is a mixture of hydrocarbons. The atomosphere that we breathe is a mixture of gases, primarily nitrogen and oxygen.