Matter in Combination
In the real world, we rarely encounter pure substances. Most of the matter around us, from the air we breathe to the milk we drink, is a mixture. A mixture is made of two or more substances that are physically combined but not chemically bonded. This means each substance in the mixture keeps its own original properties.
A key feature of a mixture is that its components can be separated by physical means (like filtering, evaporation, or using a magnet).
Heterogeneous vs. Homogeneous Mixtures
There are two main types of mixtures.
Heterogeneous Mixture: A mixture in which the different components are unevenly distributed and are easy to see. 'Hetero-' means 'different'.
Examples: A salad (you can see the lettuce, tomatoes, cucumbers), sand and water (the sand settles at the bottom), a chocolate chip cookie.
Homogeneous Mixture: A mixture in which the components are evenly distributed throughout, and you cannot see the individual parts. 'Homo-' means 'same'. These mixtures appear to be a single substance.
Examples: Salt water, air (a mixture of gases), brass (a mixture of metals called an alloy).
Solutions: A Special Type of Mixture
A solution is another name for a homogeneous mixture. A solution has two main parts:
Solvent: The substance that does the dissolving. It is the component present in the largest amount. Water is known as the 'universal solvent' because it can dissolve so many things.
Solute: The substance that gets dissolved. It is the component present in a smaller amount.
Example: When you make salt water, water is the solvent and salt is the solute.
Solubility: How Much Can Dissolve?
Solubility is a measure of how much solute can dissolve in a a given amount of solvent at a specific temperature.
A solution is unsaturated if it can still dissolve more solute.
A solution is saturated when it has dissolved the maximum amount of solute possible at that temperature. If you add any more solute, it will just sink to the bottom.
A solution is supersaturated if it holds more dissolved solute than is normally possible. These are unstable.
Factors Affecting Solubility:
Temperature: For most solid solutes, solubility increases as the temperature of the solvent increases. You can dissolve more sugar in hot water than in cold water. For gases, the opposite is true: solubility decreases as temperature increases.
Pressure: Pressure mainly affects the solubility of gases. The solubility of a gas in a liquid increases as the pressure of the gas above the liquid increases. This is why soda is carbonated under high pressure. When you open the can, the pressure is released, and the dissolved CO₂ gas comes out of solution as bubbles.