Matter & Energy


Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Solutions

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Solutions, in chemistry, homogeneous (uniform) mixtures of two or more substances. The substance present in largest quantity usually is called the solvent, and the substance present in smallest quantity is called the solute. Although a solvent can be a gas, liquid, or solid, the most common solvent is liquid water. A solution with water as the solvent is called an aqueous solution. The solute can be a gas, liquid, or solid. Carbonated water is an example of a gas solute (carbon dioxide) dissolved in a liquid solvent (water). Sugar water is an example of a solid solute (sugar) dissolved in a liquid solvent (water). Many metals are soluble in one another, forming solid solutions known as alloys. Mixtures of gases, such as the atmosphere, are sometimes referred to as solutions as well. Solutions are distinct from colloids (see Colloid) and suspensions in that the particles of the solute are of molecular size and are evenly dispersed among the molecules of the solvent. Solutions appear homogeneous under the microscope, and the solute cannot be separated by filtration. Salts, acids, and bases ionize when they are dissolved in water (see Acids and Bases; Ionization).

SOLUBILITY

Some liquids, such as water and alcohol, can dissolve in each other in any proportion. If sugar is dissolved in water, however, new sugar added to the solution above a certain amount will not dissolve any more; the solution is then called saturated. The solubility of a compound in a given solvent at a given temperature and pressure thus is defined as the maximum amount of that compound that can be dissolved in the solution. Solubility increases with the increasing temperature of the solvent for most substances. For some substances, such as gases or the organic salts of calcium, solubility in a liquid increases with a lowering of temperature. In general, solutions with molecules that are structurally similar to the molecules of the solvent have the highest solubility. For example, ethyl alcohol and water have structurally similar molecules and are highly soluble in one another.

PHYSICAL PROPERTIES OF SOLUTIONS

When a solute is added to a solvent, several physical properties of the solvent change. Its boiling point is raised and its freezing point lowered with increasing concentrations of solute. For example, cooling water in an automobile engine can be prevented from freezing by adding an antifreeze such as ethylene glycol as a solute. In addition, the vapor pressure of a solvent is lowered when a solute is added.

Another important property of a solution is its ability to exert osmotic pressure. If two solvents are separated by a semipermeable membrane (a membrane that allows the passage of the solvent molecules but prevents passage of the solute molecules), solvent molecules will migrate from the solution of lower concentration to the solution of higher concentration, making the latter more dilute (see Osmosis).