A water molecule consists of two hydrogen atoms and an oxygen atom with bonding and electron pairs in a tetrahedral arrangement. This results in a bent molecular arrangement, with 105° between the hydrogen atoms. Electrons spend more time around the oxygen, producing a polar molecule, with centers of negative and positive charge. Polar water molecules interact. The force of attraction is called a hydrogen bond. The hydrogen bond accounts for the decreased density of ice, the high heat of fusion, and the high heat of vaporization of water. The hydrogen bond is also involved in the dissolving process.

A solution is a homogeneous mixture of ions or molecules of two or more substances. The substance present in the large amount is the solvent, and the solute is dissolved in the solvent. If one of the components is a liquid, however, it is called the solvent.

Fluids that mix in any proportion are called miscible fluids, and immiscible fluids do not mix. Polar substances dissolve in polar solvents, but not nonpolar solvents, and the general rule is like dissolves like. Thus oil, a nonpolar substance, is immiscible in water, a polar substance.

The relative amount of solute in a solvent is called the concentration of a solution. Concentrations are measured (1) in parts per million (ppm) or parts per billion (ppb), (2) percent by volume, the volume of a solute per 100 volumes of solution, (3) percent by weight, the weight of solute per 100 weight units of solution, and (4) salinity, the mass of salts in 1 kg of solution.

A limit to dissolving solids in a liquid occurs when the solution is saturated. A saturated solution is one with equilibrium between solute dissolving and solute coming out of solution. The solubility of a solid is the concentration of a saturated solution at a particular temperature.

Water solutions that carry an electric current are called electrolytes, and nonconductors are called nonelectrolytes. In general, ionic substances make electrolyte solutions, and molecular substances make nonelectrolyte solutions. Polar molecular substances may be ionized by polar water molecules, however, making an electrolyte from a molecular solution.

The boiling point of a solution is greater than the boiling point of the pure solvent, and the increase depends only on the concentration of the solute (at a constant pressure). For water, the boiling point is increased 0.521°C for each mole of solute in each kg of water. The freezing point of a solution is lower than the freezing point of the pure solvent, and the depression also depends on the concentration of the solute.

Acids, bases, and salts are chemicals that form ionic solutions in water, and each can be identified by simple properties. These properties are accounted for by the modern concepts of each. Acids are proton donors that form hydronium ions (H₃O⁺) in water solutions. Bases are proton acceptors that form hydroxide ions (OH^-) in water solutions. The strength of an acid or base is measured on the pH scale, a power of ten notation of the hydronium ion concentration. On the scale, numbers from 0 up to 7 are acids, 7 is neutral, and numbers above 7 and up to 14 are bases. Each unit represents a tenfold increase or decrease in acid or base properties.

A salt is any ionic compound except those with hydroxide or oxide ions. Salts provide plants and animals with essential elements. The solubility of salts varies with the ions that make up the compound. Solutions of magnesium or calcium produce hard water, water in which it is hard to make soap lather. Hard water is softened by removing the magnesium and calcium ions.