6 Acids and Bases

How acids and bases behave

In the Brønsted–Lowry model, an Acid donates a proton, while a Base accepts a proton. In water, acids increase the amount of hydronium ions, written as $\mathrm{H_3O^+}$; many familiar bases produce hydroxide ions, written as $\mathrm{OH^-}$.

Understanding pH

The pH of an aqueous solution describes how acidic or basic it is. At room temperature, a pH of $7$ is neutral; values below $7$ are acidic, and values above $7$ are basic, or alkaline. A common definition is:

The scale is logarithmic: a solution at pH $3$ has ten times the hydrogen-ion concentration of a solution at pH $4$. Thus, each pH unit represents a tenfold change in hydrogen-ion concentration.

The familiar scale from $0$ to $14$ is a useful guide for many everyday solutions. Lemon juice and vinegar are acidic; baking-soda solution and many soaps are basic. Exact pH depends on concentration and conditions.

Strength and concentration describe different things. Strength is how extensively an acid or base ionizes in water; concentration is how much of it is present in a given volume.

Using indicators

An acid–base indicator changes color over a particular pH range, helping estimate whether a solution is acidic or basic. Blue litmus turns red in acid, while red litmus turns blue in base. Universal indicator or pH paper shows a broader range of colors that can be used to estimate pH.

Phenolphthalein is colorless in acidic solutions and pink in sufficiently basic solutions. Indicators estimate pH; a pH meter can measure it more precisely.

Neutralization in reactions and everyday life

When an acid and a base react, they neutralize each other’s acidic and basic properties. In a typical aqueous reaction, hydrogen ions and hydroxide ions form water:

For example, hydrochloric acid reacts with sodium hydroxide to form water and sodium chloride, which is a salt:

The resulting mixture is not automatically at pH $7$; its pH depends on the amounts and on the acid and base involved. Equal reacting amounts of a strong acid and a strong base produce a neutral solution under typical conditions.

Neutralization has everyday uses. Antacids such as magnesium hydroxide help relieve heartburn by reacting with some of the excess acid in the stomach. Baking soda, a base, also reacts with vinegar, an acid. Never mix household chemicals unless the product labels specifically say it is safe.