Acids and bases

Acids and bases are two complementary classes of chemical substances that play a central part in everyday life and in the laboratory. In the common Bronsted–Lowry description, an acid is a substance that donates hydrogen ions (protons) and a base is one that accepts them. When an acid dissolves in water it releases hydrogen ions, while a base increases the concentration of hydroxide ions. The relative strength of an acid or base depends on how completely it dissociates in solution.

The acidity or basicity of a solution is measured on the pH scale, which typically runs from 0 to 14. A pH below 7 indicates an acidic solution, a pH of 7 is neutral, and a value above 7 indicates a basic, or alkaline, solution. The scale is logarithmic, so each whole-number step represents a tenfold change in hydrogen-ion concentration. Indicators such as litmus or universal indicator change colour according to pH and are used to estimate it.

When an acid and a base are combined, they undergo a neutralisation reaction that generally produces a salt and water. Familiar examples include the hydrochloric acid in stomach fluid, the citric acid in fruit, and bases such as sodium hydroxide or the bicarbonate used in baking. Interactive simulations help learners see these ideas, animating the dissociation of molecules into ions, the movement of protons, and the way pH changes as solutions are mixed or diluted.

Frequently asked questions

What does the pH scale measure?

It measures how acidic or basic a solution is, based on its hydrogen-ion concentration. Values below 7 are acidic, 7 is neutral and above 7 is basic.

What happens when an acid reacts with a base?

They neutralise each other in a reaction that typically yields a salt and water, reducing the acidity or basicity of the mixture.

What is the difference between a strong and a weak acid?

A strong acid dissociates almost completely in water, releasing nearly all its hydrogen ions, whereas a weak acid only partly dissociates.