Acids, Bases and Salts

Acids, bases and salts: how indicators and the pH scale tell them apart, how they react, and the salts used every day.

Key ideas

• Acids give H⁺ ions in water; bases that dissolve in water (alkalis) give OH⁻ ions.
• Litmus: acids turn blue litmus red, bases turn red litmus blue. Phenolphthalein is pink in a base and colourless in an acid.
• pH runs from 0 to 14: below 7 is acidic, 7 is neutral, above 7 is basic. The lower the pH, the stronger the acid.
• Acid + base → salt + water (neutralisation).
• Acid + metal → salt + hydrogen gas. Acid + metal carbonate → salt + carbon dioxide + water.
• When diluting, add the acid slowly to water, never water to acid: the mixing gives out a lot of heat.
• Some salts hold water of crystallisation: blue copper sulphate crystals turn white when heated as the water leaves.

Remember

Watch out

• Thinking pH 2 is weaker than pH 5 (a lower pH is more acidic).
• Adding water to concentrated acid.
• Expecting phenolphthalein to turn pink in an acid (it stays colourless).

Worked example