Acidic and alkaline solutions

Acids

Acids form acidic solutions in water. Acids produce hydrogen ions, H+ in aqueous solution. For example:

HCl(aq) → H+(aq) + Cl-(aq)

Acidic solutions have pH values less than 7.

Alkalis

Alkalis form alkaline solutions in water. Alkalis produce hydroxide ions, OH- in aqueous solution. For example:

NaOH(aq) → Na+(aq) + OH-(aq)

Alkaline solutions have pH values greater than 7.

Neutral solutions

A neutral solution is neither acidic, nor alkaline. A neutral solution has a pH value of 7.

Indicators and the pH scale

The pH scale measures the acidity or alkalinity of a solution. The pH of a solution can be measured using a pH probe, or estimated using universal indicator and a colour chart.

Universal indicator colours.Universal indicator colours

Universal indicator is one example of an acid-alkali indicator. Indicators show whether a solution is acidic, neutral (pH 7) or alkaline. The table shows the colours for litmus paper.

IndicatorAcidicNeutralAlkaline
Red litmus paperStays redStays redTurns blue
Blue litmus paperTurns redStays blueStays blue