Acidic and alkaline solutions


Acids form acidic solutions in water. Acids are sources of hydrogen ions, H+. For example, hydrochloric acid produces hydrogen ions:

HCl(aq) → H+(aq) + Cl-(aq)

Acidic solutions have pH values less than 7.


Alkalis form alkaline solutions in water. Alkalis are sources of hydroxide ions, OH-. For example, sodium hydroxide produces hydroxide ions:

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.


Acid-alkali indicators can show whether a solution is acidic, neutral (pH 7) or alkaline. The table shows the colours for two different common indicator solutions.

Litmus paperRedPurpleBlue
Methyl orangeRedYellowYellow

Describe the limitations of methyl orange when determining if a solution is alkaline.

Methyl orange is yellow in neutral solutions as well as in alkaline solutions, so you can only be sure that a solution is acidic if methyl orange turns it red.


Describe the limitations of phenolphthalein when determining if a solution is acidic.

Phenolphthalein is colourless in neutral solutions and in acidic solutions, so so you can only be sure that a solution is alkaline if phenolphthalein turns it pink.

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 and pH values