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
Question

A few drops of universal indicator solution are added to solution X. The resulting mixture is yellow.

Estimate the pH of solution X, and state whether it is acidic, alkaline or neutral.

Solution X has a pH value of 5, and is acidic.