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Acids, alkalis and salts - AQAAcidic and alkaline solutions

Indicators are used to determine whether a solution is acidic or alkaline. Acids react with metals, bases and carbonates to produce salts. Neutralisation is the reaction between an acid and a base.

Part of Chemistry (Single Science)Chemical changes

Acidic and alkaline solutions

Acids

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

HCl(aq) 鈫 H+(aq) + Cl-(aq)

Acidic solutions have values less than 7.

Alkalis

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

NaOH(aq) 鈫 Na+(aq) + OH-(aq)

Alkaline solutions have pH values greater than 7.

Neutral solutions

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

Learn more on the pH scale in this podcast.

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 and a colour chart.

Universal indicator colours.
Figure caption,
Universal indicator colours

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

IndicatorAcidicNeutralAlkaline
Red litmus paperStays redStays redTurns blue
Blue litmus paperTurns redStays blueStays blue
IndicatorRed litmus paper
AcidicStays red
NeutralStays red
AlkalineTurns blue
IndicatorBlue litmus paper
AcidicTurns red
NeutralStays blue
AlkalineStays blue