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.
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 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.
A neutral solution is neither acidic, nor alkaline. A neutral solution has a pH value of 7.
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 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.
Indicator | Acidic | Neutral | Alkaline |
---|---|---|---|
Red litmus paper | Stays red | Stays red | Turns blue |
Blue litmus paper | Turns red | Stays blue | Stays blue |