Indicators and the pH scale

Solutions can be acidic, alkaline or neutral:

• we get an acidic solution when an acid is dissolved in water
• we get an alkaline solution when an alkali is dissolved in water
• solutions that are neither acidic nor alkaline are neutral

Pure water is neutral, and so is petrol.

An indicator is a substance that changes colour when it is added to acidic or alkaline solutions. You can prepare homemade indicators from red cabbage or beetroot juice - these will help you see if a solution is acidic or alkaline.

Litmus and universal indicator are two indicators that are commonly used in the laboratory.

Litmus

Litmus indicator solution turns red in acidic solutions and blue in alkaline solutions. It turns purple in neutral solutions.

Litmus paper is usually more reliable, and comes as red litmus paper and blue litmus paper. The table shows the colour changes it can make.

 Red litmusBlue litmus Acidic solution Stays red Turns red Neutral solution Stays red Stays blue Alkaline solution Turns blue Stays blue

Notice how we say 'stays red'. This is better than saying 'nothing' or 'stayed the same', because it tells us the colour we actually see.

Acids turn blue litmus paper red

Alkalis turn red litmus paper blue

Universal indicator and the pH scale

Universal indicator is supplied as a solution or as universal indicator paper. It is a mixture of several different indicators. Unlike litmus, universal indicator can show us how strongly acidic or alkaline a solution is, not just that the solution is acidic or alkaline. This is measured using the pH scale, which runs from pH 0 to pH 14.

Universal indicator has many different colour changes, from red for strongly acidic solutions to dark purple for strongly alkaline solutions. In the middle, neutral pH 7 is indicated by green.

Colour chart of universal…