Acids, Bases and Indicators
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flagWhat you'll discover
- arrow_forwardDefine an acid and a base and give everyday examples of each
- arrow_forwardDescribe the colour change of litmus, phenolphthalein and methyl orange in acid, base and neutral solutions
- arrow_forwardRead a pH value and place a solution on the pH scale
- arrow_forwardExplain what a universal indicator is and why it is more useful than litmus
- arrow_forwardPredict the products of an acid-base (neutralisation) reaction
Acids and bases at home
An acid gives hydrogen ions (H⁺) in water and tastes sour: lemon juice (citric acid), vinegar, curd, tamarind, sour milk. A base gives hydroxide ions (OH⁻) in water, feels soapy and tastes bitter: soap, lime water, milk of magnesia, baking soda solution, ash water.
Some substances are neutral, neither acidic nor basic — pure water, common salt solution and sugar solution. pH 7 is neutral; below 7 is acidic and above 7 is basic. Strong acids have pH near 1 and strong bases near 13.
Indicators and their colours
An indicator is a substance that changes colour when it is put into an acid or a base.
Blue litmus turns red in an acid; red litmus turns blue in a base. Phenolphthalein is colourless in acid and neutral solutions but pink in a base. Methyl orange is red-orange in an acid but yellow in a base. Universal indicator is a mixture and shows a whole range of colours, so it tells you roughly how strong the acid or base is, not only which one it is.
The colours are worth memorising in a table — a question that asks for "the colour of phenolphthalein in lime water" is a free mark if you know it, and a lost one if you guess.
Neutralisation
When an acid reacts with a base they cancel each other out and form a salt and water: HCl + NaOH → NaCl + H₂O. Because the product is often neutral, the reaction is called neutralisation.
This is why lime water soothes an acid burn, and why an antacid tablet (a mild base) settles acidity in the stomach. In the lab you can follow the neutralisation by adding a drop of phenolphthalein: the pink colour of the base disappears exactly at the end point.