Acids, Bases and Salts In this section we look at the properties of acids and bases, find out what salts are, and how to prepare them.

Acids: General Properties: Corrosive. Have a sour taste – DO NOT TRY THIS. Turn blue litmus red. Are neutralised by bases (more about this later). Have a pH of less than 7. Produce hydrogen ions, H+, in solution.

Bases: General Properties: Soluble bases are called ‘alkalis’. Are neutralised by acids. Alkalis have a ‘soapy’ feel – DO NOT TRY THIS. Have a pH of more than 7. Turn red litmus blue. Alkalis produce hydroxide ions, OH—, in solution.

Alkalis and Bases: Alkalis and bases will both neutralise acids, but there is a difference! Some bases are soluble in water. Soluble bases are called alkalis. A diagram will help:

The diagram below shows the relationship between alkalis and bases: Alkalis are a ‘sub-set’ of bases.

Indicators: Indicators are substances that change colour depending on whether the solution is acid or alkaline. Some indicators are more useful than others as they also indicate the strength of the acid or alkali. You need to know the names and colours of some common indicators.

Common Indicators: Litmus, red in acid solution, blue in alkaline solution. Phenolphthalein, colourless in acid solution, pink in alkaline solution. Universal indicator, has a range of colours depending on the strength of the acid or alkali.

Strength of Acids and Alkalis: The strength of acids and alkalis are measured using the pH scale. This is a number scale ranging from zero to 14. pH 7 = neutral, neither acid nor alkali pH less than 7 = acid, the lower the number the stronger the acid. pH more than 7 = alkaline, the higher the number the stronger the alkali.

Measuring the Strength of Acids and Alkalis: pH meter. An electronic device that gives a direct reading of the pH value. Very accurate meters give readings to two decimal places. Universal Indicator. Available as papers or solution. A few drops are added to the solution or a strip of paper is dipped in the solution and the colour compared with a chart. Not very accurate, but cheap and easy to use.

Universal Indicator Colours: pH numbers of ‘everyday’ substances: Dilute hydrochloric acid, 1.5 Lemon juice, 2.5 Orange juice, 3.5 Beer, 4.5 Pure water, 7.0 Blood, 7.5 Milk of magnesia, 10.5 Lime water, 12.5 Dilute sodium hydroxide, 13.0

Neutralisation: This is when an acid and a base react together to give a solution with a ph of 7 (a neutral solution).