b Plant extracts to indicate whether a substance is acidic or basic i Put a spot of the coloured flower extract on to a filter paper and leave to dry Put one drop of lemon juice on to the spot then note any change of colour Repeat the experiment with other fruit juices and vinegar They are acidic substances Note any colour change with dilute hydrochloric acid Different colours suggest that some substances are more acidic than others ii Put some of the original filtrate on to another piece of filter paper When dry note the colours given by sodium bicarbonate solution washing soda limewater and a dilute solution of sodium hydroxide These are alkaline or basic substances Note whether they all give the same colour Plant extracts can act as indicators to test whether a substance is acidic or basic iii Add a few drops of sodium bicarbonate solution to 1 mL of flower extract indicator in a testtube Then add lemon juice and note any colour change iv Repeat the experiment with limewater and indicator followed by dilute hydrochloric acid Note any colour change Note whether you can get back the original colour by adding more limewater Note how many times you can change the indicator colour before the testtube is full v Litmus an extract of lichens is another plant indicator An acidic solution turns blue litmus red An alkaline solution turns red litmus blue v Universal Indicator can be in the form of a solution or dried on filter paper Universal indicator not only indicates whether a substance is acidic or basic but also how acidic it is Investigate the effect of Universal Indicator on the solutions above To avoid using the name of a colour to indicate acidity we use a scale of numbers from 0 to 14 called the pH scale We can use the pH scale to express the degree of acidity Acid solutions have a pH value less than 7 Alkaline or basic solutions have a pH value greater than 7 Solutions with a pH value of 7 are neither acidic nor basic they are neutral On the bottle or packet of Universal indicator a colour chart shows the colour and the pH value associated with this colour as follows a Colour b pH c AcidBase a Red b 13 c very acidic a Orange b 45 c weak acid a Yellow b 6 c very weak acid a Green b 7 c neutral a Blue b 8 c very weak base a Indigo b 910 c weak base a Violet b 11 14 c very basic Use 2 drops of Universal Indicator to 10 mL of solution to be tested Test the pH value of lemon juice vinegar sodium bicarbonate solution washing soda lime water sodium hydroxide solution tap water distilled water