S1 Substances and Uses summary notes

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1 S1 Substances and Uses summary notes The ph scale LI 1 To develop our understanding of the ph scale Acids and alkalis are very common substances in a science lab and at home. The ph scale measures the acidity and alkalinity of a substance in a solution. (The substances must be dissolved in water) There are different pieces of equipment for measuring the ph of a substance. Universal indicator ph meter ph paper The ph meter gives a digital value between 0 and 14 which tells us how acidic or alkaline a substance is. The ph paper and universal indicator will show a colour when tested with a substance. This can then be compared with the ph scale to determine how acidic or alkaline the substance is.

2 Measuring ph of a substance LI 2 To explore the uses of acids and alkalis in everyday life The diagram below shows how we can easily measure the ph of any substance with a piece of ph paper. Substance to be tested small piece of ph paper dimple tile Below are some examples of everyday acids and alkalis and their ph: Name of Substance Laboratory or Household Colour of ph paper ph number Acid, Alkali or Neutral hydrochloric acid lab red 1 acid sodium hydroxide lab blue 14 alkali vinegar house red 3 acid baking soda house blue 9 alkali nitric acid lab red 1 acid ammonia solution lab blue 11 alkali pure water house green 7 neutral soda water house red 5 acid

3 dishwasher powder house blue 10 alkali lime water lab blue 9 alkali cola house red 5 acid lemon juice house red 4 acid sulphuric acid lab red 1 acid oven cleaner house blue 13 alkali lemonade house red 4 acid salt water house green 7 neutral bleach house blue 13 alkali Common Household Acids vinegar lemon juice cola Common Household Alkalis dishwasher powder oven cleaner bleach Laboratory Acids hydrochloric acid sulphuric acid nitric acid Laboratory Alkalis sodium hydroxide ammonia solution calcium hydroxide

4 Dilution of acids LI 3 To investigate the effect of dilution on acids and alkalis A concentrated acidic solution is one that contains a lot of acid compared with water. A dilute solution contains more water than acid. Think of diluting orange juice:- More orange and less water = concentrated solution Less orange and more water = diluted solution Diluting an acid by adding more water increases the ph of the solution towards 7 making it less acidic. As the acid is diluted the ph increases towards 7

5 Diluting an alkali by adding more water decreases the ph of the solution towards 7 making it less alkaline As the alkali is diluted the ph decreases towards 7 It is not possible to dilute an acid so that its ph increases to greater than 7 nor an alkali so that its ph decreases below 7. This is because we are diluting them with water which is a neutral substance (with a ph of 7) so adding more and more water will simply maintain the ph of the diluted acid or alkali at 7. Neutralisation LI 4 To investigate the reactions of acids and why these reactions are useful Acids and alkalis neutralise each other to form a salt and water. Remember that the acid has a low ph and the alkali a high ph, but when they are mixed in precise quantities they form water (a neutral substance) plus salt. Alkali

6 When performing a neutralisation experiment it is important not to add too much alkali to the acid as the ph will increase beyond 7 creating an alkaline solution. Remember that neutralisation is not the same as dilution. In dilution water is added to the acid so the ph of the solution cannot increase above 7 but in neutralisation it is an alkali that is being added so it is quite easy to go beyond 7 and produce a high ph alkali. Everyday examples of neutralisation: Indigestion tablets help with heartburn by neutralising stomach acid Toothpaste neutralises acid from food and so helps to prevent tooth decay

7 Naming Salts made by Neutralising Acids and Alkalis LI 5 To apply knowledge to name the products of a neutralisation reaction Alkali The first part of the name is always the metal that is part of the alkali and the second part comes from the acid. If the acid is hydrochloric acid, the salt will be a chloride. If the acid is nitric acid, the salt will be a nitrate. If the acid is sulphuric acid, the salt will be a sulphate. In the example below the alkali sodium hydroxide is added to hydrochloric acid to produce sodium chloride. eg. alkali acid sodium hydroxide hydrochloric acid salt name sodium chloride

8 In this example the alkali lithium hydroxide is added to sulphuric acid and the salt lithium sulphate is produced. eg. alkali acid lithium hydroxide sulphuric acid salt name lithium sulphate A general word equation can be used to describe this reaction: ACID + ALKALI SALT + WATER More examples: hydrochloric acid + copper oxide copper chloride + water sulphuric acid nitric acid + sodium hydroxide sodium sulphate + water + calcium hydroxide calcium nitrate + water

9 Acid Rain Non-metal elements can react with air or oxygen to form compounds called oxides. carbon + oxygen sulphur + oxygen nitrogen + oxygen carbon dioxide sulphur dioxide nitrogen dioxide Non-metal oxides which can dissolve in water (like those above) produce acids. e.g. Sulphur dioxide dissolves in water to produce sulphuric acid Nitrogen dioxide dissolves in water to produce nitric acid Acid Rain is an environmental problem which can: damage buildings made from carbonate rocks (e.g. marble)

10 corrode metals structures made of iron and steel (e.g. bridges) affect plant life by changing the ph of soil (e.g. less crops grown) affect animal life by changing the ph of rivers/lakes (e.g. kills fish)

11 Acid rain could be reduced if the gases that cause acid rain were reduced: Carbon dioxide is released when fossil fuels like coal, petrol and gas are burned If we burn less fossil fuels, then less carbon dioxide will be released Sulphur dioxide is released when fossil fuels like coal are burned Use low-sulphur fuels instead of high sulphur fuels Remove sulphur dioxide fumes before release into atmosphere Nitrogen dioxide is produced in car engines by the sparking of air with petrol to set the petrol on fire. Fit catalytic converters to cars to remove nitrogen dioxide from car exhaust fumes. Neutralisation with indigestion powder LI 6 To investigate how changing certain variables affects the rate of the reaction between an acid and calcium carbonate Acids and alkalis neutralise each other to form a salt and water. Other substances also neutralise acids to form a salt. stirring rod Indigestion powder spatula 10cm 3 hydrochloric acid + 3 drops of universal indicator In the experiment, indigestion powder is used to neutralise hydrochloric acid just like an alkali neutralises an acid. Only this time it is not just salt and water that is produced.

12 This time we see that the red hydrochloric acid turns green as it is neutralised but we also notice that a gas is given off. Acid + Metal Carbonate Salt + Water + Carbon Dioxide Indigestion powder is a form of metal carbonate which means it will give off carbon dioxide gas as well as salt and water during neutralisation. e.g. sulphuric acid + lithium carbonate lithium sulphate + water + carbon dioxide