The pH scale is used to measure how acidic or alkaline a solution is. Only chemicals that dissolve in water can have a pH. A pH of less than 7 is an acid; a pH of more than 7 is an alkali. Only a neutral substance would have a pH of exactly 7. Pure water has a pH of 7. An indicator such as Universal Indicator can be used to find out whether a solution is acidic or alkaline. The indicator changes colour to show what the pH of the solution is. Universal indicator is red in an acid, green in a neutral solution & blue in an alkali.
When reactive metals react with oxygen (or water) they form alkaline substances (always solids when not in solution). When non-metals react with oxygen they form acidic substances (usually gases).
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Reactive metals |
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sodium |
+ |
oxygen |
→ |
sodium oxide |
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magnesium |
+ |
oxygen |
→ |
magnesium oxide |
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calcium |
+ |
water |
→ |
calcium hydroxide |
+ |
hydrogen |
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Non-metals |
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carbon |
+ |
oxygen |
→ |
carbon dioxide |
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|
sulphur |
+ |
oxygen |
→ |
sulphur dioxide |
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Neutralisation reactions
When acids and alkalis meet a neutralisation reaction occurs. Any acid will react with any alkali to produce a salt + water.
Here are some examples…
|
sodium hydroxide |
+ |
hydrochloric acid |
→ |
sodium chloride |
+ |
water |
|
calcium hydroxide |
+ |
hydrochloric acid |
→ |
calcium chloride |
+ |
water |
|
calcium hydroxide |
+ |
sulphuric acid |
→ |
calcium sulphate |
+ |
water |
Some things, which are not alkalis, can also neutralise acids. When a metal reacts with oxygen an oxide is formed. If the metal is very reactive then the oxide will react with water to form an alkali (hydroxide). If the metal is unreactive (an earth metal) its oxide will not dissolve in water – we call these oxides bases and they can neutralise acids. The main earth (or base) metals you will meet are copper, iron & zinc. Reacting these basic oxides with different acids can make some interesting salts. Here are some examples…
|
copper oxide |
+ |
sulphuric acid |
→ |
copper sulphate |
+ |
water |
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|
iron oxide |
+ |
hydrochloric acid |
→ |
iron chloride |
+ |
water |
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|
zinc oxide |
+ |
sulphuric acid |
→ |
zinc sulphate |
+ |
water |
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Reactions with metals
Acids will also react with metals to produce a salt + hydrogen. The more reactive the metal, the faster the gas is produced. Metals like tin and lead hardly react at all.
|
magnesium |
+ |
sulphuric acid |
→ |
magnesium sulphate |
+ |
hydrogen |
The test for hydrogen is that it burns with a squeaky pop.
Reactions with carbonates
When an acid reacts with any carbonate, a salt, carbon dioxide gas & water are produced.
|
calcium carbonate |
+ |
hydrochloric acid |
→ |
calcium chloride |
+ |
carbon dioxide |
+ |
water |
The test for carbon dioxide is that it turns limewater milky.
|
sodium bicarbonate |
+ |
sulphuric acid |
→ |
sodium sulphate |
+ |
carbon dioxide |
+ |
water |
They always follow the same pattern so you can predict the outcome. Sulphuric acid produces sulphates and hydrochloric acid produces chlorides.
Which salt is produced in the following examples?
|
1. copper oxide |
+ |
hydrochloric acid |
→ |
_____________ _____________ |
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|
2. magnesium hydroxide |
+ |
sulphuric acid |
→ |
_____________ _____________ |
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|
3. copper oxide |
+ |
sulphuric acid |
→ |
_____________ _____________ |
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|
4. sodium hydroxide |
+ |
hydrochloric acid |
→ |
_____________ _____________ |
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|
5. lead oxide |
+ |
sulphuric acid |
→ |
_____________ _____________ |
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|
6. potassium hydroxide |
+ |
nitric acid |
→ |
_____________ _____________ |
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