Year 8 Chemical Reactions Topic 9
Summary Sheets
Chemical reactions
In a chemical reaction a new substance is always formed. Most chemical changes are not easily
reversed; they are irreversible. In a physical change no new substance is formed. Melting and
evaporation are examples of physical changes. Physical changes are usually reversible.
You can tell that a reaction has occurred if there is a colour change or when a gas is given off.
Most chemical reactions also involve an energy change. This is usually in the form of heat, but can
also involve light being given off (for example, when something burns).
Fire – a very important chemical reaction
A fire needs three things to keep burning: fuel, oxygen and heat. We show these three things on the
Fire Triangle.
If any one of these three things runs out, the fire will go out.
Fire extinguishers are used for putting out fires. There are different types of fire extinguisher, and
it is important always to use the correct sort for a particular fire. Sand or fire blankets can also be
used to put out fires.
Water is often used to put out fires, because it takes away the heat. However, water should never be
used on oil or petrol fires, because it makes the burning fuel spread out.
Foam, powder or carbon dioxide (CO2) extinguishers work by stopping oxygen getting to the
flames.
Burning Metals Reactions with Air
When a metal burns, the metal combines with oxygen from the air to form a chemical called an
oxide. We can show this using a word equation. The chemicals that you start with are called the
reactants. The chemicals at the end are called the products.
magnesium + oxygen
reactants

magnesium oxide
products
Fossil fuels contain a lot of carbon and hydrogen. When they burn they use up oxygen from the air
and produce water and carbon dioxide. We can show the reaction using a word equation. Energy is
in brackets in this equation because it is not a chemical substance.
fuel + oxygen  carbon dioxide + water (+ energy)
Reactions of Metals and Water
The metals that react quickly with air also tend to react with water. When metals react with water
they form hydrogen gas and a metal hydroxide.
metal + water  metal hydroxide + hydrogen
Reactions of Metals and Acids
Some metals react with acids, and hydrogen gas is produced. When acids react with chemicals
called carbonates, carbon dioxide gas is given off. Carbonates are found in rocks such as limestone
or marble, and in some cooking ingredients and indigestion tablets.
You can test the gas made in a reaction to find out what it is:
•
Hydrogen burns with a squeaky pop if a lighted splint is held near the test tube.
•
Carbon dioxide will put out a lighted splint, and it makes limewater turn milky.
•
Oxygen makes flames burn more brightly, and will relight a glowing splint.
Some gases are more reactive than others. In the air, oxygen is the most reactive gas. Nitrogen is
not very reactive. When metals react with the oxygen in the air they form oxides.
metal + oxygen  metal oxide
The metals that react with water also react very quickly with acids. Some metals that don’t react
with water do react with acids. When metals react with acids, they produce hydrogen and a salt.
metal + acid  salt + hydrogen
The name of the salt formed depends on the name of the acid:
•
•
•
sulphuric acid makes sulphates
nitric acid makes nitrates
hydrochloric acid makes chlorides.
Potassium Hydroxide + sulphuric acid → potassium sulphate + water.
The metal part of the alkali comes first and second part comes from the acid.
Reactivity Series
K Na Ca Mg Al C Zn Fe Sn Pb H Cu Ag Au Pt
Metals can be arranged in a Reactivity Series. The most reactive metals are placed at the top of the
table.
More reactive metals can displace less reactive metals from their compounds. In a displacement
reaction, the more reactive metal will form a compound, and the less reactive metal is left on its
own as the pure element.
For example, iron is more reactive than copper, so it will displace copper from a compound.
iron + copper chloride  iron chloride + copper
Zinc is more reactive than iron, so iron will not displace zinc from a compound.
iron + zinc chloride  no reaction
You can use displacement reactions to work out the position of a metal in the Reactivity Series. For
instance, zinc will displace lead from a compound, so we know that zinc is more reactive than lead.
The Reactivity Series can also be used to predict whether reactions will occur.
The reactivity of metals can be linked to their uses. Metals used for construction need to have a low
reactivity, otherwise they will corrode away. Some metals, such as aluminium, have a natural
protective oxide layer. Others, such as iron, have to be protected from corrosion, e.g. by painting.
Many low reactivity metals have been known for hundreds or thousands of years. They can be
extracted by heating their compounds in a fire.
More reactive metals are extracted by electrolysis. This means that they have
only been discovered in the last two hundred years, since the invention of the
electric battery.
Rusting of Iron
For rusting to occur both water and oxygen are
needed. Stopping either of these reaching the metal
prevents corrosion.
Methods of rust prevention remove one or both.
Method of rust
prevention
Paint
Used for
Advantages
Cars, bridges, boats.
grease
Moving parts –bicycle
chains.
Hard shiny silver surface
Motorway crash barriers
Cheap, can be coloured,
but scratches easily.
Needs to be checked
often and re-applied.
expensive
Chemical and physical
protection.
Tough
Chrome plating
Galvanized (zinc coating)
Plastic powder coating
Sacrificial anode
Motorcycle frames
Bicycle frames
Boats
Chemical protection
Energy from reactions: Burning Fuels
Hydrocarbon fuels are made from the elements carbon and hydrogen. Coal, oil and natural gas
(methane) are all hydrocarbons. When hydrocarbons burn they form carbon dioxide and water, and
release heat energy.
methane + oxygen  carbon dioxide + water
petrol + oxygen → carbon dioxide + water
If there is not enough oxygen available, carbon monoxide or even soot is produced.
Carbon dioxide is a greenhouse gas, it causes global warming..
More on Chemical Reactions
Bases are compounds which react with acids. All metal oxides, metal hydroxides and metal
carbonates are bases. Bases which dissolve in water are called alkalis (e.g. sodium hydroxide).
Metal oxides and hydroxides and acids
A metal oxide or a metal hydroxide reacts with an acid to form water and a salt. This reaction is
called neutralisation.
The general equation is:
acid + metal oxide  salt + water
(or hydroxide)
For example:
hydrochloric acid + potassium hydroxide  potassium chloride + water
sulphuric acid + copper oxide  copper sulphate + water
We can check to see if neutralisation has occurred using universal indicator. The pH of the solution
gets closer to neutral (pH7).
Formula of Acids
Sulphuric acid
H2SO4
Nitric Acid
HNO3
Hydrochloric Acid
HCl