A chemical reaction ALWAYS forms a new substance.

Any chemical reaction can be represented by a word equation
example: burning hydrogen form water
Word equation: hydrogen +  oxygen → water

For a summary of all word equations shown on this page and more information on word equations click this link

Signs that show a chemical reaction has taken place are:

i. A change in colour
ii. A change in temperature (usually gets hot)

Types of reaction

 


1. Burning

This is when a substance reacts with oxygen and produces a flame.

Burning always produces an OXIDE.

Burning an element produces ONE oxide.

Burning a compound produces TWO or more oxides.

The mass of the oxides formed will always be MORE than the mass of the substance being burned. (Why?)
Note: in order to show an increase in mass it is important to weigh all of the products. If one of the products is smoke or a gas this will be difficult.

 Burning magnesium
Burns with bright white flame. The magnesium combines with oxygen too leave a white ash called magnesium oxide

Word equation: Magnesium + oxygen —-> magnesium oxide

 Burning hydrogen
Hydrogen is a flammable gas that burns to form water
(water is hydrogen oxide)
Hydrogen can explode when lit and this reaction is known as the ‘squeeky pop’ test to identify hydrogen

Word equation: Hydrogen + oxygen —->  water

Burning Methane (natural gas)
Methane is a COMPOUND of carbon and hydrogen. (ie a hydrocarbon. other hydrocarbons are wax, petrol)

It burns with a smoky flame to form carbon dioxide and water

Word equation: Methane + oxygen ——> carbon dioxide and water

Alcohol will also burn to form carbon dioxide and water.


2. Decomposition

This is when a compound splits apart and forms two (or more) new substances.. (Each of the new substances contains atoms that was there to start with)

note : Elements can NEVER decompose because they are only contain ONE kind of atom.

Each new substance formed will have LESS mass than the original substance

eg i. Heating copper sulphate

Copper sulphate DECOMPOSES and forms water vapour, leaving white anhydrous copper sulphate.

Word equation: Hydrated copper sulphate —-> anhydrous copper sulphate + water

NOTE: This reaction is reversible. If water is added to anhydrous copper sulphate then the solution turns blue again accompanied by a rise in temperature.

Heating copper carbonate The copper carbonate DECOMPOSES forming carbon dioxide gas and leaving copper oxide.

Word equation: Copper carbonate —-> copper oxide + carbon dioxide

iii. Heating limestone

The limestone decomposes and forms LIME and carbon dioxide gas

Word equation: Calcium carbonate —–> calcium oxide + carbon dioxide

or Limestone —–> lime + carbon dioxide

 
3. Oxidation and Reduction

Oxidation is when a substance reacts with (or joins up with) oxygen

Reduction is when a substance has oxygen taken away from it.

eg i. Heating carbon with copper oxide

The carbon REDUCES the copper oxide, forming carbon dioxide gas and leaving pink copper metal.
This reaction happens because carbon is more reactive than copper so takes its place, pushing the copper away from the oxygen.

Word equation: Carbon + copper oxide —–> carbon dioxide + copper


Displacement reactions
This is when a reactive metal displaces another metal from one of it’s compounds.
This reaction happens because iron is more reactive than copper and ‘pushes’ the copper out of the solution, taking its place.

1.   Reacting iron with copper sulphate solution
Word equation: Iron+ copper sulphate iron sulphate + copper

 

Placing burning magnesium in carbon dioxide

Word equation: magnesium + carbon dioxide —–> magnesium oxide + carbon


4.  Reactions with an acid
: Neutralization

i. Adding dilute hydrochloric acid to marble

The acid will react with the marble and produce carbon dioxide gas.

Word equation
Hydrochloric acid + calcium carbonate ——> calcium chloride + carbon dioxide

Adding dilute hydrochloric acid to dilute sodium hydroxide solution.

The hydrochloric acid will NEUTRALISE the sodium hydroxide (a strong alkali) forming water and common salt.

Word equation
Hydrochloric acid + sodium hydroxide ——> sodium chloride + water

Reactions in Biology


Respiration
Respiration is the reaction that takes place in ALL living organisms that releases energy from sugar. This energy is used by the organisms for essentials like growth, movement and warmth

Word equation: Glucose + oxygen —-> carbon dioxide + water


Photosynthesis
This is the reaction that takes place in the chloroplasts (found mainly in leaves) of plant cells

It converts carbon dioxide and water into sugar and starch, using energy from the sun.

Word equation    Carbon dioxide + water ———–> glucose + oxygen 
(Chlorophyll+ sunlight)

For a more detailed explanation see ‘Photosynthesis‘ in the biology section


Fermentation
Fermentation is the name of the process that turns sugar into alcohol and carbon dioxide

It is carried out by a fungus called YEAST and is used a lot in the baking and brewing industries

Word equation: sugar —-> carbon dioxide + alcohol


How to identify chemicals:

Water Chemical test: Add it to anhydrous cobalt chloride which will turn from blue to pink
(This shows that a liquid contains water)
Physical test: Measure the boiling point which should be100 C
(this shows that a liquid is pure water)
Hydrogen:  Hydrogen burns with a squeaky ‘pop’.
Carbon dioxide:  Bubble the gas through lime-water which will turn cloudy.
Oxygen Oxygen will re-light a glowing splint.