Summary Science - Chemistry - Chemical Changes

Chapter 18 - Chemical Changes

I. Physical and Chemical Changes


Physical Change Chemical Change

No new substances New substances formed

Little heat (usually no) A lot of heat

Substance’s properties do not change Substance’s properties change


Helpful chemical changes:


Decomposition Growth Reproduction

Photosynthesis Cooking Metal alloys

Digestion Cleaning products Medicines

Cellular respiration Fuel combustion Making products in industry


Harmful chemical changes:
 Burning: Burning releases toxic gases (e.g: sulphur dioxide) into the atmosphere, this
can cause breathing problems and acid rains which kill fish and other aquatic life.
 Rusting
 Decomposition

These changes are brought about by interactions between matter and different forms of
energy:
 Heat (section V.)
 Light (section VII.)
 Electricity (section VIII.)
 Mixing matter (section IX.)

Indicators of chemical changes:
 Color change
 Precipitate formation: a solid settled after mixing certain liquids
 Gas bubbles: caused when gas released
 Odor change
 Energy change (Temperature and Light)
II. Chemical Reaction
Reactant A + Reactant B → Product C + Product D
Reactant (s) - (Condition) → Products (s)



III. Different Phases
Solid (s) ←→ Liquid (l) ←→ Gas (g)
 Expansion and Contraction
 Phase shifts are physical changes because phases are reversible and no new
substances are formed.



IV. Diatomic Elements
 Hydrogen H2
 Nitrogen N2

,  Fluorine F2
 Oxygen O2
 Iodine I2
 Chlorine Cl2
 Bromine Br2



V. Chemical Changes Caused by Heat


Combination  Two or more substances form a new substance
Iron + Sulphur → Iron Sulphur
Fe + S → FeS

Combustion  Combination of a substance with oxygen with heat
 In the combustion of Hydrocarbon (a combination of carbon and
hydrogen atoms), CO2 and H2O are formed
 Combustion cannot take place on the moon, unless there is a supply
of oxygen


Complete Combustion Incomplete Combustion

 Plenty of O2  Not enough O2
 Produce CO2  Forms a soot
 Forms CO
 Combustion → Pollutants




 Hydrocarbon + Oxygen → Carbon dioxide + Water

Oxidation  A process where a substance reacts with O2
(rusting)  Examples:
o Rusting - oxidation of iron in the presence of water
o Burning of magnesium in O2 - product is Magnesium oxide
(MgO)
o Respiration
o Combustion

Thermal  Substance breaks down into 2 or more substances by heat.
Decomposition  NO OXYGEN



VI. Cellular Respiration
 Breakdown of glucose in the presence of oxygen to release energy
 It is a combustion reaction (product is CO2 and H2O)
 Formula:
o C6H12O6 + O2 → CO2 + H2O (reverse of photosynthesis)
 ATP
o Adenosine triphosphate
o Complex organic chemical - provided energy