1. Pages 47 – 51, Science Dimension 3
STUDY NOTES
UNIT 2.2
CHEMICAL REACTIONS
Pages 47 – 51, Science Dimension 3

2. PHYSICAL vs CHEMICAL CHANGES
PHYSICAL CHANGE
A physical change occurs when a new substance IS NOT formed.
The clues to a physical change are :
A change in shape, eg. A rubber band is stretched.
Breaking into smaller pieces, eg. Shatter a window.
Dissolving, eg. add sugar to water.
Mixing with another substance, eg. Addn cordial to water.
Changing state, eg. Ice melts.
CHEMICAL CHANGE
A chemical change has occurred when a new substance IS formed.

3. 5 Signs of a Chemical Reaction
A chemical reaction has definitely occurred if any of the following are observed :
A permanent colour change occurs, eg. Metal rusting.
A gas is given off, eg. Berocca tablet dissolving.
There is a change in temperature, indicating that energy has been
produced or absorbed.
eg. Gas burns in a bunsen burner.
4. A precipitate (solid ) forms when two clear solutions are mixed.
5. One metal deposits on another.

4. 1. A Permanent Colour Change
A permanent colour change may also indicate that a chemical reaction has occurred.
A reaction has occurred if two solutions are mixed and there is a permanent colour change because a new substance has been produced.
( Not simply the colour changing because of the result of a the dilution of one of the reactant solutions, eg. Cordial and water.)

5. 1. A Permanent Colour Change
Other examples.
Metal Car Rusting
Autumn Leaves
Banana Rotting

6. 2. Gas Production
The formation of a gas is the third sign that a chemical reaction may have occurred.
The formation of bubbles when two liquids are mixed usually indicates
that a gas has formed.
A gas can also be formed when a solid is added to a solution.

7. 2. Gas Production
Other examples
Alka - Seltzer
Making Foam

8. 3. Temperature Change
An increase or decrease in temperature both indicate that a chemical reaction
is occurring. Here the temperature increased when two liquid samples were mixed.
If a thermometer is not available, a change in temperature can be detected by
feeling the reaction vessel. If it feels hot, the temperature increased.
If it feels cold, the temperature decreased. Since some reactions generate large
amounts of heat, you should be very cautious when touching a reaction vessel.

9. 3. Temperature Change Other examples Wood Burning
(Endothermic Reaction
Fireworks Exploding
(Endothermic Reaction
Instant Ice Pack
(Endothermic Reaction)
Instant Heat Pack
(Exothermic Reaction)

10. 4. Formation of a Precipitate
A precipitate is a solid formed in a chemical reaction that is different from either of the reactants.
This can occur when solutions containing ionic compounds are mixed and an insoluble product is formed.
The identity of the precipitate can often be determined by examining solubility rules.

11. 5. Metal Deposition
Sometimes one metal will be deposited on another metal.
eg. Copper coating zinc metal when the zinc is inserted in a copper sulphate
solution.