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UNIT 2.2 CHEMICAL REACTIONS Pages 47 – 51, Science Dimension 3

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

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UNIT 2.2 CHEMICAL REACTIONS Pages 47 – 51, Science Dimension 3

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  1. 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. Autumn Leaves Metal Car Rusting 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 Making Foam Alka - Seltzer

  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 Fireworks Exploding (Endothermic Reaction Wood Burning (Endothermic Reaction Instant Heat Pack (Exothermic Reaction) Instant Ice Pack (Endothermic 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.

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