3.1 Characteristics of chemical reactions

a) This is not a chemical process. It is only a phase change, after cooling down the candle grease can return to its original form.

b) As soon as dynamite catches fire (lit by a fuse) you will hear a loud bang. This is the result of an explosion. The dynamite is gone. New, different substances were formed instead. An explosion of dynamite is therefore a chemical process.

c)This is not a chemical process. The properties of the substance sugar have not changed. For example, the solution will still taste sweet.

d) Dye in ink is broken down by (ultraviolet) light. The colour will fade as a result. This breaking down of dye is a chemical reaction.

e) Nail polish is not broken down. Nail polish is removed from nails by dissolving it in nail polish remover. Dissolving a substance is not a chemical process.

5 a)Figure 3.8b represents a chemical reaction, because new molecules are formed.

b) In figure 3.8a molecules are only sorted. This figure represents the separation of a

mixture.

6. a) Explosions are very fast exothermic reactions that often create a lot of gas. Explosions occur only when the reagents are finely divided and mixed in a (near) optimal ratio.

b) Dust explosions are dependent on the reactants being finely divided and mixed in a (near) optimal ratio. So the surface area between the sawdust and the air is the most important factor defining the risk of sawdust explosions.

7.The solid has disappeared. The fact that a gas has formed is an indication that the

substance did not just dissolve, but that the original substance has changed.

8 a) The reactions of the rotting process are slowed by the low temperature. This causes the food to spoil at a lower speed. Keeping it in edible condition for a longer period of time.

b) Twigs have a much smaller mass and are more easily heated up to their ignition temperature.

OR

Twigs have a higher surface area with the air, because they are more finely divided.

9. a) For melting candle grease energy (heat) is needed. It is therefore an endothermicprocess.

b) To evaporate a certain amount of water, heat is needed. When condensing the same amount of water vapour the same amount of heat is released! Condensation is therefore an exothermic process.