CHEMICAL CHANGECH 3.1 PAGE 68

DEFINITION OF A CHEMICAL CHANGE

Chemical change a new substance is formed – the change is almost impossible to undo or reverse.• Indications of a chemical change

• change in colour,

• gas or bubbles produced,

• a new solid (precipitate) forms,

• energy is either released (exothermic) heat, sound or light or absorbed (endothermic)

• Further information is still required though to be sure a reaction has taken place.

CONSERVATION OF MATTER• Matter can be neither created nor destroyed, but transformed.• The amount of reactants (the substances you start with) must balance with

the amount of products (the new substances formed by the reaction) eg

Hydrochloric acid + sodium hydroxide = sodium chloride + water

HCl + NaOH = NaCl + H20

However, If you weigh 30 g of HCl and 10 g of Zinc, after they are reacted the test tube weighs much less. Why?

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BALANCED EQUATION – COPPER AND SULFUR DIOXIDELaw of Conservation of Mass - Mass of reactants must equal the mass of products. P 69

Copper + Sulfur Dioxide = Copper Sulfide + Oxygen gas

(the valencies are: Copper 2+, Sulfur 4- and Oxygen 2-)

• Cu + SO2 = Cu2S + O2

• 2Cu2+ + S-4 2(O2-) = 2Cu2+ S-4 + O2

• 2Cu2+ + SO2 = Cu2S + O2

COMBUSTION REACTIONS P 70• Combustion occurs whenever something reacts with oxygen.• Exothermic - Heat is released - combustion of wood, gas from a

Bunsen burner• Endothermic - Heat is absorbed - Ammonium nitrate + water

(instant cooling pack)

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