1st March 2011

Today we started on a new topic, chemical changes.

Chemical changes:
- permanent
- new substances formed

- irreversable

- involves chemical change
- heat/light energy may be taken in or given out

- properties of starting reactants differ from products
- for example, cooking, burning of charcoal and steel rusting

Physical change:

- temporary
- no new substances formed
- reversable

- does not involve chemical change

- no/little light or heat energy is given in or taken out

- properties of substances do not change
- for example, changes in state and dissolving salt

Chemical reactions:
- changing reactants into products
- provides identies of reactants and products
- provides possible reaction possibilities
- represented using chemical equations

- can be written using word or chemical equation

Balanced chemical equation:

1) write down chemical formulae of reactants and products

2) check atoms of each elements on both sides of the reaction

3) balance the chemical equation by placing numbers in front of the element in the question

- for example, N3+H2=NH3
N2+3H2=2NH3

Chemical equations involving heat

- combustion

- combination

- thermal decomposition

Combustion:

- burning

- substance combine with oxygen in the air
- products are heat and light

- octane+oxygen+heat=carbon dioxide+water

Combnation:
- two or more reactants combine chemically to form a new product

- sodium+chlorine=sodium chloride

Thermal decompositon:

- a substance breaks down into two or more simpler substances(decomposition)
- a substance breaks down by the effect of heat(thermal decomposition

-sugar+heat=carbon+water vapour

chemical reactions involving light:

- photosynthesis

- photography

Photosynthesis:

- green plants in the presence of chlorophyll and light energy manufacture carbohydrates(glucose) from carbon dioxide and water

- carbon dioxide+water+light energy+chlorophyll=glucose+water

Photography:

- photographic films are coated with silver bromide

- when films are exposed to light, silver bromide decomposes to form silver particles

- silver bromide+light energy=silver+bromine

Electrolysis;

- passing electric currents may change the propeties of the substance temporarily or permanently

- some substances decomposes into simpler ones when electricity is passed through

- electroplatimg

Chemical reactions involving matter:
- digestion

- neutralisation
- rusting

Digestion:

- involves chemical reactions caused by mixing

- when saliva and starch are mixed, the enzymes in the saliva break down the food

Neutralisation

- when an acid and alkali is mixed, salt and water is formed

- hydrochloric acid+sodium hydroxide=sodium chloride+water

Rusting
- iron comes in contact with oxygen in the air and water

- combination

- iron+oxygen+water=hydrated iron(III)oxide