Physical changes involve no change in chemical composition and are reversible. Examples include changes in state from solid to liquid to gas. Chemical changes produce new substances and are usually irreversible. Examples include burning and rusting. Chemical reactions involve the release or absorption of energy in the form of heat. Exothermic reactions release heat while endothermic reactions absorb heat. Light can trigger chemical reactions like photosynthesis and developing photographs. Proper storage and disposal of chemicals and batteries is important to avoid pollution.

2. No change in chemical
content and properties
Reversible and temporary
Change of characteristics
and physical properties
No new substances
produced
Physical
change
Examples of physical changes :
•The condensation of steam
•The evaporation of water
•The sublimation of ammonium salt
•The change in the shape of a ball when pressed
•The melting of ice into water

3. Example picture of physical change
The change in the shape of a ball
when pressed.
The melting of ice into water
The evaporation of water

4. Irreversible and
permanent
New substance(s) is
formed
Release or absorption of
energy
Change in chemical
content and properties
Chemical
Change
Example of chemical changes include :
•The rusting of iron nails : iron + oxygen + water  rusted iron
•Photosynthesis : carbon dioxide + water  food + oxygen
•The burning of magnesium in air : magnesium + oxygen  magnesium oxide
•The reaction of an acid with an alkali : Sulphuric acid + Calcium hydroxide  Calcium
sulphate + water
chlorophyll
sunlight

5. Example of chemical change
Rusting of iron nails Burning of magnesium in air

6. Similarities and differences between chemical
change and physical change

7. Heat change in chemical reactions
• All chemical reactions which take place involve energy change, whether it
is the absorption or the release of energy.
• The main energy change is heat energy.
• In chemical reactions:
– The energy is absorbed to break the chemical bonds between atoms
– The energy is released when chemical bonds between atoms are formed
• If the energy released is more than the energy absorbed in a particular
chemical reaction, the overall reaction would release heat energy into the
surrounding. The total energy content of the products is lower than the
total energy content of the reactants. For example, heat is released when
sodium hydroxide and dilute hydrochloric acid solutions undergo
neutralisation : Sodium Hydroxide + Hydrochloric acid  Sodium Chloride
+ water
• On the other hand, if the energy absorbed is more than the energy
released, the overall reaction would absorb heat energy from the
surroundings. For example, heat is absorbed when calcium carbonate
decomposes into calcium oxide and carbon dioxide : Calcium carbonate 
Calcium oxide + Carbon dioxide
Heat

8. Reactions involving heat loss and heat gain
• Based on the heat change which occurs, chemical reactions can be
classified into
– Exothermic reactions
– Endothermic reactions
• An exothermic reaction is a reaction which releases heat(heat loss) into
the surroundings. This type of reaction causes the surrounding
temperature to increase.
• An endothermic reaction is a reaction which absorbs heat(heat gain) from
the surroundings. This reaction causes the surrounding temperature to
decrease.

9. Comparison between exothermic and
endothermic reactions

10. Energy level diagrams for exothermic and
endothermic reactions

11. Haber Process

12. More details about Haber Process
© Chris Goodman (http://www.youtube.com/watch?v=CZDaJ7do6Go)

13. Contact Process

14. More details about Contact Process
© Chris Goodman (http://www.youtube.com/watch?v=_zj3bMjFclA)

15. Reactivity series of metals
• The reactivity of a metal is the activity of a particular metal in a chemical
reaction.
• Different metals have different reactions towards water, dilute acids and
oxygen.
• There are metals which are very reactive, reactive and not reactive. For
example :
– Potassium and sodium --- very reactive
– Magnesium and zinc --- reactive
– God and platinum --- not reactive

16. Comparing and contrasting the reactivity of
metals with water, acids and oxygen

17. Position of carbon in the reactivity series of
metals
• Although carbon is not a metal element, it is also inclined to react with
oxygen to produce carbon dioxide. (carbon + oxygen  carbon dioxide)
• Carbon is also a reducing agent because of its high position in the
reactivity series. Thus, carbon can reduce metal oxide K to metal K and
release carbon dioxide when two substances are heated together. (carbon
+ metal oxide K  Carbon dioxide + metal K)
• The position of carbon in the reactivity series can be determined by
heating a mixture of carbon and metal oxide. Carbon reduces metal oxide
to its metal if carbon is more reactive than the metal. Carbon does not
react with metal oxide if carbon is less reactive than the metal.
• Therefore, carbon can only be used to extract metals which are less
reactive than carbon itself.
• Carbon is less reactive than potassium, sodium, calcium, magnesium and
aluminium. Thus, carbon is unable to reduce the oxides of these metals.
• Carbon is more reactive than zinc, stannum, lead, copper, mercury, silver
and gold. Thus, carbon is able to reduce (remove the oxygen) oxides of
these metal.
Heat

18. Two methods of extraction of metals

19. Electrolysis
• Electrolysis is a process of breaking down an electrolyte using electric
current.
• In electrolysis, electrical energy is changed into chemical energy. (
electrical energy  chemical energy )
• The apparatus used in electrolysis as shown in Figure 5.24 is made up of :
– Electrolyte
– Electrodes
– Electrical source
– Ammeter

20. Anode Cathode
Chloride ions move to the anode Copper ions move to the cathode
Chloride ions release electrons to
the anode and form chlorine atoms
Copper ions receive electrons from
the cathode and form copper atoms
Chloride ions  chloride atoms +
electrons
Copper ions + electrons  copper
atoms
Two chlorine atoms combine to
form a chlorine gas molecule
The copper metal is deposited at
the cathode

22. Various Types of Cells and Their Uses
• Chemical energy can be changed into electrical energy through chemical
reactions in electrochemical cells such as simple cells, lead- acid
accumulators and nickel- cadmium batteries. ( chemical energy 
electrical energy)
Electrochemical cell

23. Advantages and disadvantages of various
electrochemical cells

24. Chemical reactions which require light
• Light energy can start a chemical reaction.
• Light energy can also be changed into chemical energy through chemical
reactions.
• Examples of chemical reactions which are stimulated by using light energy:
– Photosynthesis process
– Effect of light on photographic film and sliver chloride

25. Photosynthesis
• Photosynthesis is a process in which green plants produce food using
water, carbon dioxide and light as shown in Figure 5.37.
• Photosynthesis involves two stages :
– Light reaction : Chlorophyll in the leaf cells absorbs light energy. The light energy is used
to break down the water molecule absorbed from the roots into its components,
hydrogen and oxygen. This reaction is called the photolysis of water. ( Water 
hydrogen + oxygen)
– Dark reaction : Hydrogen gad produced from the light reaction will react with carbon
dioxide which is absorbed through the stomata of the leaf to produce food (Glucose)
and oxygen.
Light energy
• Photosynthesis can be summarised by this equation : water + carbon
dioxide  glucose + oxygen
Light energy
chlorophyll

26. Effect of light on photosensitive chemicals
• Light plays an important role in photography.
• Photographic film is made from plastic material and is wrapped with a thin
layer of emulsion. This layer is made up of gelatin and white sliver
compounds like sliver bromide, silver chloride or silver iodine which
known as photosensitive chemicals because they change when exposed to
light.
• When a picture is taken with a camera, light enters the camera through
the camera lens as shown in Figures 5.38.

27. • Light sparks off a chemical reaction in the photographic film as follows:
– Light breaks down sliver bromide into black silver spots and bromine vapour. (silver
bromide  silver + bromide)
• The reaction between light and sliver bromide causes the film to change
from white to black.
• The photographic film is then processed with sodium thiosulphate
solution in a dark room to dissolve any white bromide which did not react
with light. The end product is a negative image formed on the
photographic film as shown in Photograph 5.6.
• The image on the photographic film is projected onto a piece of white
photographic paper. Finally, the paper is processed with chemicals.
light

28. Storing chemical substances
• Chemical substances like chlorine water , sodium hypochlorite solution
and silver salt are very sensitive to light.
• These chemical substances will decompose to other substances if exposed
to sunlight.
• As a result, photosensitive chemicals must be stored in dark condition.
• Chlorine water and sodium hypochlorite solution must be stored in dark
bottles.
• Photographic paper is also stored in a black bag or black box.

29. Effects of disposing electrochemical cells in the
environment
• Disposing electrochemical cells (batteries) in the environment has causes
pollution which endangers human health.
• This is because most of the components in these electrochemical cells are
poisonous substances.
• For example, the lead-acid accumulator contains toxic metals like lead,
nickel, cadmium, mercury and manganese. These toxic substances pollute
water and soil.

30. Disposal of electrochemical cells
• The best way to dispose electrochemical cells is to recycle them.
• Used batteries are collected and then reprocessed so that the raw
materials used like zinc and mercury can be reused.
• These batteries can also reused after the chemical substances in them are
replaced by the new substances.