4S/E Chemistry Notes

Part I Planet Earth

Ch2 The atmosphere

 Matter can be classified into pure substances and mixtures. A mixture consists of two or more pure
substances which have not chemically combined together

 A pure substance can either be an element or a compound.

 *An element is a pure substance that cannot be broken down into anything simpler by chemical
methods.

 A compound is a pure substance made up of two or more elements chemically combined together.

 Differences between mixtures and compounds

Mixture Compound

1. variable (the substance in the mixture Fixed (e.g. in water, the ratio of
Composition can be mixed together in any proportion) hydrogen to oxygen by mass is always
by mass 1 : 8)

2. Changes in no chemical reaction takes place; usually a chemical reaction takes place; heat is
formation no heat change in making a mixture usually given out or absorbed when a
compound is made

3. Melting melts or boils over a wide range of melts or boils at a definite temperature
point (m.p.) temperatures (i.e. has a sharp m.p. and b.p.)
and boiling (i.e. does not have a sharp m.p. or b.p.)
point (b.p.)

4. General each constituent substance retains its properties are entirely different from
properties own properties those of its constituent elements

5. Separation constituents can be separated by physical constituent elements can only be

of constituents methods, based on differences in separated by chemical methods, not by
physical properties physical methods

 Physical properties of a substance are those properties that can be determined without the substance

changing into another substance. E.g. Appearance, odour, Hardness, Density, solubility, M.p. and b.p.,

electrical & thermal conductivity, ductility and malleability.

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 Chemical properties of a substance are those properties that describe the ability of that substance to
react with other substance(s) or to change from one substance to another.

 Composition of air
Gases Percentage composition
Nitrogen 78%
Oxygen 21%
Noble gases (mainly argon) ~ 0.9%
Carbon Dioxide ~0.03%
Water variable

 Fractional distillation of liquid air

 Oxygen and nitrogen can be obtained by this method
 There are three main stages in the process:
1. Dust particles are first filtered. The filtered air is cooled to –80°C carbon dioxide and water

vapour become solid and are removed

2. Purified air is then compressed and cooled.

3. The liquid air is passed into a fractionating tower. Different gases in the air boil at different
temperatures and can be collected separately.

 Uses of oxygen
 Making oxygen cylinders
 Metals welding

 Uses of nitrogen
 Food packaging (prevent Rancidification)
 Refrigerant
 Making ammonia (Haber process)

 Test for oxygen

 Oxygen relights a glowing splint.

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Ch3 Ocean

 *A saturated solution is a solution in which the solvent has dissolved the maximum amount of the
solute it can at a particular temperature.
 Filtration is a method that separates an insoluble solid from a liquid or a solution. The liquid that

passes through the filter paper is called filtrate. Insoluble substances remain on the filter paper as

residue.

 Evaporation 蒸發 refers to the change of a liquid to a vapour at a temperature below its boiling point.

 Crystallisations is a process of formation of solid crystals from a saturated solution.

 Distillation

 Using ‘Quickfit’ apparatus

 Using simple apparatus

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 Flame test
 Some metals and metal compounds, when burnt or heated strongly, produce a characteristic
coloured flame.
1. Moisten a clean platinum wire with concentrated hydrochloric acid.
2. Dip the wire into a crushed sample (or solution) of the substance to be tested.
3. Heat the end of the wire strongly in a blue flame.
Compound of colour
Potassium Lilac
Sodium Golden yellow
Calcium Brick red
Copper Bluish green

 Silver nitrate test

 To show that chloride ion is present in common salt
1. Dissolve the common salt sample in deionized water
2. Add excess dilute nitric acid to the solution of the sample
3. Add silver nitrate solution to the sample
4. The appearance of a white precipitate (silver chloride which is insoluble in acid) indicates the presence
of chloride in the sample.

 Test for the presence of water in a sample

 Dry cobalt(II) chloride paper turns from blue to pink.

 Anhydrous copper(II) sulphate turns from white to blue

 Test for pure water

 If the liquid boils at 100°C (at 1 atmospheric pressure), it should be pure water.

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 Electrolysis of sea water
 Hydrogen gas and chlorine gas form at the negative electrode and the positive electrode
respectively.

 The sea water left behind becomes sodium hydroxide solution.

 Uses of the products

Chlorine Hydrogen Sodium hydroxide
Sterilizer Rocket fuel Drain cleanser
PVC Margarine Soap

Ch 4 Rocks and minerals

 A rock is a solid mass of mineral or a mixture of minerals. Minerals may be elements or compounds

 An ore is the type of rocks that contains minerals

 Iodine solid sublimes to form vapour on heating.

 Test for the presence of carbon dioxide

 Carbon dioxide turns lime water to milky from colourless.

 Test for carbon dioxide content

% of CO2 Colour of hydrogencarbonate indicator
> 0.03 Yellow
0.03 Red
< 0.03 purple

 Limestone, chalk and marble are common rocks which contain calcium carbonate.

 Express the above reactions with chemical equations

1. 4.

2.

3.
 Tests for the presence of calcium and carbonate in a sample of limestone/ chalk/ marble

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  Test for the presence of calcium

Calcium compounds give a brick red flame in the flame test.

 Test for the presence of carbonate

Dilute hydrochloric acid is added to a sample. If carbonate is present, carbon dioxide is produced,
which turns limewater milky.

Public Exam Questions

MC Questions
1. The table below gives some information about certain components in a sample of liquefied air.
Component Boiling point / oC
Argon -186
Nitrogen -196
Oxygen -183
In what order are these components distilled out when the sample undergoes fractional distillation?
A. nitrogen, oxygen, argon
B. nitrogen, argon, oxygen
C. oxygen, argon, nitrogen
D. oxygen, nitrogen, argon
[HKCEE 2005 Section A #3]
2. Nitrogen, instead of air, is used to fill the packets of potato chips. It is because
A. air supports combustion but nitrogen does not
B. the density of air is higher than the nitrogen
C. argon in air contaminates the chips but nitrogen does not
D. oxygen in air makes the chips go bad but nitrogen does not
[HKCEE 2008 Section A #8]

3. Which of the following combinations concerning the change of physical state of a substance is
INCORRECT?
Change of physical state Process
A. Liquid to gas Evaporation
B. Liquid to solid Precipitation
C. Solid to gas sublimation
D. Gas to liquid condensation
[HKCEE 2006 Section A #4]

4. Which of the following correctly describes the sequence of procedures to separate sand, salt and water
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 from a mixture of sand and salt solution?
A. filtration, evaporation
B. filtration, distillation
C. crystallization, filtration
D. crystallization, filtration, distillation
[HKCEE 2005 Section A #19]
5. When flame test is performed on copper(II) chloride, what is the colour of the flame observed?
A. golden yellow
B. pale purple
C. brick-red
D. bluish-green
[HKCEE 2005 Section A #5]
6. Some physical properties of a compound X are listed below:
melting point : 82 ℃
boiling point : 221 ℃
solubility in water : soluble
Which of the following is the most appropriate method to obtain X from a solution of X in water?
A. decantation.
B. crystallization.
C. fractional distillation.
D. paper chromatography.
[HKCEE 1998 #26]

7. Which of the following substances contain calcium carbonate as the main chemical constituent?
(1) limestone
(2) chalk
(3) marble
A. (1) and (2) only
B. (1) and (3) only
C. (2) and (3) only
D. (1),(2) and (3)
[HKCEE 2006 #25]

8. 2013 Paper 1A Q.19

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9. 2015 Paper 1A Q.23

10. 2016 Paper 1A Q.1

11. 2017 Paper 1A Q.14

12. 2017 Paper 1A Q.19

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Conventional questions

1. Draw a labeled diagram to show the laboratory set-up for the electrolysis of brine and the collection of
the gaseous products.
(3 marks)
[HKCEE 1999 Paper I 9biv]

2. Describe the procedure for a flame test.

(3 marks)
[HKALE 1999 Paper I 7ci / HKALE 2002 Paper II 1ciI]

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3. Explain why filtration can be used to remove mud particles from muddy water, but cannot be used to
remove sodium chloride from sea water.
(3 marks)
[HKCEE 2003 Paper I 7c]

4. Suggest a test to show that marble is a calcium containing substance.

(2 marks)
[HKCEE 2005 Paper I 1aii]

5. 2013 Paper 1B Q.1b

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6. 2015 Paper 1B Q.1c

7. 2015 Paper 1B Q.2a

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Answer

1. B 2. D 3. B 4. B 5. D 6. B 7. D 8. B 9. D 10. A
11. A 12. D

1. [HKCEE 1999 Paper I 9biv]

(3)

2. Clean a Pt wire/use a clean Pt wire (silica rod for flame test). (1)
Stick a sample of the salt onto the Pt wire with concentrated HCl. (1)
Heat the wire with the sample in a non-luminous (bunsen) flame (1)
[HKALE 1999 Paper I 7ci / HKALE 2002 Paper II 1ciI]

3. Mud particles cannot dissolves in water. As the size of mud particles is much larger than that of water
particles. Mud particles cannot pass through the filter but water molecules can. (1)
Sodium chloride is an ionic compound which can dissolve in polar solvent such as water to form sodium
cation and chloride anions. (1)
The ions have size comparable to that of water molecules. All water molecules, Na+ and Cl- are small
enough to pass through the filter, and therefore they cannot be separated by filtration. (1)
[HKCEE 2003 Paper I 7c]
4. Flame test. (1)
A brick-red flame will be observed if calcium is present. (1)
[HKCEE 2005 Paper I 1aii]
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5. 2013 Paper 1B Q.1b

6. 2015 Paper 1B Q.1c

7. 2015 Paper 1B Q.2a

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