O Level – Chemistry

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  • #2965

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    Chapter 13 – Salts

    1. Salts can be prepared using the following methods:

    Solubility of the salt in water – soluble
    Solubility of the starting materials in water – one is insoluble
    Method of preparation – reaction of aids, with metals, insoluble bases or insoluble carbonates
    _________________________________________________________________________

    Solubility of the salt in water – soluble
    Solubility of the starting materials in water – soluble
    Method of preparation – titration of solids with alkalis or soluble carbonates
    __________________________________________________________________________

    Solubility of the salt in water – insoluble
    Solubility of the starting materials in water – soluble
    Method of preparation – precipitation
    __________________________________________________________________________

    2. Filtration and crystallisation are important laboratory techniques used for the separation and purification of salt crystals

    Contact Mr Ong @9863 9633 for much key ideas on salts

    #2981

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    Chapter 14 – Metal

    1. The physical properties of metals are as follows:
    – Usually have high densities, melting points and boiling points.
    – Can be bent, stretched or beaten into very thin sheets without breaking
    – Good conductors of heat and electricity

    2. Any alloy is a mixture of a metal with one or few other elements

    3. There are four main reasons for making alloys
    – To improve the strength and hardness of metals.
    – To improve the appearance of metals.
    – To improve the resistance of metals against corrosion.
    – To lower the melting points of metals.

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    #2991

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    Chapter 15 – Electrolysis

    1. An electrolyte is a compound that conducts electricity in the molten state or in aqueous state.

    2. A cathode is a negatively charged electrode. An anode is a positively charged electrode.

    3. Positively charged ions are called cations and negatively charged ions are called anions.

    4. The decomposition of a compound by electricity is called electrolysis.

    5. During the electrolysis, cations move towards the cathode while the anions move towards the anode.

    6. Redox reactions take place at the electrodes. Oxidation occurs at the anode and reduction at the cathode

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    #3008

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    Chapter 16 – Energy Changes

    What are the characteristics of exothermic reactions?

    When an exothermic reaction occurs,

    a) heat is liberated and is transferred from the chemicals to the surroundings, and

    b) the temperature of the reaction mixture rises. The container feels warm.

    Examples of exothermic reactions

    • Combustion of fuels
    • Rusting of iron
    • Corrosion of metals
    • Reaction between acid and alkali neutralisation),
    • Respiration

    What are the characteristics of endothermic reactions?

    When an endothermic reaction occurs,

    a) heat is absorbed and is transferred from the surroundings to the reactants, and

    b) the temperature of the reaction mixture falls. It feels cold.

    Examples of endothermic reactions

    • Photosynthesis

    • Action of light on silver bromide in photographic film

    • Thermal decomposition
    Example CaCO3 —–> CaO + CO2

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    #3053

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    Chapter 17 – Speed of Reaction

    1. Different chemical reactions take place at different speeds

    2. Speed of reaction = change in amount of reactant or product/Time taken

    3. Speed of reaction = change in volume gas/Time taken

    4. For reactions involving solutions, an increase in the concentration of a reactant increases the speed of reaction.

    5. For reactions involving gases, an increase in the pressure of a gas increases the speed of reaction.

    5. Reactions take place faster when the solid is broken into smaller pieces.

    6. The higher the temperature, the faster the movement of the particles and the greater the number of collisions. Hence, reactions take place faster when the temperature is increased.

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    #3074

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    Chapter 18 – Atmosphere and Environment

    Greenhouse Gases

    Carbon dioxide is released to the atmosphere when solid waste, fossil fuels (oil, natural gas, and coal), and wood and wood products are burned.

    Methane is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from the decomposition of organic wastes in municipal solid waste landfills, and the raising of livestock.

    Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of solid waste and fossil fuels.

    What are the effects of global warming?
    1. Heat waves and periods of unusually warm weather
    2. Sea level rise and coastal flooding
    3. Glaciers melting
    4. Arctic and Antarctic warming
    5. Spreading disease
    6. Earlier spring arrival
    7. Plant and animal range shifts and population declines
    8. Coral reef bleaching
    9. Downpours, heavy snowfalls, and flooding
    10. Droughts and fires

    What is causing the depletion of ozone?
    1. Chlorofluorocarbon compounds are also known as CFCs – made of chlorine, fluorine and carbon.
    2. These compounds are unreactive and do not burn.
    3. They are compressed to form liquids which are used in aerosol propellants and as coolant fluids for refrigerators and air conditioners.
    4. When an aerosol can is utilised, the CFC molecules are released into the air. At higher altitudes in the atmosphere, these molecules are decomposed by sunlight to produce chlorine atoms.
    5. The chlorine atoms react with the ozone molecules and thus destroy the ozone layer which protects the earth from the direct rays of the sun.
    6. Exposure to direct radiation of the sun can cause skin cancer to humans and also destroy the agriculture.

    Effects of Ozone Depletion
    1. There is an increase in the temperature due to the ozone depletion as more UV rays are entering the earth’s surface.
    2. There would be a no.of skin cancers. 3.Crop yields would be adversely affected.Vegetation land becomes dessert.
    4. North and South Pole melts causing the ocean level to rise and flood the low-lying countries such as Netherlands. 5.Rapid evaporation would occur and cause droughts especially in India.Thus, CO2 dissolved in the oceans rise into the atmosphere adding further to the greenhouse effect.

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    #3105

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    Chapter 18 – Organic

    Macromolecules

    Macromolecules (polymers) are formed by linking together many small repeating units known as monomers.

    Polymerisation is the process of joining together a large number of small molecules to form a macromolecule.

    2 Classes of macromolecules

    Synthetic macromolecule
    Man-made polymers
    Eg. Polyethene, nylon, terylene

    Natural macromolecule
    Naturally occurring
    Eg. Proteins, Fats, Carbohydrate, Cellulose, Wool, Cotton etc…

    2 Types of Synthetic Polymerisation

    Addition Polymerisation
    is a process by which many small unsaturated molecules (monomers) are added onto one another to form one large molecule (polymer).

    Condensation Polymerisation
    is a process where two monomers react together to produce a large molecule, with the elimination of a small molecule (ie. Water or HCl)

    Addition Polymerisation

    For ONLY Unsaturated monomers
    Conditions : Heat, High pressure, Catalyst
    Examples:
    Ethene -> Poly(ethene)
    Propene -> Poly(propene

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    #3129

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    Stop and Think, are the following questions True or False

    1. Temperature, volume and mass are physical quantities.

    2. All measurements in chemistry are made using SI units.

    3. It is not possible to measure precise volumes of liquids.

    4. Clocks and watches are accurate to only 1 second.

    5. Many physical quantities can be measured with senors connected to
    computers.

    Ans: TFFFT

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    #3148

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    Purification of Substances

    Stop and Think, are the following questions True or False

    1. In filtration, the filtrate is always a pure liquid.

    2. Drinking water can only be obtained from seawater by distillation.

    3. The fractional distillation of miscible liquids is only possible if the liquids have different boiling points.

    4. Paper chromatography is a physical method for separating mixutres.

    5. Mixtures have fixed melting and boiling points.

    Ans: FFTTF

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    #3166

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    Kinetic Particle Theory – Common Error

    Wrong Concept
    During melting, the temperature rises because heat is absorbed

    Correct Concept
    During melting, the temperature remains constant at the melting point because the heat absorbed is used to overcome the attractive forces between the particles.

    Wrong Concept
    Diffusion does not occur in solid

    Correct Concept
    Diffusion does take place in solids, it occurs extremely slowly in solids because the solid particles are less energetic and move with less speed

    For more common errors please contact Mr Ong @98639833

    #3213

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    Experimental Techniques – Common Error

    Wrong Concept
    The accuracy of burette is +- 0.01

    Correct Concepts
    The accuracy of bureete is +-0.05.
    Thus the burette reading can be 23.80 or 23.85 cm^3 but not 23.84cm^3

    Wrong Concept
    Lemon juice is a pure substance

    Correct Concepts
    It is a mixture of different substance such citric acid and water

    For more common errors please contact Mr Ong @98639833

    #3238

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    Experimental Techniques – Key Points

    1. Fractional distillation is used to separate a mixture of liquids with different boiling points

    2. The starting line in paper chromatography should be drawn with pencil and not ink. Ink contains dyes that will dissolve in the solvent.

    For more key points and exam based questions with full worked solutions please contact Mr Ong @98639833

    #3268

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    Elements, Compounds and Mixtures – Key Points

    1. All metals exist as atoms. Most non-metals as molecules, examples hydrogen and oxygen

    2. Tap water is a mixture that contains water and dissolved substances such as chlorine and other minerals.

    For more key points and exam based questions with full worked solutions please contact Mr Ong @98639833

    #3317

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    Atomic Structure – Key Points

    1. Not all element have isotopes. Elements like Beryllium, Fluorine and Phosphorus do not have isotopes.

    For more key points and exam based questions with full worked solutions please contact Mr Ong @98639833

    #3339

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    Chemical Bonding – Important Definitions

    1. An ion is a charged particle formed from an atom by loss or gain of electrons

    2. A cation is a positively charged ion formed when an atom loses valence electrons(s)

    3. An anion is a negatively charged ion formed when an atom gains valence electrons(s)

    4. A simple ion is an ion formed from single atoms eg Na+, A polyatomic ion is an ion containing more than one kind of atom eg NH4+ or SO42-

    5. An ionic bond is the strong electrostatic fore of attraction between positive and negative ions in an ionic compound.

    6. A covalent bond is the bond formed by sharing of electrons between two non-metal atoms.

    7. A molecule is a small group of atoms held together by covalent bonds.

    8. A metallic bond is the force of attraction between positive metal ions and the ‘sea of delocalised electrons’

    For more key points and exam based questions with full worked solutions please contact Mr Ong @98639833

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