O Level – Chemistry

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EXPERIMENTAL CHEMISTRY – Do you Know?

1. Experimental Design

Burettes and pipettes are both used to measure accurate volumes of liquids because they have been accurately calibrated.

Pipettes are calibrated to measure fixed volumes such as 10.0 cm3, 25.0 cm3. Burettes measure up to an accuracy of 0.1 cm3.

2. Uses of filtration

Insoluble salts such as copper(II) oxide, lead(II) iodide are separated from water by filtration.
The filter paper has tiny holes (called pores) that enable the particles of liquid (e.g. water, ink dyes, dissolved sodium chloride) to pass through, retaining behind the larger solid particles (e.g. sand, copper(II) oxide).

Solutions such as aqueous sodium chloride can be collected as a filtrate as the sodium and chloride ions are small enough to pass through the pores of the filter paper.

3. Crystallisation

A saturated solution is one that contains the maximum amount of solute that can possibly dissolve in it at a given temperature. A hot solution can dissolve more solute than a cold one. Hence, on cooling, the bulk of the solute is obtained as crystals.

A hot saturated solution gives large crystals when cooled slowly e.g. cooling at room temperature gives larger crystals compared to freezing. This is because in freezing, particles in a saturated solution have a shorter period of time to pack closer together to form a larger crystal.

Crystals can be re-crystallised (that is dissolved again in the same solvent, then repeating the entire crystallisation process) in order to obtain purer crystals.

After crystallisation, the crystals can be weighed and the percentage purity of the impure salt can be calculated using:

Purity = (Mass of salt obtained/ Initial mass of impure salt )x 100%

If you need help in the O level Chemistry, please contact Angie @96790479 or Mr Ong 98639633

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EXPERIMENTAL CHEMISTRY – Do you Know?

Use of distillation

To obtain pure water from sea water

The porcelain boiling stones are used to smoothen the boiling.
The first few drops of liquid are discarded to make sure that any possible impure liquid that may have a boiling point slightly lower than that of the required liquid is not collected.
The running tap must not be turned off before the flame is extinguished to avoid breakage of the condenser if overheated.
The thermometer shows a constant temperature during the distillation process when pure solvent is being collected at the boiling point of the solvent.

Fractional distillation

Separation of a mixture of miscible liquids with different boiling points can be done through fractional distillation.

Uses of fractional distillation

1. Separation of liquid air into oxygen, nitrogen and other useful gases.
2. Separation of crude oil into petrol, kerosene and other useful components of crude oil.
3, Separation of femented liquor into ethanol and water

Do you Know?
The pure liquid that has the lowest boiling point will distil off first. This is because the glass beads in the fractionating column condense the liquids with the higher boiling points back into the flask, allowing the pure liquid with the lowest boiling point to vapourise and distil off as the first distillate

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Elements, Compounds and Mixtures

Do you know?
All metals exist as atoms. Most non-metals exist as molecules

Do you know?
Many non-metallic elements exist as molecules. For example, hydrogen and oxygen exist as H2, and 02 molecules respectively

Do you know?
Tap water is not a pure compound. It is a mixture that contains water and dissolved substances such as chlorine and other minerals.

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31-Jul Thu 5pm to 7pm 2 hrs Atomic Structure & Chemical Bonding

1-Aug Fri 6pm to 7.30pm 1.5 hrs Atomic Structure & Chemical Bonding

2-Aug Sat 7.30pm to 9.30pm 2 hrs Atomic Structure & Chemical Bonding

3-Aug Sun 9am to 10.30am 1.5 hrs Atomic Structure & Chemical Bonding

3-Aug Sun 2.30pm to 4pm 1.5 hrs Atomic Structure & Chemical Bonding

7-Aug Thu 5pm to 7pm 2 hrs Mole Calculation & P2 Exam Practice

8-Aug Fri 6pm to 7.30pm 1.5 hrs Mole Calculation & P2 Exam Practice

9-Aug Sat 7.30pm to 9.30pm 2 hrs Mole Calculation & P2 Exam Practice

10-Aug Sun 9am to 10.30am 1.5 hrs Mole Calculation & P2 Exam Practice

10-Aug Sun 2.30pm to 4pm 1.5 hrs Mole Calculation & P2 Exam Practice

14-Aug Thu 5pm to 7pm 2 hrs Acids and Bases & P1 Exam Practice

15-Aug Fri 6pm to 7.30pm 1.5 hrs Acids and Bases & P1 Exam Practice

16-Aug Sat 7.30pm to 9.30pm 2 hrs Acids and Bases & P1 Exam Practice

17-Aug Sun 9am to 10.30am 1.5 hrs Acids and Bases & P1 Exam Practice

17-Aug Sun 2.30pm to 4pm 1.5 hrs Acids and Bases & P1 Exam Practice

21-Aug Thu 5pm to 7pm 2 hrs Salts & P2 Exam Practice

22-Aug Fri 6pm to 7.30pm 1.5 hrs Salts & P2 Exam Practice

23-Aug Sat 7.30pm to 9.30pm 2 hrs Salts & P2 Exam Practice

24-Aug Sun 9am to 10.30am 1.5 hrs Salts & P2 Exam Practice

24-Aug Sun 2.30pm to 4pm 1.5 hrs Salts & P2 Exam Practice

28-Sep Thu 5pm to 7pm 2 hrs Organic Chemistry & P1 Exam Practice

29-Aug Fri 6pm to 7.30pm 1.5 hrs Organic Chemistry & P1 Exam Practice

30-Aug Sat 7.30pm to 9.30pm 2 hrs Organic Chemistry & P1 Exam Practice

31-Aug Sun 9am to 10.30am 1.5 hrs Oxidation and Reduction & P1 Exam Practice

31-Aug Sun 2.30pm to 4pm 1.5 hrs Organic Chemistry & P1 Exam Practice

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TOPIC 1 EXPERIMENTAL CHEMISTRY

Exam Tip 1
Burettes and pipettes are both used to measure accurate volumes of liquids because they have been accurately calibrated.
Pipettes are calibrated to measure fixed volumes such as 10.0 cm3, 25.0 cm3. Burettes measure up to an accuracy of 0.1 cm3.

Exam Tip 2
When mixture gasses passed into sodium hydroxide, sodium hydroxide reacts with the sulphur dioxide to form a salt and water. Aqueous bromine reacts with ethene to form a saturated compound. What remains is oxygen that is collected over water. Oxygen supports combustion, hence relights a glowing splint.

Exam Tip 3
Oxygen can be collected over water because it is only slightly soluble in water.

Exam Tip 4
Insoluble salts such as copper(II) oxide, lead(II) iodide are separated from water by filtration.
The filter paper has tiny holes (called pores) that enable the particles of liquid (e.g. water, ink dyes, dissolved sodium chloride) to pass through, retaining behind the larger solid particles (e.g. sand, copper(II) oxide).

Exam Tip 5
A saturated solution is one that contains the maximum amount of solute that can possibly dissolve in it at a given temperature. A hot solution can dissolve more solute than a cold one. Hence, on cooling, the bulk of the solute is obtained as crystals.

A hot saturated solution gives large crystals when cooled slowly e.g. cooling at room temperature gives larger crystals compared to freezing. This is because in freezing, particles in a saturated solution have a shorter period of time to pack closer together to form a larger crystal.

Crystals can be re-crystallised (that is dissolved again in the same solvent, then repeating the entire crystallisation process) in order to obtain purer crystals. After crystallisation, the crystals can be weighed and the percentage purity of the impure salt can be calculated using:

Mass of salt obtained Purity =
(Mass of the salt obtained / Initial mass of impure salt) x 100%

If you need help in the O level Chemistry, please contact Angie @96790479 or Mr Ong 98639633

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2015 Schedule – Chemistry

S3 1.5 hrs SAT 9pm – 10.30pm

S3 1.5 hrs SAT 12.30pm – 2pm

S3 1.5 hrs SUN 12.30pm – 2pm

S4 2 hrs FRI 5pm – 7pm

S4 1.5 hrs SAT 12.30pm – 2pm

S4 1.5 hrs SUN 9am – 10.30pm

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Kinetic Particle Theory – Concise Notes

1.1 State of Matter

Solids:
• Have a fixed shape, fixed volume; cannot be compressed.
• Particles vibrate and rotate about fixed positions.

Liquids:
• Have no fixed shape but have fixed volumes; cannot be compressed.
• Particles move.

Gases:
• Have no fixed shape, no fixed volume; can be compressed easily.
• Particles move about rapidly.

1.2 Kinetic Particle Theory and the Changes of State
• The kinetic particle theory states that
(a) all matter is made up of tiny particles, and
(b) all particles are in constant, random motion.
• Particles have kinetic energy.
• When matter is heated or cooled, heat energy is taken in or given out. This causes the kinetic energy of the particles to change, leading to a change of state.

1.3 Diffusion
• Diffusion provides evidence that the particles in gases and liquids are constantly moving.
• Examples of diffusion include: the spreading of the smell of perfume, the spreading of bromine in a gas jar of air, and the spreading of potassium manganate(VII) in water.
• The lower the molecular mass of the particles, the faster the rate of diffusion.
• The higher the temperature, the faster the rate of diffusion.

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Experimental Techniques – Concise Notes

2.1 Measuring Physical Quantities
• An electronic balance is used to measure the mass of a substance.
• A stopwatch is used to measure time.
• Measuring cylinders, burettes, and pipettes are used to measure volumes of liquids.
• Degree of accuracy: Measuring cylinder (to the nearest 0.5 cm3), burette (to the nearest 0.05 cm3).
• A gas syringe is used to measure the volume of a gas.

2.2 Method for collecting gases
• Displacement of water is used for insoluble or slightly soluble gases, e.g. hydrogen, oxygen and carbon dioxide.
• Downward delivery is used for soluble gases that are denser than air, e.g. chlorine, hydrogen chloride and sulfur dioxide.
• Upward delivery is used for soluble gases that are less dense than air, e.g. ammonia. Drying agents
• Concentrated sulfuric acid — used for most gases except ammonia.
• Fused calcium chloride — used for most gases.
• Quicklime — used for ammonia gas.

2.3 Criteria of Purity
• Pure solids have fixed melting points.
• Pure liquids have fixed boiling points.
• An impurity will lower point and increase the boiling of a substance.

2.4 Methods of Separation and Purification
• Decanting and filtration are used to separate a solid from a liquid.
• Separating a mixture of solids can be carried out using a solvent, sublimation, or a magnet.
• Simple distillation is used to separate a liquid from a solution.
• A separating funnel is used to separate immiscible liquids.
• Fractional distillation is used to separate miscible liquids with different boiling points.
• Chromatography is used to separate two or more soluble components in a sample.

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Elements, Compounds and Mixtures – Concise Notes

Elements

· Elements can be classified as metal, metalloid or non-metal.
· Elements can exist as atoms (as the noble gases) or as diatomic/polyatomic molecules

Compounds

· Compounds are formed when elements react together, e.g. water (a compound) is formed when the elements hydrogen and oxygen react together.
· Compounds can exist as molecules (e.g. H2O molecule) or as ions.
· Some compounds decompose to give their elements, e.g. mercury(II) oxide
decomposes to give mercury and oxygen

Mixtures

· A mixture can be made up of elements (e.g. hydrogen gas and oxygen gas).
· A mixture can be made up of compounds (e.g. hydrogen chloride and sodium chloride).
· A mixture can be made up of elements and compounds (e.g. oxygen gas and iron(II) sulfide).

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Atomic Structure – Concise Notes

The Structure of An Atom

· The nucleus of an atom consists of protons and neutrons.
· Protons are positively charged particles and neutrons are particles with no electric charge.
· Electrons are negatively charged particles moving around the nucleus.

The Proton Number and the Neutron Number

· Proton number is the number of protons in an atom. You can identify an atom by its proton number.
· Nucleon number is the number of protons and neutrons in the nucleus.
· Atoms have no net charge because every atom has the same number of protons and electrons.

Isotopes

· Most elements occur naturally as a mixture of isotopes.
· Isotopes have the same number of protons and electrons but different numbers of neutrons.
· Isotopes have similar chemical properties but different physical properties.

Arrangement of Electrons in the Atom

· Electrons are arranged in electron shells around the nucleus.
· Each electron shell can hold only a limited number of electrons.
· The electrons in the outer shell of an atom are called valence electrons

· Radioisotopes have uses in the medical field and in archaeology.

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Secondary 3 Year End Exams and Secondary 4 Prelim Preparatory Classes

for Chemistry.

Open for Registration Now!

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Learning outcomes:

Suggest suitable apparatus, given relevant information, for a variety of simple experiments, including collection of gases and measurement of rates of reaction

Collection of Gases
The method of gas collection depends on:
(i) Solubility of gas in water
(ii) Density of gas as compared to air
· Density of gas is proportional to relative molecular mass [ Mr ] of gas.
· Density of gas, as compared to density of air, can be obtained by comparing the approximate ‘Mr’ of air about 29

A) Displacement Method
– Hydrogen
– Oxygen
– Carbon Dioxide

B) Downward delivery
-Chlorine
-Hydrogen chloride
-Sulfur dioxide

C) Upward delivery
-Ammonia

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Chromatography

So how does chromatography separate a mixture?
The components in a mixture have
(1) different adsorption by the chromatography paper, and
(2) different solubility in the moving solvent.

– Solutes which are more soluble in the solvent will travel further while the less soluble solutes are found closer to the base of the chromatography paper.

Note. To identify and locate colourless spots, the chromatogram is sprayed a locating agent which can reacts with the substances on the paper to produce a coloured product.

Chromatography can be used
(a) To separate very complex mixtures which contain many components.
(b) When only very small amount of sample.
(c) To identify unknown substances ***

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