O Level – Combined Science

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Key Points – Electromagnetism Part 2

7. Magnetic field lines are drawn to represent magnetic fields. Lines that are closer together indicate a greater magnetic field strength where a charged particle experiences a greater magnetic force.

8. An induced magnetic field is strongest near the conductor. It weakens as the distance from the conductor increases. Magnetic field strength also increases as the current in the conductor increases.

9. A charged particle moving in a magnetic field perpendicular to its direction of motion experiences a magnetic force that causes it to
deflect in a circular path.

10. A pair of parallel wires attracts each other if the currents in both wires are in the same direction. A pair of wires repels each other if the currents they carry are in opposite directions.

11. A rectangular coil carrying an electric current in a magnetic field experiences a turning effect due to the magnetic forces acting on each side of the coil. This turning effect is used in the action of a simple d.c. motor.

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The Manufacture of Alkenes by Cracking

1. Alkenes are obtained by the cracking of petroleum fractions, such as the naphtha fraction.

2. Cracking is the process in which large hydrocarbon molecules are broken down into smaller molecules.

3. When an alkane molecule is cracked, a mixture of short-chain alkanes, alkenes and hydrogen are formed.
Example:
When the alkane C13H28 in naphtha is cracked, it is broken up into smaller alkane and alkene molecules

3. When an alkane molecule is cracked, a mixture of short-chain alkanes, alkenes and hydrogen
are formed.

Example:
When the alkane C13H28 in naphtha is cracked, it is broken up into smaller alkane and alkene molecules

4. The conditions used for cracking
(a) In industries, cracking is carried out by passing the petroleum fraction over the catalyst at high temperature. The process is called catalytic cracking.
(b) The conditions for catalytic cracking are:
• Temperature: 600°C
• Catalyst: Silicon(IV) oxide, SiO2, or aluminium oxide, A/203

5. In the school laboratory, catalytic cracking can be carried out using the apparatus as shown in the following diagram.

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Key definitions – Alcohols and Carboxylic Acids

1. Alcohols are organic compounds that contain the hydroxyl (—OH) group.

2. Fermentation is the process of converting sugars into ethanol and carbon dioxide, catalysed by the enzymes in yeast.

3. Carboxylic acids are organic compounds that contain the carboxyl (—COOH) group.

4. Esters are organic compounds produced from the reactions between carboxylic acids and alcohols.

5. Esterification is the reaction between a carboxylic acid and an alcohol to produce an ester and water.

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Key definitions – Kinematics

Kinematics

Scalars and vectors

Scalar quantities are physical quantities that have magnitude only.

Vector quantities are physical quantities that have both magnitude and direction.

Example Scalar Quantities

Distance
Speed
Mass
Energy
Time

Example Vector Quantities

Displacement
Velocity
Acceleration
Force

Distance
• The total length covered by a moving object regardless of the direction of motion
• A scalar quantity (i.e. has magnitude only)
• SI unit: metre (m)

Displacement
• The distance measured in a straight line in a specified direction
• A vector quantity (i.e. has both magnitude and direction)
• SI unit: metre (m)

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Kinematics

Basic Kinematic Quantities

1. Distance is the total length covered between two points. Distance is a scalar quantity.

2. Displacement is the measurement of distance from one reference point to another in a certain direction. Displacement is a vector quantity.

3. The SI unit of distance and displacement is metre (m).

4. Speed is the rate of change of distance. Speed is a scalar quantity.

5. Velocity is the rate of change of displacement. Velocity is a vector quantity. Speed is the magnitude of velocity.

6. The SI unit of speed and velocity is m s–1 or m/s.

7. For constant (or uniform) speed, its value is given by:

Speed v = (distance travelled s)/(time taken t)

8. For non-constant speed, the average speed is given by:

Average speed = (total distance travelled)/(total time taken)

9. If the speed of an object is zero, the object is not moving (or at rest).

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Kinetic Particle Theory

Important Definitions

1. The melting point is the temperature at which a solid changes into a liquid.

2. The boiling point is the temperature at which a liquid turns rapidly into a gas.

3. The freezing point is the temperature at which a liquid becomes a solid.

4. Condensation is the process by which a gas or vapour changes into a liquid or a solid.

5. Sublimation is the process by which a solid changes directly into a gas without going through the liquid state.

6. Evaporation is the process by which a liquid changes into a vapour at any temperature below its boiling point.

7. Diffusion is the mixing process in gases or solutions due to the random motion of particles.

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Mass,Weight and Density

KEY TERMS AND CONCEPTS

1. The mass of an object is a measure of the amount of matter in the object. The SI unit of mass is kilogram (kg).

2. Inertia is the resistance of an object to a change in its state of motion or rest. The larger the mass, the greater is its inertia.

3. The attractive force between any two masses is known as gravitational force.

4. The gravitational field is a region in which a mass experiences
a gravitational force.

5. Gravitational field strength g is the gravitational force acting per unit mass. The unit for g is m s-2 or N kg-1. The gravitational field strength is also known as the acceleration due to gravity.
F = mg

6. Gravitational field strength g is usually taken to be a constant at 10 N kg-’ on the surface of the Earth. It varies slightly according to the location.

7. Weight is the gravitational force acting on an object. It is equal to the product of mass and gravitational field strength. The SI unit for weight is the newton (N).
W = mg

8. Density p of an object is its mass per unit volume.

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Turning Effect of Forces

1. The moment of a force about a pivot is the product of the force F and the perpendicular distance d from the pivot to the line of action of the force.
Moment of a force = F x d

2. The SI unit for moment of a force is newton metre (N m).

3. Moment of a force is a vector. The direction of the moment of a force is either clockwise or anticlockwise about the pivot.

4. There are two conditions for an object in equilibrium:
(a) Net external force F = 0
(b) Net moment due to external forces = 0 (also known as the Principle of Moments)

5. The Principle of Moments states that when an object is in equilibrium, the sum of the clockwise moments equals the sum of the anticlockwise moments.

6. The centre of gravity of an object is the point at which its entire weight appears to act. The
centre of gravity of regular-shaped objects can be determined by geometrical symmetry. The centre of gravity of irregular-shaped objects can be determined by the plumb line experiment.

7. The stability of an object increases when the base area increases or when the centre of gravity is lowered.

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Energy,Work and Power

1. Energy is the ability to do work.

2. Energy can exist in many forms. It can be converted from one form to another.

3. Gravitational potential energy Ep is the energy possessed by an object due to its position in a gravitational field.

Ep = mgh

4. Kinetic energy Ek is the energy possessed by an object due to its motion.

Ek = 1/2 m v^2

5. The Principle of Conservation of Energy states that the total amount of energy in an isolated system is constant.

Total initial energy = total final energy

6. Work done on an object W is the product of force F acting on the object and distance s travelled by the object in the direction of the force.

W = Fs

7. The SI unit of energy and work done is the joule (J).

8. Power P is the rate of work done W. Power can also be defined as the rate of energy conversion E.

P = E/t

9. The SI unit of power is the watt (W). It can also be expressed in joule per second (J s-’).

10. Efficiency is the ratio between useful energy output Eoutput. and energy input Einput.

Efficiency = Eoutput/Einput

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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%

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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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Pressure

1. Pressure p is the force F acting on a surface divided by the area A of the surface.

P = F/A

2. The SI unit of pressure is pascal (Pa) or newton per square metre (N m-2).

3. Pressure p at a certain depth h of a liquid with density p under the influence of gravitational field strength g is given by the following relationship:

p = hpg

4. According to Pascal’s Principle, an extra pressure applied at any part of a body of liquid will be experienced throughout the liquid. In a hydraulic press, an extra pressure applied at one end will be experienced by the other end.

5. According to the Principle of Conservation of Energy, the work done at one end of a hydraulic press is equal to the work done at the other end.

Fx x dx= Fy x dy
(where F is the force applied on/by the piston d is the distance moved by the piston)

6. Atmospheric pressure is exerted due to the weight of the atmosphere. Its average value at sea level is 1.013 x 10^5 Pa.

7. The barometer is a simple device used to measure atmospheric pressure. The height of liquid level in the vacuum-filled tube increases if the atmospheric pressure increases

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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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2014 Aug Combine Chemistry Intensive Revision

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

2014 Aug Combine Physics Intensive Revision

29-Jul Tue 7pm to 9pm 2 hrs Sound & P1 Exam Practice

1-Aug Fri 4.30pm to 6pm 1.5 hrs Sound & P1 Exam Practice

3-Aug Sun 10.30pm to 12pm 1.5 hrs Kinematics and Dynamics & P1 Exam Practice

3-Aug Sun 1pm to 2.30pm 1.5 hrs Sound & P1 Exam Practice

5-Aug Tue 7pm to 9pm 2 hrs Light & P2 Exam Practice

8-Aug Fri 4.30pm to 6pm 1.5 hrs Light & P2 Exam Practice

10-Aug Sun 10.30pm to 12pm 1.5 hrs Forces and Moments & P2 Practice

10-Aug Sun 1pm to 2.30pm 1.5 hrs Light & P2 Exam Practice

12-Aug Tue 7pm to 9pm 2 hrs Electromagnetic Waves & Sound

15-Aug Fri 4.30pm to 6pm 1.5 hrs Electromagnetic Waves & Sound

17-Aug Sun 10.30pm to 12pm 1.5 hrs Energy, Work and Power & P1 Exam Practice

17-Aug Sun 1pm to 2.30pm 1.5 hrs Electromagnetic Waves & Sound

19-Aug Tue 7pm to 9pm 2 hrs Current Electricity & P2 Exam Practice

22-Aug Fri 4.30pm to 6pm 1.5 hrs Current Electricity & P2 Exam Practice

24-Aug Sun 10.30pm to 12pm 1.5 hrs Pressure & P2 Exam Practice

24-Aug Sun 1pm to 2.30pm 1.5 hrs Current Electricity & P2 Exam Practice

26-Aug Tue 7pm to 9pm 2 hrs D.C. Circuits & P1 Exam Practice

29-Aug Fri 4.30pm to 6pm 1.5 hrs D.C. Circuits & P1 Exam Practice

31-Aug Sun 10.30pm to 12pm 1.5 hrs Thermal Physics & P1 Exam Practice

31-Aug Sun 1pm to 2.30pm 1.5 hrs D.C. Circuits & 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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