O Level – Combined Science

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Chapter 1 Measurement

Exam Tip 1
It is important to use SI units. A physical quantity without the appropriate unit is ambiguous, e.g. 5 units of length can represent 5 cm, 5 m or 5 km.
Many students are not careful enough in writing symbols. Do not use Kg (capital letter K for kilogram and M for metre.)

Exam Tip 2
Common e.g. of SI units using prefixes: millimetre (mm), centimetre (cm), kilometre (km), etc. Candidates must remember all the prefixes, their symbols and meanings.

Exam Tip 3
When using graphical method to find the resultant force of two forces.
The length of the diagonal in the diagram is not the actual resultant in most cases. Students need to refer to the scale chosen to calculate the resultant.

Exam Tip 4
A student has been asked to calculate the volume of a piece of wire, which is about 80 cm long and about 0.2 cm in diameter.
Which measuring instruments should the student use?

To answer a question like this, candidates need to know the accuracy of different measuring instruments

Exam Tip 5
The INSIDE jaws of a pair of vernier calipers can be used to measure the internal diameter of measuring cylinder and beaker.

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2015 Schedule – Combine 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

2015 Schedule – Combine Physics

S3 1.5 hrs SAT 10.30am – 12pm

S3 1.5 hrs SAT 2pm – 3.30pm

S3 1.5 hrs SUN 1pm – 2.30pm

S4 1.5 hrs FRI 4.30pm – 6pm

S4 1.5 hrs SAT 2pm – 3.30pm

S4 1.5 hrs SUN 10.30am – 12pm

Please contact Angie @ 96790479 or Mr Ong 98639633

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Chapter 2 Kinematics

Exam Tip 1
An example to show non-zero distance but zero displacement: An object is under a circular motion. It completes one revolution and goes back to the starting point. The distance travelled is the circumference of the circular path but the displacement is zero.

Exam Tip 2
An example of constant speed but changing velocity: When an object is under circular motion, the direction of motion changes with time. Its speed can be the same but the displacement keeps changing when the object is moving along the circle.

Exam Tip 3
A misconception for acceleration is that many students think that acceleration must be zero if an object is at rest. This is not always true.

A case of zero velocity but non-zero acceleration: When an object is thrown upwards, it is momentarily at rest at the highest point. The velocity is zero. However it is a free falling body as gravity is the only force acting on it.

For free falling body, the acceleration is always 10 m/s^2 (if there is no air resistance) no matter whether it is moving downwards, upwards or at rest momentarily

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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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Chapter 2 Kinematics Part 2

Exam Tip 4
Uniform acceleration means uniform increasing speed, i.e. the change of velocity (or speed) is a constant.

Exam Tip 5
Students should be able to describe the different kinds of motion of an object by reading distance-time or speed-time graph. They should be able to describe the change in distance, speed and acceleration

Exam Tip 6
Apply area formula for triangle, rectangle or trapezium according to the graph to find the distance travelled under the Velocity-Time graph

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Chapter 2 Kinematics Part 3

Exam Tip 7
An important point for a free falling body:
A free falling body may also move up-wards. When a free falling body is rising instead of falling, its direction of motion is upwards but the acceleration is -still 10 m 5 2 downwards.
This upward motion will slow down even-tually because the acceleration due to gravity is opposing its direction of motion.
In fact, this free falling body is deceler-ating. When its velocity becomes zero, it will stop at that position momentarily and then start falling again. The free falling body is now accelerating.

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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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Chapter 2 Kinematics Part 4

Exam Tip 8

EFFECT OF AIR RESISTANCE

When an object is in motion (either on the ground or in the air), there is always air resistance acting on the moving object in a direction OPPOSITE the direction of motion.

– Air resistance is a FRICTIONAL FORCE which:
– Increases with the surface area (size) of the object;
– Increases with the speed of the object;
– Increases with the density of air.

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Secondary 3 Year End Exams and Secondary 4 Prelim Preparatory Classes

for Combined Physics, Chemistry and Biology

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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Measurement

Question 1

Which pair of units both measure the same quantity?
A km/h and kg/m3
C V and J/C
B N/m3 and Pa
D W and J

Ans C
Potential Difference V = Work done per unit charge = J/C

Question 2

Which instrument is used to measure directly the circumference of a golf
ball?
A calipers
B micrometer
C rule
D tape

Ans : D
A cloth tape can be used to directly measure the length along a curved surface, such as the circumference of a golf ball.

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Light – Reflection

1. The diagram below shows a ray of light being reflected from a plane surface

2. The following terms are commonly used in the reflection of light

Normal – Imaginary line perpendicular to the surface of reflection
Angle of incidence, i – Angle between the incident ray and the normal
Angle of reflection, r – Angle between the reflected ray and the normal

3. Laws of reflection:
(a) Angle i = Angle r
(b) The incident ray, reflected ray and the normal at the point of incidence all lie on the same plane.

4. Characteristics of an image formed in a plane mirror:
(a) Upright
(b) Virtual (Cannot be captured on a screen)
(c) Laterally inverted
(d) Same size as the object
(e) Image distance from the other side of the surface of reflection is the same as the object’s distance from the surface of reflection.

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