HβSOβ β 2 H, 1 S, 4 O = 7 atoms Ca(OH)β β 1 Ca, 2 O, 2 H
Symbol
Element
Symbol
Element
Symbol
Element
H
hydrogen
Na
sodium
Fe
iron
C
carbon
Mg
magnesium
Cu
copper
N
nitrogen
Al
aluminium
Zn
zinc
O
oxygen
Cl
chlorine
Ag
silver
S
sulfur
K
potassium
Ca
calcium
βοΈ Writing simple formulae
use the combining power (valency) and cross over: Na (1) + Cl (1) β NaCl Β· Mg (2) + O (2) β MgO Β· Al (3) + O (2) β AlβOβ Β· H (1) + O (2) β HβO
π§ HOOKIons: atoms that lose electrons become positive (NaβΊ); atoms that gain electrons become negative (Clβ»).
π WHY IT WORKSWhy formulas tell you everything: the symbols say which elements are present, the small numbers say how many atoms of each. Brackets multiply everything inside.
π§ Worked example β follow the thinking
How many atoms of each element are in Ca(OH)β?
Step
Why I do this
Ca: 1
π No number = 1.
O: 1 Γ 2 = 2
π The 2 outside the bracket multiplies everything inside.
H: 1 Γ 2 = 2
π
Total = 5 atoms
π
Science Β· Sec 2 (Lower Sec Science, G3)
ποΈ
2
Forces & Pressure
contact/non-contact, mass vs weight, p = F/A
BIG IDEAA force is a push or pull, measured in newtons (N). Pressure = how spread out a force is: p = F Γ· A
Contact forces
Non-contact forces
friction
gravitational (weight)
air resistance
magnetic
tension, normal (reaction) force
electrostatic
Effects of a force
start or stop motion Β· speed up / slow down Β· change direction Β· change shape
Mass
Weight
Meaning
amount of matter
gravitational pull on it
Unit
kg
N
Measured by
beam / electronic balance
spring balance
On the Moon
same
smaller (β )
βοΈ Weight
W = m Γ g g = 10 N/kg on Earth 5 kg bag β W = 5 Γ 10 = 50 N
same weight, different area
Pressure in daily life
unit: N/mΒ² = Pa (or N/cmΒ²) high p: sharp knife, nails, stiletto heels low p: snowshoes, wide tractor tyres, school bag straps
β TRAPNever say "weight is 5 kg". Mass is in kg; weight is a force in N.
π WHY IT WORKSWhy a sharp knife cuts better: the same push is focused on a tiny area. Smaller A in p = F Γ· A means a much bigger pressure, enough to break the food apart.
π§ Worked example β follow the thinking
A 60 N box has a base 0.3 m Γ 0.2 m. Find the pressure on the floor.
Step
Why I do this
Area = 0.3 Γ 0.2 = 0.06 mΒ²
π Pressure needs the area in contact.
p = F Γ· A = 60 Γ· 0.06
π Force = weight of the box.
p = 1000 Pa
π Units: N Γ· mΒ² = Pa.
Science Β· Sec 2 (Lower Sec Science, G3)
β‘
3
Energy & Work Done
forms, conversions, conservation, W = Fd
BIG IDEAEnergy cannot be created or destroyed, only converted from one form to another. Unit: joule (J).
Form / store
Example
kinetic (KE)
moving car
gravitational potential (GPE)
raised book
elastic potential
stretched rubber band
chemical potential
food, fuel, batteries
heat, light, sound, electrical
transfers between stores
KE β GPE as it swings
Device
Energy conversion
torch
chemical β electrical β light + heat
electric fan
electrical β kinetic + sound + heat
solar cell
light β electrical
car
chemical β kinetic + heat + sound
hydroelectric dam
GPE of water β kinetic β electrical
π§ Work done
W = F Γ d F in N, d (in direction of force) in m β W in J work done = energy transferred
π± Energy sources
renewable: solar, wind, hydro, tidal, geothermal, biomass non-renewable: coal, oil, natural gas, nuclear Singapore: mostly natural gas + growing solar
β TRAPHolding a heavy bag still does no work β there is no distance moved in the direction of the force.
π WHY IT WORKSWhy "useful" energy is never 100%: every conversion also produces some heat (and sound) that spreads out into the surroundings. Energy is not lost β it is just wasted in a form we cannot use.
π§ Worked example β follow the thinking
Sam lifts a 20 N box onto a 1.5 m shelf. How much work does he do?
Step
Why I do this
Force needed = weight = 20 N (upwards)
π To lift steadily, he must push up with a force equal to the weight.
Distance in direction of force = 1.5 m
π The box moves up 1.5 m β same direction as the force.
W = F Γ d = 20 Γ 1.5 = 30 J
π This 30 J becomes GPE of the box.
Science Β· Sec 2 (Lower Sec Science, G3)
π₯
4
Heat & its Effects
expansion, conduction, convection, radiation
BIG IDEAHeat flows from hotter to colder by conduction (solids), convection (liquids and gases) and radiation (no medium needed).
π‘ Heat vs temperature
temperature: how hot (Β°C; SI unit kelvin, K) heat: energy transferred (J) K = Β°C + 273
βοΈ Expansion
heated β particles vibrate more, move further apart β expands uses: bimetallic strip (thermostat), shrink-fitting problems: gaps in railway tracks, bridges; sagging power lines
Good conductor / emitter
Bad conductor (insulator)
Conduction
metals (copper pots)
plastic, wood, air, cloth, polystyrene
Radiation
dull black surfaces absorb and emit best
shiny white/silver surfaces reflect, emit least
π§ Vacuum flask
vacuum stops conduction & convection Β· silvered walls reduce radiation Β· stopper stops convection out the top
π Climate change
burning fossil fuels β more COβ (and methane) β traps more heat (greenhouse effect) β global warming, rising sea levels, extreme weather
π WHY IT WORKSWhy convection currents form: heated fluid expands, so it becomes less dense and rises. Cooler, denser fluid sinks to take its place, making a loop. That is why an aircon is mounted high and a heater low.
π§ Worked example β follow the thinking
Why does a metal spoon feel colder than a wooden spoon at the same room temperature?
Step
Why I do this
Both are at the same temperature
π Do not say the metal is colder β it isn't.
Metal is a good conductor
π It conducts heat away from your warmer hand quickly.
Heat leaves your hand faster β it feels colder
π Wood is an insulator, so heat leaves your hand slowly.
Science Β· Sec 2 (Lower Sec Science, G3)
π₯
5
Chemical Changes
word equations, types of reactions
BIG IDEAA chemical change makes a new substance. Atoms are rearranged, never created or destroyed.
Physical change
Chemical change
New substance?
no
yes
Reversible?
usually easy
usually hard
Examples
melting ice, dissolving sugar
burning, rusting, cooking an egg
π Signs of a chemical change
gas bubbles (effervescence) Β· colour change Β· temperature change Β· precipitate forms Β· light/sound
β TRAPTotal mass before = total mass after (conservation of mass). If mass seems to drop, a gas escaped.
π WHY IT WORKSWhy mass is conserved: in a reaction atoms are only rearranged into new substances β none are created or destroyed. If mass seems to drop, a gas escaped; if it rises, something from the air (like oxygen) was gained.
π§ Worked example β follow the thinking
Magnesium ribbon is burned in air. Describe what you see and write the word equation.
Step
Why I do this
Bright white light, white powder forms
π Signs of a chemical change: light, new substance.
magnesium + oxygen β magnesium oxide
π Burning = reacting with oxygen (combustion/oxidation).
Mass of the powder > mass of the ribbon
π Oxygen from the air has been added.
Science Β· Sec 2 (Lower Sec Science, G3)
π
6
Acids & Alkalis
pH, indicators, neutralisation
BIG IDEApH below 7 = acid, 7 = neutral, above 7 = alkali. Acid + alkali β salt + water.
01234567891011121314
β very ACIDICslightly acidicNEUTRAL 7slightly alkalinevery ALKALINE βΆ
Acid
Alkali
Taste / feel
sour
bitter, soapy
Litmus
turns red
turns blue
Universal Indicator
red / orange / yellow
blue / violet
Examples
vinegar, lemon juice, HCl
soap, NaOH, ammonia
Reaction
Products
acid + metal
salt + hydrogen
acid + carbonate
salt + water + COβ
acid + alkali
salt + water
π¦· Everyday neutralisation
antacid tablets for indigestion Β· toothpaste (alkaline) vs mouth acid Β· lime for acidic soil Β· baking soda on bee stings
β TRAPAcids and alkalis are corrosive. Use goggles; add acid to water, not water to acid.
π WHY IT WORKSWhy neutralisation makes water: acids release HβΊ, alkalis release OHβ», and they join: HβΊ + OHβ» β HβO. Once they cancel out, the solution is neutral (pH 7).
π§ Worked example β follow the thinking
Dilute acid is added to marble chips (calcium carbonate). How would you identify the gas?
Step
Why I do this
Bubbles / effervescence
π acid + carbonate β salt + water + carbon dioxide.
Bubble the gas through limewater
π The test for COβ.
Limewater turns milky β COβ
π State the test AND the result.
Science Β· Sec 2 (Lower Sec Science, G3)
πΏ
7
Ecosystems
food chains/webs, interactions, energy flow, human impact
BIG IDEAArrows in a food chain show the flow of energy: producer β consumers. Decomposers recycle nutrients back to the soil.
food web: arrow = "is eaten by"
Roles
producer: makes food by photosynthesis (plants) herbivore = primary consumer carnivore = secondary/tertiary consumer decomposer: bacteria, fungi break down dead matter
Interaction
What happens
predatorβprey
one hunts the other (snakeβrat)
competition
both need the same food / space / light
mutualism
both benefit (bee & flower)
parasitism
one benefits, host is harmed (tick on a dog)
Physical factors & adaptations
factors: light, temperature, water, pH, oxygen, salinity adaptation: a trait that helps survival β cactus spines (less water loss), thick fur (cold), camouflage
π» Energy pyramid
only about 10% of energy passes to the next level β the rest is lost as heat (respiration), movement, waste β food chains are short (4β5 levels)
π WHY IT WORKSWhy the arrows point to the eater: the arrow shows where the energy goes. Energy stored in grass moves INTO the grasshopper when it eats the grass β so the arrow points to the grasshopper.
π§ Worked example β follow the thinking
In the web above, what happens to the grasshoppers if all the snakes are removed?
Step
Why I do this
Snakes eat rats, so rats increase
π No predator keeps the rat population down.
More rats eat more rice plants, but rats do not eat grasshoppers
π Trace each arrow, one link at a time.
Grasshoppers increase (snakes no longer eat them)
π Snake was also a predator of grasshoppers.
Science Β· Sec 2 (Lower Sec Science, G3)
π
8
Electrical Systems
circuits, V = IR, power, cost, safety
BIG IDEACurrent flows only in a closed loop. V = I Γ R Β· P = V Γ I Β· cost = kWh Γ price
bigger R β smaller current (for the same V) longer / thinner wire β more resistance variable resistor (rheostat) controls brightness
π° Cost of electricity
energy (kWh) = power (kW) Γ time (h) cost = kWh Γ price per kWh save energy: LED bulbs, switch off, energy-label ticks (more ticks = more efficient)
β οΈ Electrical safety
fuse melts if current is too large Β· circuit breaker trips Β· earth wire carries current safely away from a metal case Β· dangers: wet hands, damaged insulation, overloaded sockets
π WHY IT WORKSWhy homes use parallel circuits: each appliance gets the full mains voltage and has its own switch, and if one appliance breaks the rest keep working.
π§ Worked example β follow the thinking
A 2000 W heater is used 3 h a day. Electricity costs 30Β’ per kWh. Find the daily cost.
Step
Why I do this
P = 2000 W = 2 kW
π Cost uses kilowatts and hours, not W and s.
Energy = 2 kW Γ 3 h = 6 kWh
π kWh = kW Γ h.
Cost = 6 Γ 30Β’ = $1.80
π Multiply by the price per kWh.
Science Β· Sec 2 (Lower Sec Science, G3)
π
9
Human Digestive System
organs, enzymes, absorption, food tests
BIG IDEADigestion breaks large insoluble food molecules into small soluble ones that can be absorbed into the blood.
acid kills germs; protease starts protein digestion; churns
liver / gall bladder
bile breaks fat into droplets (emulsifies)
pancreas
makes amylase, protease, lipase
small intestine
finish digestion + absorb (villi)
large intestine
absorbs water; faeces pass out the anus
Enzyme
Breaks down
Into
amylase
starch
maltose (sugar)
protease
protein
amino acids
lipase
fats
fatty acids + glycerol
π§ͺ Food tests
iodine: brown β blue-black = starch Benedict's + heat: blue β brick-red = reducing sugar
π‘ Enzymes
biological catalysts (proteins) Β· work best at body temperature (~37 Β°C) Β· denatured by high temperature Β· specific to one substrate
After absorption
glucose β used in respiration for energy Β· amino acids β build cells, growth and repair Β· this is assimilation
π₯ Healthy diet
balanced: carbs, proteins, fats, vitamins, minerals, fibre, water Β· gut bacteria help digestion; harmful bacteria cause food poisoning
π WHY IT WORKSWhy the small intestine absorbs so well: its wall is folded into millions of finger-like villi β a huge surface area, thin walls and many blood capillaries, so digested food diffuses into the blood quickly.
π§ Worked example β follow the thinking
Starch + amylase is kept at 0 Β°C, 37 Β°C and 80 Β°C. Where does iodine stay brown soonest?
BIG IDEABig organisms need a transport system: diffusion alone is too slow to reach every cell. Plants: xylem + phloem. Humans: heart + blood vessels.
Diffusion
net movement of particles from high β low concentration (Oβ into blood in the lungs)
Osmosis
movement of water from a dilute to a concentrated solution through a partially permeable membrane
water lost from leaves = transpiration β it pulls water up
Vessel
Carries
Wall
artery
blood away from heart (high pressure)
thick, muscular, elastic
vein
blood back to heart
thin, has valves
capillary
exchange with cells
one cell thick
β€οΈ Heart & blood
heart pumps blood round the body Β· red blood cells carry Oβ Β· white blood cells fight germs Β· plasma carries dissolved food, COβ, waste Β· platelets clot
Health
smoking & fatty diets β blocked arteries, heart attack Β· drug abuse damages the heart, brain and liver Β· heart transplants raise ethical issues (donor consent, who gets one)
π WHY IT WORKSWhy capillaries are one cell thick: substances only diffuse a short distance quickly. A wall one cell thick keeps the distance tiny, so oxygen and food pass into cells fast.
π§ Worked example β follow the thinking
A potato strip is put in concentrated salt solution. Predict and explain.
Step
Why I do this
Potato cells are more dilute than the salt solution
π Compare the water in each.
Water leaves the cells by osmosis
π Water moves from dilute β concentrated across the cell membrane.
Strip becomes shorter, lighter and soft
π Losing water makes the cells shrink.
Science Β· Sec 2 (Lower Sec Science, G3)
πΆ
11
Human Reproduction
reproductive systems, menstrual cycle, heredity, health
BIG IDEASexual reproduction: a sperm and an egg join (fertilisation) to form a zygote β half the genes from each parent, so offspring vary.
Male part
Job
testes
make sperm and the hormone testosterone
sperm duct
carries sperm
penis
passes sperm into the vagina
Female part
Job
ovaries
release an egg each month; make oestrogen
oviduct
fertilisation happens here
uterus
embryo implants and grows
vagina
receives sperm; birth canal
the menstrual cycle (β 28 days): uterus lining breaks down, rebuilds, egg released
Fertilisation in the oviduct
Zygote divides into an embryo
Implantation in uterus lining
Foetus fed via placenta + umbilical cord
Birth β 9 months
Heredity
genes (on chromosomes, made of DNA) carry traits from parents Β· variation: eye colour, blood group, height Β· identical twins come from one zygote
Puberty
body changes caused by hormones: growth spurt; boys β voice deepens, sperm production; girls β breasts develop, periods begin
Health
STIs (HIV/AIDS, gonorrhoea, syphilis) spread by sexual contact Β· prevention: abstinence, condoms Β· bacterial STIs treated with antibiotics; HIV has no cure smoking, alcohol and drugs in pregnancy harm the foetus (low birth weight, defects)
Birth control & population
barrier (condom) Β· hormonal (pill) Β· surgical (sterilisation) world population grows due to better food, medicine and hygiene β more people survive
π WHY IT WORKSWhy children look like but not identical to parents: each sperm and egg carries a random half of each parent's genes. Every fertilisation mixes a different combination, so siblings differ.
π§ Worked example β follow the thinking
A woman's period starts on 1 May (28-day cycle). Around when is she most likely to ovulate?
Step
Why I do this
Day 1 = 1 May
π The cycle is counted from the first day of menstruation.
Ovulation β day 14
π Egg released about halfway through a 28-day cycle.
β 14 May
π Fertile window about 11β17 May (sperm and egg survive a few days).