π WHY IT WORKSWhy only change one thing: if you change two things at once and the result changes, you cannot tell which change caused it. Keeping the others controlled makes the test fair, so the IV must be the cause.
π§ Worked example β follow the thinking
Ali tests if sugar dissolves faster in hot water. Name the IV, DV and two CVs.
Step
Why I do this
IV = temperature of water
π It is the thing Ali deliberately changes.
DV = time taken for sugar to dissolve
π It is what he measures to see the effect.
CVs = mass of sugar, volume of water (also stirring, type of sugar)
π Anything else that could affect dissolving must stay the same.
Science Β· Sec 1 (Lower Sec Science, G3)
π§±
2
Physical Properties of Matter
strength, hardness, conductivity, density
BIG IDEAEvery material has physical properties you can measure without changing what it is. density = mass Γ· volume
Property
Means
Metals
Non-metals
Strength
resists breaking
strong
usually weak
Hardness
resists scratching
usually hard
varies
Flexibility
bends without breaking
malleable, ductile
brittle (solids)
Melting / boiling point
temp it changes state
high
usually low
Electrical conductivity
lets electricity flow
good
poor (graphite conducts)
Thermal conductivity
lets heat flow
good
poor (insulators)
βοΈ Density
Ο = m Γ· V units: g/cmΒ³ or kg/mΒ³ stone: 60 g, 20 cmΒ³ β 3 g/cmΒ³ water = 1 g/cmΒ³: less dense β floats
irregular object: volume = rise in water = 20 cmΒ³
π§ HOOKChoose a material by its properties: copper for wires (conducts), plastic for handles (insulator), steel for bridges (strong).
β TRAPDensity of a substance stays the same whatever the size of the piece β a small and big gold bar have the same density.
π WHY IT WORKSWhy things float or sink: density compares how much mass is packed into each cmΒ³. Water is 1 g/cmΒ³ β anything packed more tightly sinks, anything less tightly floats.
π§ Worked example β follow the thinking
A block has mass 54 g and volume 20 cmΒ³. Will it float in water?
metals: shiny, conduct, malleable (Fe, Cu, Al) non-metals: dull, insulators (C, S, O) listed in the Periodic Table
β WRITE THIS"A compound has different properties from the elements it is made of, and its elements are in a fixed ratio."
π WHY IT WORKSWhy a compound is different from its elements: in a compound the atoms are chemically bonded into a brand-new substance with new properties. In a mixture nothing is bonded, so each part keeps its own properties and can be separated physically.
π§ Worked example β follow the thinking
Classify: oxygen, water, air.
Step
Why I do this
Oxygen β element
π Only one type of atom (O).
Water β compound
π H and O chemically joined in a fixed 2 : 1 ratio.
Air β mixture
π Nβ, Oβ, COββ¦ not joined, any proportions, can be separated.
Science Β· Sec 1 (Lower Sec Science, G3)
π§
4
Solutions & Suspensions
solute, solvent, solubility, rate of dissolving
BIG IDEAsolute + solvent = solution. If it dissolves, you get a clear solution; if not, a cloudy suspension.
salt + water = salt solution
Solution
Suspension
Looks
clear (may be coloured)
cloudy
Settles?
no
yes, on standing
Filter?
passes through
solid stays on paper
Example
salt water
muddy water, chalk + water
β‘ Rate of dissolving β faster ifβ¦
π₯ solvent is hotter π₯ you stir π¨ solute is in smaller pieces (powder)
π Solubility
max mass that dissolves in 100 g of solvent at that temperature most solids: more soluble when hotter
Words
dilute: little solute Β· concentrated: lots of solute saturated: can dissolve no more at that temperature
β TRAPStirring makes sugar dissolve faster, but it does NOT make more sugar dissolve β that is solubility, which depends on temperature.
π WHY IT WORKSWhy heat and stirring speed up dissolving: hot particles move faster and hit the solute more often; stirring brings fresh solvent to the solid; smaller pieces expose more surface. All mean more particles reach the solute each second.
π§ Worked example β follow the thinking
Explain why sugar dissolves faster in hot tea than in iced tea.
Step
Why I do this
Hot tea particles have more kinetic energy
π Temperature = how fast particles move.
They collide with the sugar more often and with more energy
sea water β drinkable water by distillation or reverse osmosis
π WHY IT WORKSWhy distillation gives pure water: water boils at 100 Β°C but salt does not boil β so only water turns to vapour. The vapour is cooled back to liquid in the condenser, leaving the salt behind.
π§ Worked example β follow the thinking
How do you get pure water from salt water?
Step
Why I do this
Use simple distillation
π I want the LIQUID, not the solid β so evaporation alone would lose it.
Heat: water boils, salt stays in the flask
π Different boiling points.
Condenser cools the steam back to water
π Cold water flows in at the bottom for best cooling.
Collect the distillate
π Pure water β check: boils at exactly 100 Β°C.
Science Β· Sec 1 (Lower Sec Science, G3)
π«§
6
Particulate Nature of Matter
states, changes of state, expansion, diffusion
BIG IDEAAll matter is made of tiny particles that are always moving. Heat makes them move faster.
SOLID π§ packed Β· vibrate in place
LIQUID π§ close, random Β· slide past
GAS π¨ far apart Β· move fast, randomly
red = takes in heat Β· blue = gives out heat
Explain with particles
Expansion: heated particles move faster and further apart (particles do NOT get bigger) Diffusion: particles move from high to low concentration (smell of perfume spreads) Gas pressure: particles hit the walls Mass is conserved: no particles are made or destroyed
β TRAPWhen a solid expands, the particles stay the same size β only the spaces between them get bigger.
π WHY IT WORKSWhy things expand when heated: particles gain energy, vibrate or move more and push further apart. The particles themselves do NOT get bigger β only the spaces between them grow.
π§ Worked example β follow the thinking
Explain why a metal rod gets longer when heated.
Step
Why I do this
Particles gain kinetic energy
π Heating adds energy.
They vibrate more vigorously
π Solid particles can't move freely, only vibrate.
They move slightly further apart β rod expands
π Spacing increases; particle size does not change.
Science Β· Sec 1 (Lower Sec Science, G3)
π¦
7
The Cell Model
cell parts, plant vs animal, specialised cells
BIG IDEAThe cell is the basic unit of life. Plant cells have 3 extras: cell wall, chloroplasts, large vacuole.
plant cell vs animal cell (both have nucleus, cytoplasm, cell membrane)
Part
Job
Plant
Animal
nucleus
controls the cell; holds DNA (genetic info, in chromosomes)
β
β
cell membrane
controls what enters and leaves
β
β
cytoplasm
where chemical reactions happen
β
β
cell wall
cellulose; gives shape and support, stops bursting
β
β
chloroplast
photosynthesis (contains chlorophyll)
β
β
vacuole
stores cell sap; keeps the cell firm
large, central
small / none
mitochondria
respiration releases energy
β
β
𧱠Levels of organisation
cell β tissue (same cells, same job) β organ β system β organism e.g. muscle cell β muscle tissue β heart β circulatory system
Specialised cell
Adapted by
red blood cell
no nucleus, biconcave β more room for haemoglobin to carry Oβ
root hair cell
long hair β large surface area to absorb water
nerve cell
long β carries messages far
muscle cell
contracts to move
π¬ Using a microscope
start on the lowest power objective Β· coarse focus, then fine focus Β· magnification = eyepiece Γ objective (10Γ Γ 40Γ = 400Γ) Β· stain (iodine / methylene blue) to see the nucleus
π WHY IT WORKSWhy cells specialise (division of labour): one cell cannot do everything well. Different cells take on one job and their shape is built for it β so the whole organism works more efficiently.
π§ Worked example β follow the thinking
A cell has a cell wall and a large vacuole but no chloroplasts. Where could it come from?
Step
Why I do this
It has a cell wall + vacuole β plant cell
π Animal cells never have a cell wall.
No chloroplasts β from a part with no light
π Chloroplasts are only needed where photosynthesis happens.
Answer: a root cell (e.g. root hair cell)
π Roots are underground, so no photosynthesis.
Science Β· Sec 1 (Lower Sec Science, G3)
π‘
8
The Ray Model of Light
reflection, refraction, dispersion, colour
BIG IDEALight travels in straight lines (rays). It can be reflected (bounce), refracted (bend) or dispersed (split into colours).
angle i = angle r Β· angles measured from the normal
light slows in glass β bends
πͺ Image in a plane mirror
same size Β· upright Β· laterally inverted (leftβright) Β· virtual (cannot be caught on a screen) Β· as far behind as the object is in front
Surfaces
regular reflection: smooth (mirror) β clear image diffuse reflection: rough (paper) β scattered, no image, but lets us see objects from any angle
dispersion: ROYGBIV β violet bends most
π¨ Colour
a red object reflects red, absorbs the rest Β· white reflects all Β· black absorbs all a red filter lets only red through red object under blue light looks black
βοΈ Effects of EM radiation
UV: sunburn, skin cancer β sunscreen Β· infrared: felt as heat Β· visible: lets us see Β· too much light can damage the eyes β never look at the Sun
π WHY IT WORKSWhy light bends in glass: light travels slower in glass than in air. When a ray enters at an angle, one side of the beam slows first, so the beam swings towards the normal β like a car driving off the road onto sand at an angle.
π§ Worked example β follow the thinking
A ray hits a mirror making 30Β° with the mirror surface. Find the angle of reflection.
Step
Why I do this
Angle of incidence = 90Β° β 30Β° = 60Β°
π Angles are measured from the NORMAL, which is at 90Β° to the mirror.