Secondary 2 Science

A group of elements that are lustrous, good conductors of heat and electricity, and can be hammered into shape, e.g. iron, copper and aluminium.

An element without metallic properties — mostly dull, brittle and a poor conductor, e.g. sulphur and carbon.

The ability of a metal surface to reflect light; freshly polished metals shine brightly.

The property of being hammered or rolled into thin sheets without breaking.

The property of being drawn into thin wires without breaking, like copper wire.

A material that lets electricity or heat pass through it; metals are common conductors.

A material that does not allow electric current to flow through it, such as plastic and rubber.

A pure substance made of only one kind of atom, which cannot be broken down by chemical means.

The ability of a material to resist scratching or denting; diamond is the hardest natural material known.

A group of compounds whose solutions taste sour and have a pH below 7, e.g. citric acid and ethanoic acid.

The natural acid found in lemons, oranges and other citrus fruits.

The acid in vinegar that gives it its sour taste, commonly called acetic acid.

The natural acid that makes yoghurt slightly sour, produced by bacterial fermentation.

Able to eat away skin, metals and other materials; concentrated acids are highly corrosive.

A scale from 0 to 14 showing how acidic or alkaline a substance is: 7 is neutral, below 7 is acidic, above 7 is alkaline.

The gas given off when acids react with some metals; it burns with a pop when tested with a lit splint.

Adding plenty of water to make a solution less concentrated; dilute acids are far less corrosive than concentrated ones.

A mixture of indicators showing the whole pH range 0 to 14 through a series of colours.

A compound that dissolves in water to give a pH above 7; its solution tastes bitter and feels soapy.

A strong alkali, also called caustic soda, used in drain cleaners and soap making.

A weak alkali used in window cleaners and as a raw material for fertilisers.

Also called slaked lime; its clear solution, limewater, is used to test for carbon dioxide.

A common indicator: it turns red in acids and blue in alkalis.

An indicator: colourless in acidic or neutral solutions, turning pink in alkalis.

A natural indicator: yellow in acid or neutral solutions, turning red-brown in alkalis.

The clear solution of calcium hydroxide, which turns milky when carbon dioxide passes through it.

A highly alkaline and corrosive alkali, such as sodium hydroxide, requiring great care.

The chemical reaction in which an acid and an alkali cancel each other out, forming a salt and water.

Neither acidic nor alkaline; pH 7, like pure water.

The general name for compounds formed when an acid neutralises an alkali, e.g. sodium chloride — not just table salt.

The dilute hydrochloric acid made by the stomach to digest food; too much causes discomfort.

A medicine containing an alkaline substance, such as milk of magnesia, that neutralises excess stomach acid.

Sodium hydrogencarbonate, a mildly alkaline powder used to neutralise acids and in baking.

Calcium hydroxide, the alkaline powder farmers spread on fields to neutralise acidic soil.

A tooth-cleaning paste containing mild alkalis that neutralise acids made by mouth bacteria, protecting enamel.

The build-up of electric charge on a surface where it does not flow, e.g. a comb after brushing hair picking up paper bits.

A basic property of matter, either positive or negative; like charges repel and unlike charges attract.

The basic unit of matter, made of a central nucleus with electrons moving around it.

Tiny negatively charged particles moving round the nucleus; some electrons in metals are free to move.

The positively charged centre of an atom, made of protons and neutrons.

The flow of electric charge in one direction; in metal conductors it is the flow of free electrons.

A source that converts chemical energy into electrical energy, driving electrons round a circuit.

The sudden release of accumulated charge, e.g. lightning.

A facility that generates electricity on a large scale and sends it to homes and factories through cables.

A complete closed loop through which current flows, made of a source, wires and components.

A circuit joined all the way round from the source through components and back; break it anywhere and the current stops.

A single unit that converts chemical energy to electrical energy; several joined together form a battery.

A component that opens or closes a circuit: closed lets current flow, open stops it.

A component that converts electrical energy into light and heat.

A drawing that represents a circuit using standard symbols.

Standard shapes standing for components: a cell is one long and one short line, a switch a breakable gate, a bulb a circle with a cross.

Metal wire joining the circuit for current to flow through, usually copper with an insulating plastic coat.

The rate at which charge flows, measured in amperes (A) with an ammeter.

The unit of current, symbol A.

The push a source gives to drive charge round a circuit, measured in volts (V) with a voltmeter.

The unit of voltage, symbol V; a dry cell supplies about 1.5 V.

The property of a material that opposes the flow of current, measured in ohms (Ω).

The unit of resistance, symbol Ω.

The instrument that measures current; it must be connected in series.

The instrument that measures voltage; it must be connected in parallel across a component.

Components joined end to end so that current has only one path.

A connection where the current splits and flows through two or more branches.

One of the separate paths available to the current in a parallel circuit.

The point in a circuit where the current splits or rejoins.

How strongly a bulb shines: bulbs dim in series but each keeps near-normal brightness in parallel.

A break that stops the current: in series everything stops, in parallel only that branch stops.

The parallel connection of home appliances so each can be switched independently.

In parallel the total current divides at a junction, each branch taking a share.

A fault where current bypasses the appliance through a low-resistance path, making the current dangerously large.

Connecting too many appliances to one socket or circuit so the total current exceeds what the wiring can safely carry.

A safety device containing a thin wire that melts when the current is too large, breaking the circuit.

A safety switch that trips automatically when the current is too large and can be reset after the fault is cleared.

A safety wire that carries leaked current into the ground so a metal case never shocks anyone.

The plastic coat round wires that prevents shocks and short circuits; damaged leads must be replaced at once.

The idle state where appliances still draw power after being switched off, quietly wasting energy over time.

A graded label showing an appliance's energy efficiency — Grade 1 is the most efficient and should be preferred.

A push or pull between objects that can change shape, speed or direction of motion, measured in newtons (N).

The unit of force, symbol N; a medium apple weighs about 1 N.

The force with which the Earth pulls objects towards it, making them fall.

The size of the gravitational pull on an object; the same object weighs less on the Moon.

The force between two surfaces in contact that opposes relative motion.

The pull a magnet exerts on magnetic materials, or the attraction and repulsion between magnetic poles.

An instrument that measures the size of a force by how far a spring stretches.

A change of shape or size when a force acts, e.g. squeezing a sponge flat.

The force between two surfaces in contact that opposes relative motion or attempted motion.

How uneven a surface is; the rougher the surface, the greater the friction.

A substance, such as machine oil, placed between surfaces to reduce friction.

A ring of balls that replaces sliding with rolling to cut friction, found in bicycle hubs.

The friction between soles or tyres and the ground that stops slipping when walking or braking.

The gradual loss of surface material caused by friction, e.g. soles growing thinner.

A shape that reduces air or water resistance, like the nose of a racing car or a fast train.

The drag an object meets moving through air — a form of friction that grows with speed.

The distance travelled per unit of time — distance divided by time.

The unit of speed, symbol m/s: 1 m/s means 1 metre each second, about 3.6 km/h.

The everyday unit of speed, symbol km/h, shown on car speedometers.

A graph with time on the horizontal axis and distance on the vertical axis; the slope of the line gives the speed.

How steep a line is: on a distance-time graph steeper means faster, and a horizontal line means stationary.

Motion at constant speed; its distance-time graph is a straight sloping line.

The total distance divided by the total time taken.

The gauge that shows a vehicle's current speed.

Forces acting on one object that are equal in size and opposite in direction, cancelling each other out.

The overall effect of all the forces combined; when it is zero the state of motion stays unchanged.

The upward push that a support such as a table exerts on an object.

The tendency of an object to resist changes in its motion; the greater the mass, the greater the inertia.

The amount of matter in an object and the measure of its inertia, in kilograms.

Motion at an unchanging speed in a straight line; it occurs when the net force is zero.

The restraint that holds the passenger during a crash or sudden braking, countering the forward lunge caused by inertia.

The pulling contest over a rope — a classic example for studying balanced forces and net force.

The systematic grouping of organisms according to their features.

An observable characteristic used for identification, e.g. having feathers or being able to flower.

A high rank of classification, e.g. the animal, plant and fungi kingdoms.

The system dividing life into prokaryotes (monera), protists, fungi, plants and animals.

An animal with a backbone, e.g. fish, frogs, snakes, birds and humans.

An animal without a backbone, e.g. insects, snails, shrimps and corals.

A group of similar organisms that can interbreed to produce offspring — the basic unit of classification.

The variety of living things found in an area.

A tool that identifies organisms step by step, offering two mutually exclusive choices at each step.

A pair of parallel statements (a and b) in a key, of which only one can apply.

The key feature used to tell organisms apart, e.g. feathers or the number of wings.

The path taken after a choice, leading either to the next step or to the answer.

The process of finding out what an organism is by using a key.

Two options that cannot both apply — one must be chosen; this is what makes a key work.

A common first step in animal keys: 'backbone or not?', splitting animals into two large groups at once.

Testing a finished key on real users to confirm every organism has exactly one path.

The natural place where an organism lives and finds food, e.g. a mangrove, a stream or woodland.

Salt-tolerant plant communities on intertidal shores, best known in Hong Kong around Deep Bay.

The muddy shore exposed at low tide, where mudskippers, fiddler crabs and migratory birds feed.

Birds that migrate long distances with the seasons, e.g. black-faced spoonbills wintering at Deep Bay.

A species found only in one region; Romer's tree frog is endemic to Hong Kong.

Legally protected countryside covering about 40 per cent of Hong Kong's land.

Dense tree-covered habitat such as Tai Po Kau Nature Reserve, home to mammals and forest birds.

The shore zone covered at high tide and exposed at low tide, whose life shows special adaptations.

A species so reduced in numbers that it faces the risk of extinction.

The permanent disappearance of a species when every individual has died.

The loss of homes through clearing, reclamation and pollution — the leading cause of endangerment.

Killing faster than the species can breed, collapsing its population.

A species introduced to a new area that spreads fast and crowds out native life.

The measures and actions protecting species and their habitats, such as reserves and legal controls.

A protected, managed area set aside for habitats, e.g. the Mai Po Nature Reserve.

The international agreement controlling trade in endangered wild species and their products.

Seafood caught or farmed in ways that do not harm fish stocks or ocean ecosystems.

A line representing light's path.

Light bouncing off a surface.

Angle of incidence equals angle of reflection.

Reflection from a smooth surface.

Reflection from a rough surface.

An image that cannot be formed on a screen.

White light splitting into colours.

To take in light rather than reflect it.

Rapid back-and-forth movement; the source of all sound.

The substance sound travels through.

Empty space: no medium, no sound.

Vibrations per second; decides pitch.

How high or low a sound is.

How loud a sound is; depends on amplitude.

The unique quality of a sound.

Sound above human hearing; used in scans.