LOWKEY SMART

Read big numbers in groups of three digits: first the thousands, then the hundreds, tens and ones.

Everything is made of tiny particles. In a solid, the particles are packed tightly in fixed places, so a solid keeps its own shape. In a liquid, the particles are close together but can slide past each other, so a liquid flows and takes the shape of its container. In a gas, the particles are far apart and move quickly, so a gas spreads out to fill any space. Heating a solid can make it melt into a liquid: ice melts at 0°C. Heating a liquid makes it evaporate into a gas, and water boils at 100°C. Cooling a gas turns it back into a liquid, which is called condensation, and cooling a liquid turns it into a solid, which is called freezing.

The Sun is a star at the centre of our solar system. Eight planets orbit (travel around) the Sun. In order from the Sun they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Earth takes about 365 days, which is one year, to orbit the Sun once. The Moon orbits the Earth and takes about 28 days to go round once. The Sun, Earth and Moon are all roughly spheres (ball shapes). The Moon does not make its own light: we see it because it reflects light from the Sun.

The Earth spins on its axis, an imaginary line through the North and South Poles. It turns round once every 24 hours. The half of the Earth facing the Sun has day, and the half facing away from the Sun has night. Because of the way the Earth spins, the Sun appears to rise in the east and set in the west. The Sun is not really moving across our sky: it only seems to move because the Earth is turning. At midday the Sun is at its highest in the sky, so shadows are at their shortest.

We choose a material for a job because of its properties. A transparent material, like glass, lets light through so you can see through it; an opaque material, like wood, does not let light through. A thermal insulator, like wool or plastic, slows down heat moving through it; a thermal conductor, like metal, lets heat through easily. Metals also let electricity flow through them, while plastic is an electrical insulator, so it stops electricity. Magnetic materials are pulled towards a magnet: iron and steel are magnetic, but most other metals, such as copper and aluminium, are not. A material is flexible if it bends without breaking.

When a solid such as sugar or salt mixes into water and seems to disappear, it has dissolved, and the mixture is called a solution. A material that dissolves is soluble. Sand does not dissolve in water, so it is insoluble. Stirring, or using warmer water, makes sugar dissolve faster. You can separate sand from water by filtering: the water passes through filter paper and the sand is left behind. You can separate big solids, like stones, from small ones, like sand, with a sieve. To get salt back from salty water, you evaporate the water by leaving it somewhere warm, and the salt is left behind.

Every question here takes more than one step, so write the steps down. Read it twice, decide what is being asked, then work through. It pulls together angles (straight line 180°, around a point 360°).

A force is a push or a pull, and forces are measured in newtons (N). Gravity is a pull force: it pulls everything towards the centre of the Earth, which is why things fall when you drop them. Your weight is the force of gravity on you, while your mass is how much matter you are made of, measured in kilograms (kg). Air resistance is a force that slows down things moving through air. A parachute has a large surface area, which gives lots of air resistance. Water resistance slows down things moving through water, so fish and boats have streamlined shapes that let them move easily. Friction acts when two surfaces rub together, and a rough surface gives more friction than a smooth one.