Matter and particles, Year 9：全部卡片

2 g/cm³

提示Use the ruler measurements to find how much space the block takes up first.

为什么The volume is 2 × 3 × 5 = 30 cm³. Density = mass ÷ volume, so 60 ÷ 30. The unit follows from the sum: grams divided by cubic centimetres.

27

提示The density tells you what each single cubic centimetre weighs in at.

为什么Density = mass ÷ volume rearranges to mass = density × volume, so 2.7 × 10. Each cubic centimetre has a mass of 2.7 g, and there are ten of them.

volume

提示It is the amount of space the object takes up.

为什么Density compares how much matter there is with how much room it fills. Lead is dense because a lot of mass is packed into a small space; expanded polystyrene is the opposite.

30 cm³; 3 g/cm³

提示The stone pushes aside exactly its own amount of space in the water.

为什么An awkward shape cannot be measured with a ruler, but it displaces its own volume of water: 70 − 40. Then density is mass divided by that volume, 90 divided by the difference in the two readings.

Block B

提示Work out the mass of a single cubic centimetre of each.

为什么A has 200 ÷ 100 = 2 g in each cubic centimetre; B has 150 ÷ 50 = 3 g in each. B is lighter overall but denser, because its mass is packed into much less space.

More particles hit the walls each second

提示Picture hail on a roof, and then a much heavier shower.

为什么Each air particle that strikes the inside of the tyre gives it a tiny push. With more particles in the same space there are more collisions every second, so the total push on each bit of the tyre wall is bigger.

Its particles move randomly in all directions

提示Ask which way any one of them is heading at a given instant.

为什么Gas particles fly about in straight lines until they hit something, with no preferred direction. At any moment about as many are heading up as down or sideways, so every part of the inside surface is struck equally often.

Gas pressure

提示A gauge at a garage measures it inside your tyres.

为什么One collision is far too small to notice, but there are countless millions every second, and together they add up to a steady outward force on every square centimetre of the wall.

Ice is less dense than liquid water

提示Compare how much mass is packed into each cubic centimetre of the two.

为什么Anything less dense than the liquid around it floats. A lump of ice has slightly less mass than the same volume of water, so it rides at the surface with about a tenth of it showing above.

Water expands as it freezes

提示The same mass needs more room once it has turned solid.

为什么Most liquids shrink a little when they solidify. Water does the opposite: the ice takes up about a tenth more space than the water it formed from, and a sealed pipe has no room to give.

further apart

提示The same particles fill more space as a solid, so think about the gaps.

为什么In most solids the particles pack tighter than in the liquid. In ice they lock into a fixed, roomy pattern with more empty space between them, which is why the same mass of water takes up more volume frozen.

The water beneath stays liquid

提示Think about what the floating layer does for everything underneath it.

为什么Because ice floats, it forms a lid on the surface and insulates the water below from the cold air. If ice sank instead, ponds would fill with it from the bottom and freeze solid, and little could survive the winter.

Sink

提示In most solids the particles are packed more tightly than in the liquid.

为什么Solid wax drops to the bottom of molten wax, and solid iron to the bottom of molten iron, because the solid is the denser form. Water behaves the other way round, which is why its freezing is called an anomaly.