Matter and particles, Year 8：全部卡片

Jitter about randomly

提示They never keep still or keep to a straight path.

为什么Each speck shows as a bright dot that darts a tiny distance one way, then another, with no pattern. This is called Brownian motion.

It is made of moving particles too small to see

提示Something invisible must be doing the pushing.

为什么The specks cannot push themselves about. Their jittering was the first direct evidence that the air is made of fast-moving particles.

The collisions come unevenly from all sides

提示No two instants bring an identical set of knocks.

为什么Air particles hit the speck from every direction. By chance the hits never quite balance, and the imbalance keeps changing.

No — the air particles never stop moving

提示Ask whether the cause has gone away.

为什么As long as the air is at any ordinary temperature, its particles keep moving and keep colliding with the specks.

The metal lid expands and loosens

提示Think about what heating does to the size of a solid.

为什么Heated particles vibrate more and push each other slightly further apart. The lid grows a little wider than the glass and loses its grip.

No — they move further apart

提示A hot iron bar is longer, yet it has no extra atoms.

为什么The particles vibrate more strongly and so take up more room each. The solid grows because the spacing grows.

To give the steel room to expand on hot days

提示Think about what a long strip of metal does on a hot day.

为什么A long bridge can grow by several centimetres between winter and summer. The gaps, called expansion joints, let it do so safely.

Neither — they have the same mass

提示Read the two amounts carefully.

为什么A kilogram is a kilogram. The feathers take up far more space because they are much less dense, but the two masses are equal.

Iron is denser: more mass is packed into the same space

提示Compare how much matter fits in each cubic centimetre.

为什么Density compares mass with volume. In iron the particles are heavier and more closely packed, so each cubic centimetre has more mass.

Its particles are much further apart

提示Think about the amount of empty space there is.

为什么The particles are identical in both states, but in the gas they are spread through a far larger volume, so each cubic centimetre holds very little mass.