Particles and states of matter, Year 8: evidence and solutions

Brownian motion

HintIt is named after a nineteenth-century botanist who saw it in grains from pollen.

The whyRobert Brown described it in 1827 while looking at tiny grains in water. Decades later it was explained by collisions with particles too small to see.

Collisions with moving air particles

HintThe cause is far too small to see, even under the microscope.

The whyAir particles move fast in every direction and keep striking each speck. The speck is light enough for these knocks to push it about.

They are far too small

HintCompare their size with a speck of smoke.

The whyA smoke speck is made of a huge number of particles. A single air particle is far smaller than anything a light microscope can show.

It is hit unevenly from different sides

HintAt any instant, the knocks do not balance out.

The whyBy chance, more particles strike one side than the other, so the speck is pushed that way. An instant later the imbalance is somewhere else.

That it is made of tiny, moving particles

HintSomething unseen must be doing the knocking.

The whyNobody can see the particles of air, but the jiggling of the specks is exactly what the particle model predicts.

A saturated solution

HintThink of a sponge that can soak up no more.

The whyEvery solvent has a limit. Once it is reached, any extra solid simply stays undissolved at the bottom.

More of it dissolves

HintCompare how sweet you can make hot tea and iced tea.

The whyFor most solids, solubility rises with temperature. The cold tea was saturated; warm tea has room for extra sugar.

72

HintYou have twice as much water.

The whySolubility is given for each 100 g of water. Twice the water can hold twice the solute: 36 × 2.

Pencil does not dissolve in the solvent

HintThink about what would happen to a line drawn in ink as the liquid rises.

The whyPencil 'lead' is graphite, which stays where it is drawn. An ink line would separate into its own dyes and spoil the result.

They are made of the same dyes

HintEach dye travels its own fixed distance.

The whyA dye always travels the same distance under the same conditions, so matching patterns mean matching mixtures of dyes.