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

Brownian motion

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

为什么Robert 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

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

为什么Air 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

提示Compare their size with a speck of smoke.

为什么A 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

提示At any instant, the knocks do not balance out.

为什么By 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

提示Something unseen must be doing the knocking.

为什么Nobody can see the particles of air, but the jiggling of the specks is exactly what the particle model predicts.

A saturated solution

提示Think of a sponge that can soak up no more.

为什么Every solvent has a limit. Once it is reached, any extra solid simply stays undissolved at the bottom.

More of it dissolves

提示Compare how sweet you can make hot tea and iced tea.

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

72

提示You have twice as much water.

为什么Solubility 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

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

为什么Pencil '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

提示Each dye travels its own fixed distance.

为什么A dye always travels the same distance under the same conditions, so matching patterns mean matching mixtures of dyes.