Particles and states of matter, Year 10: mixtures and changes of state

Fractional distillation

HintCrude oil is split into petrol, diesel and other parts this way; the name comes from a word for a part of a whole.

The whyIn the column the vapours cool and condense many times over, so the liquid with the lower boiling point reaches the top first and the rest runs back down. Simple distillation, with no column, is enough when one substance boils far below the other, as with water and dissolved salt.

Ethanol

HintThe one that turns to vapour at the lower temperature gets to the top of the column first.

The whyAs the flask warms, the liquid with the lower boiling point evaporates more readily, so its vapour is the first to pass over the top of the column and into the condenser. Most of the water vapour condenses in the column and runs back into the flask.

physical

HintThe opposite kind of process would be a reaction.

The whyThe substances in a mixture are not chemically joined, so no reaction is needed to part them. Each method uses a difference in a property such as particle size, solubility or boiling point. A compound is different: its elements can only be separated by chemical reactions.

Simple distillation

HintOne of these methods lets the solvent escape into the room; the other catches it.

The whyBoth methods separate a dissolved solid from its solvent, but they keep different parts. Crystallisation lets the water evaporate away and keeps the solid. Simple distillation boils the water off, then cools the vapour in a condenser so that the water itself is collected.

It keeps its own chemical properties

HintCompare with water, where the oxygen is combined with hydrogen and nothing will burn in it.

The whyIn a mixture each substance is still itself, so it reacts just as it would alone; mixing only dilutes it. In a compound the elements are chemically joined, and the compound has new properties of its own.

Liquid

HintPlace 25 °C on a number line with the two change points marked on it.

The whyBelow its melting point a substance is solid, above its boiling point it is a gas, and in between it is a liquid. 25 °C lies between −7 °C and 59 °C. (These are the figures for bromine.)

They are much stronger in A

HintAsk which one needs more energy before its particles can break out of their fixed positions.

The whyThe stronger the forces between the particles, the more energy is needed to separate them, so the higher the melting and boiling points. How strong the forces are depends on the type of bonding and structure. (A is sodium chloride; B is methane.)

These are bulk properties of many atoms together, not of one atom

HintThink about what you would need a large crowd of before such features could show up at all.

The whyColour, shine, hardness, melting point and conductivity all come from the way huge numbers of atoms are arranged and held together. One atom on its own has none of them.

forces

HintIt is whatever was holding the particles close to one another.

The whyHeating gives the particles more energy. At the boiling point this energy goes into separating the particles from one another, which is why substances with strong attractions between their particles need more energy and boil at higher temperatures.

Every particle is shown as a sphere, and the spheres are solid

HintBoth are about how each one is drawn: its outline, and what it is like inside.

The whyReal particles are atoms, ions or molecules. Many are not round, and none is a solid ball. Leaving these details out keeps the model simple, but it limits what the model can explain.

It has no forces between the particles

HintAsk what decides how much energy it takes to pull one sphere away from its neighbours.

The whyMelting and boiling points depend on how strongly the particles attract each other. A model in which the spheres do not attract at all cannot say why one substance needs more energy to change state than another.