Layers, vapours and alloys

Oil and water do not mix: they are immiscible, and they form two layers. To separate them, use a separating funnel. Pour in 5 mL of mustard oil and 20 mL of water and let them settle. The oil is less dense, so it floats on top.

• Open the tap (stopcock) at the bottom and let the water run out.
• Throw away the small part where the two meet.
• Then collect the oil.

Gases mix easily and evenly, so a mixture of gases is usually homogeneous: for example, hydrogen and oxygen in rocket fuel. But tiny bits floating in air make heterogeneous mixtures:

• smoke: solid particles in air;
• fog: water droplets in air;
• dust in air.

Sublimation: some solids turn straight into vapour when heated, without first melting into a liquid. The reverse, vapour turning straight back into solid, is deposition.

Separating camphor from sand: heat the mixture under an upside-down funnel. The camphor sublimes, and its vapour deposits as solid on the cool funnel. The sand stays behind. Naphthalene (mothballs) and dry ice (solid carbon dioxide, used to keep ice cream frozen) also sublime.

Alloys. Melted metals can mix into a solution that hardens into an alloy: a homogeneous mixture of metals (or a metal and a non-metal). Alloys are made to be stronger or to resist rust. You cannot separate them by physical methods.

• Brass: about 80% copper, 20% zinc.
• Bronze: about 80% copper, 20% tin.
• Stainless steel: iron, with carbon, chromium (16 to 18%), nickel and molybdenum.