Separating Immiscible Liquids — and the Story of Alloys

Pour mustard oil into a glass of water and you'll always see the same thing: the oil floats on top, water sits at the bottom. They form two clear layers with a sharp boundary in between.

There are two reasons for this:

1. Different densities. A litre of mustard oil weighs less than a litre of water. The lighter liquid naturally floats on the heavier one.
2. No attraction between the molecules. Water molecules pull strongly on each other; oil molecules don't fit into that pull. Each kind of molecule prefers its own company.

Knowing this, can you think how to drain only the water out of an oil-water mixture without disturbing the oil floating above? The answer is one of the simplest pieces of equipment in any chemistry lab — the separating funnel.

You will need: a 50 mL separating funnel mounted on a lab stand, a glass stopper, a conical flask, 5 mL mustard oil, 20 mL water.

Steps:

1. Pour the 5 mL of mustard oil and 20 mL of water into the separating funnel. Place the glass stopper on top so nothing splashes out.
2. Set the funnel undisturbed on the lab stand. Watch it for a minute.
3. Two clear layers will appear — golden mustard oil on top, clear water below — with a sharp horizontal boundary between them. (Why oil on top? Because it is less dense than water.)
4. Place a clean conical flask under the funnel and slowly open the stopcock at the bottom. The water (the lower layer) will start trickling out.
5. The moment the boundary line reaches the stopcock, close it carefully. You now have water in your flask.
6. There may be a tiny mixed strip near the boundary — let that next small portion drip into a separate beaker and discard it. (Better safe than mixed.)
7. Open the stopcock again and collect the oil layer in a fresh clean container.

Done. Two pure liquids, separated by gravity and a tap.

Most mixtures of gases are homogeneous — gas molecules zoom around so fast and freely that they mix evenly almost the moment they meet. The air you are breathing right now is one such mixture. Even rocket fuel in some space launches is a homogeneous mixture of hydrogen and oxygen gases.

But not every gas mixture is homogeneous. Look around you:

• Smoke — tiny solid carbon particles floating in air. Heterogeneous.
• Fog — tiny liquid water droplets floating in air. Heterogeneous.
• Dust in a sunbeam — solid bits of dirt, fabric and skin floating in air. Heterogeneous.

These are everywhere in our cities. They are also why our skies are not always clear blue.

Here is a strange thought to end this page with: can a solid dissolve into another solid? At room temperature, no — iron and copper sitting next to each other on a table will not dissolve into one another. But melt them at high temperature, mix the molten metals, and let the mixture cool slowly, and something remarkable happens. The result is a solid that looks and behaves like a single new metal — but it is actually a homogeneous mixture of two or more metals.

This kind of solid mixture is called an alloy. You have used alloys all your life without knowing it.

Why bother making alloys instead of using pure metals? Because alloys are usually stronger, harder, or more rust-resistant than the pure metals they are made from. Pure iron rusts; stainless steel doesn't. Pure copper is too soft for a door handle; brass is firm but easy to shape. The exact recipe lets engineers design materials with exactly the properties they need.

One catch: unlike a separating funnel for oil-water, you cannot un-mix an alloy by simple physical methods. The atoms are too well blended. Once an alloy is made, it stays an alloy.