Elements, Compounds and Mixtures: Eight Substances Sorted

Three questions that settle almost every case

Rather than memorising definitions, work through the substance with three questions in order. How many kinds of atom are there? Are they chemically bonded? Could you separate them without a chemical reaction?

One kind of atom and you have an element, whatever shape it takes. Two or more kinds, bonded, in a fixed ratio, and you have a compound. Two or more substances not bonded, in whatever ratio happens to be there, and you have a mixture.

There is a fourth question that decides borderline cases in the laboratory, and it is examinable in its own right. A pure substance — an element or a compound — melts and boils at a single sharp temperature. A mixture melts and boils over a range. If a sample softens gradually across several degrees rather than turning to liquid at one, it is a mixture, no matter how uniform it looks.

Eight substances, classified with the reasoning

Definitions become useful the moment they are applied, so here are eight ordinary things put in their boxes and the reason given each time.

Notice what the compounds have in common. Sodium is a soft metal that reacts violently with water. Chlorine is a poisonous green gas. Bonded in a fixed one-to-one ratio, they make something you sprinkle on food. Hydrogen burns and oxygen supports burning, and together they put fires out. A compound does not average the properties of its elements — it has its own.

• Oxygen gas, O₂ — element. Two atoms bonded, but both are the same kind of atom, so it is still one element.
• Diamond — element. Carbon only, arranged in a giant covalent lattice. Structure has no bearing on the classification.
• Water, H₂O — compound. A fixed two-to-one ratio, and only electrolysis will separate it.
• Carbon dioxide, CO₂ — compound. One carbon to two oxygens, always.
• Sodium chloride, NaCl — compound. A metal and a non-metal ionically bonded in a fixed ratio.
• Air — mixture. Roughly 78 per cent nitrogen and 21 per cent oxygen by volume, plus argon, carbon dioxide and water vapour, none of it bonded together. Fractional distillation of liquid air separates it.
• Brass — mixture. An alloy of copper and zinc, unbonded, and the proportions vary between grades.
• Seawater — mixture. Water with dissolved salts, separable by distillation or by simple evaporation.

The two that almost every student misplaces

Air is the first. It is invisible, it behaves as one substance, it has no visible layers or lumps, and every one of those observations is irrelevant. The nitrogen molecules and oxygen molecules in a room are not bonded to one another, and the proportions are not fixed — they differ between a sealed classroom and a hillside. Air is a mixture, and the fact that it is separated industrially by cooling it to a liquid and distilling it is proof: distillation is a physical process.

Alloys are the second. Brass, bronze, steel and solder look and behave like single metals, take a polish, and have their own names, so students file them with compounds. But there is no fixed formula for brass and no chemical bond between the copper and the zinc, and the mixture can be made in different proportions to change the properties deliberately. That is exactly why alloys exist.

A third misreading is worth heading off. Oxygen gas is written O₂, and students see two of something and reach for compound. Count the kinds of atom, not the number of atoms. A molecule made of identical atoms is still an element.

Why the classification is worth having at all

The reason this is taught early is that the answer predicts what you can do to the substance. Mixtures come apart with physical methods, and the components survive unchanged: filtration for an insoluble solid in a liquid, evaporation or crystallisation for a dissolved solid, simple distillation to get the solvent back, fractional distillation to separate liquids by boiling point, chromatography to separate dyes, a magnet for iron filings in sand.

None of those will touch a compound. Getting an element out of a compound requires a chemical reaction: electrolysis of molten aluminium oxide to obtain aluminium, reduction with carbon in a blast furnace to obtain iron from its ore. That difference in effort — a filter paper against a furnace — is the practical meaning of a chemical bond, and it is what separation-technique exam questions are really testing.