Difference between mixtures and compounds

Two ways to put substances together

Substances can come together in two very different ways. They can simply be mixed, like sugar stirred into water or sand into gravel, with no new substance formed. Or they can react, joining chemically into something new, as hydrogen and oxygen do in water. The first gives a mixture, the second a compound. They can look alike, since both may seem uniform, so we need tests to tell them apart.

Five differences

A mixture forms by mixing alone, while a compound forms by a chemical reaction. A mixture can contain its parts in any proportion: a little or a lot of sugar in water. A compound always has the same fixed ratio: water is always two hydrogen atoms to one oxygen atom. In a mixture each part keeps its properties, but a compound has new ones. Mixtures can be separated by physical methods such as a magnet, filtering or evaporation; compounds cannot. Air, seawater and brass are mixtures; water, common salt and sugar are compounds.

The iron and sulfur test

Activity 8.5 shows the difference with two familiar substances. About 5.6 g of grey-black iron filings and 3.2 g of yellow sulfur powder are mixed well on a watch glass: this is Sample A. Half of it is heated gently in a china dish, with stirring, until it forms a black mass, which is cooled and ground: this is Sample B. Because hot sulfur gives off fumes, the teacher shows this step where the fumes can escape safely, under a hood or beside open windows. The two samples are then compared step by step.

Reading the results

In Sample A, black and yellow specks are both visible, so it is a non-uniform mixture, and a magnet pulls the iron filings out. With a few drops of dilute hydrochloric acid, the iron gives hydrogen, a colourless gas with no smell that burns with a pop, while the sulfur stays as a yellow solid at the bottom: iron + dilute hydrochloric acid → iron chloride + hydrogen. Sample B is black throughout, and the magnet has no effect on it. With dilute hydrochloric acid, iron sulfide gives iron chloride and hydrogen sulfide, a gas that smells like rotten eggs. Heating made a new compound: iron + sulfur → iron sulfide. Smell such gases only by wafting them gently towards your nose, never directly. The two different gases are the clearest sign that Sample B is no longer iron and sulfur side by side, but a single new substance.

Mixtures and compounds around us

The same oxygen can be in a mixture or a compound. In air it is mixed with nitrogen and keeps its properties; in water it is joined to hydrogen, and the compound puts out fire instead of feeding it. If water were only a mixture of hydrogen and oxygen, it would burn and explode instead of quenching thirst and fires. Alloys like brass and bronze are mixtures; rust and iron sulfide are compounds. See mixtures and pure substances, elements and compounds, the Chapter 8 notes and the NCERT solutions.