Mixtures and components

What makes something a mixture. A mixture is two or more substances combined physically, in any proportion, with no chemical reaction between them. The substances that go into a mixture are its components. Because nothing chemically new is created, every component keeps behaving exactly as it did on its own — iron filings stay magnetic, sugar stays sweet, oxygen in air still supports burning.

Why "no fixed ratio" matters. Unlike a compound, which always combines its elements in one fixed mass ratio (water is always $\text{H}_2\text{O}$, never a little more or less oxygen), a mixture has no required proportion. You can make lemonade weak or strong, salty or sweet, and it is still a mixture of the same components. This is a genuinely testable difference, not just a definition to memorise: if changing the amount of one ingredient still leaves you with "the same kind of substance," you are looking at a mixture; if it does not, you may be looking at a compound.

The properties test. The single most reliable way to decide "mixture or not" is to ask: after combining these substances, does each one still behave the way it did before? If salt still tastes salty inside a soup, if sand still feels gritty inside a sandcastle, if iron filings are still pulled by a magnet after being stirred into sulfur powder — you are looking at a mixture. The moment a substance stops behaving like itself and something with genuinely new properties appears (like a new colour, a new melting point, a gas being released that was not there before), a chemical reaction has likely occurred instead, and the result is not simply a mixture of the starting materials.

Component count and blending. A mixture can have as few as two components or many — air, for instance, is a mixture of nitrogen, oxygen, carbon dioxide, water vapour and more. Components do not even need to look mixed: sugar dissolved in water looks perfectly uniform to the eye, yet it is every bit as much a mixture as visibly jumbled iron filings and sand. What matters is not appearance but whether each component has kept its own chemical identity, which is why the properties test above is more reliable than a visual check.

where: the double arrow reads "if and only if" — this is a two-way test. If a combination satisfies it, it is a mixture; if either condition fails (a new substance appears, or a component's original properties vanish), it is not.

The properties test works because it directly checks the physical definition of a mixture: combination without chemical reaction. When two substances are simply placed together — stirred, shaken, dissolved, or ground — their atoms and molecules are not rearranged into new chemical bonds. Iron atoms stay iron atoms, sulfur atoms stay sulfur atoms, so every physical property that depends on that identity (magnetism, colour, melting point, taste) survives unchanged. This is also exactly why mixtures can usually be pulled apart again using ordinary physical methods, such as a magnet, filtration, or evaporation, without needing any chemical process to undo what was done. A chemical reaction, by contrast, breaks and re-forms bonds, producing a substance whose atoms are arranged completely differently from either starting material. Heat iron filings and sulfur powder strongly enough and they combine chemically into iron sulfide ($\text{FeS}$), a new compound with its own melting point, its own grey-black colour, and — crucially — no magnetism at all. A magnet passed over iron sulfide picks up nothing, because there is no longer any free iron to attract; the iron atoms are now chemically bonded to sulfur. This is the sharpest possible illustration of the mixture-versus-reaction boundary: identical starting substances, but a mixture at room temperature and a genuinely new compound once enough heat drives a reaction. The "any proportion" rule follows the same logic. Since no fixed chemical bond is formed between the components of a mixture, there is no fixed ratio they must obey — you could mix a little iron with a lot of sulfur, or the reverse, and both are still valid mixtures of iron and sulfur. A compound like iron sulfide, however, always has iron and sulfur combined in one specific ratio by mass, because that ratio is fixed by how the atoms actually bond together.

Question: A student mixes iron filings with sulfur powder at room temperature (without heating) and later, as a separate trial, heats a similar mixture strongly until it glows. Compare the two results and state which one is a mixture, giving the components.

Step 1. Examine the cold, unheated sample. You can still see grey iron filings and yellow sulfur powder lying side by side — the two substances have not blended into one uniform material, and each still looks like itself.

Step 2. Test the cold sample with a magnet. The iron filings are attracted and pull out cleanly, leaving the sulfur powder behind. This is a purely physical separation, requiring no chemical process.

Step 3. Examine the heated sample. Once heated strongly enough, the mixture glows and, on cooling, forms a dull grey-black solid, iron sulfide ($\text{FeS}$), that looks nothing like either starting material.

Step 4. Test the heated sample with a magnet. Nothing is attracted — the iron has chemically combined with the sulfur and lost its individual magnetic property, so no physical method can separate the two back out.

Step 5. Compare and conclude. The cold sample shows both original properties intact and separates with a simple physical method, so it is a mixture of two components, iron and sulfur. The heated sample has lost those original properties and cannot be separated physically, so it is a compound (iron sulfide), not a mixture — heating caused a chemical reaction. Answer: only the cold, unheated combination of iron filings and sulfur powder is a mixture, with iron and sulfur as its two components; the heated sample is a new compound, iron sulfide.

• Thinking a mixture must look visually jumbled — sugar dissolved in water looks completely uniform, yet it is still a mixture, not a compound, because sugar and water each keep their own properties.
• Confusing mixing with a chemical reaction — stirring or heating two substances together is not automatically a chemical change; always check whether new properties (or a new substance) appear.
• Assuming a mixture must have exactly two components — a mixture can have two, three, or many, such as air with its several gases.
• Forgetting that a mixture's components can be combined in any proportion, while a compound like iron sulfide always has a fixed mass ratio of its elements.

Ask the tutor: want to walk through more examples of separating everyday mixtures, or see how this idea connects to elements and compounds later in the chapter?