Iron Filings and Sulphur Can Be Two Entirely Different Things

Yes — and it is the clearest experiment in the whole chapter.

Stir iron filings and sulphur powder together and you get a speckled grey-and-yellow powder. Draw a magnet across it and the iron leaps out, leaving the sulphur behind. Nothing has really happened to either substance: that is a mixture.

Now heat the same two powders together. They glow, give out heat, and cool into a black solid. A magnet does nothing at all to it, and no amount of stirring or sieving will get the iron back. That is a compound — iron sulphide.

Same two elements, two completely different results. This page covers the second part of the ICSE Class 8 Chemistry chapter on elements, compounds and mixtures: what a compound really is, the two kinds of mixture, and how to tell them apart.

Because in a compound the elements are chemically combined, so the original substances no longer exist as themselves — only the new one does.

A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio by mass, with the formation of a new substance whose properties are entirely its own.

The iron sulphide comparison, point by point:

- Iron is grey, lustrous, magnetic, and dissolves in dilute acid giving hydrogen gas.
- Sulphur is yellow, brittle, non-magnetic, and insoluble in water.
- Iron sulphide is black, non-magnetic, and gives a foul-smelling gas with dilute acid — a gas neither element produces.

Not one of the compound's properties was predictable from its ingredients.

The fixed ratio, calculated. Iron sulphide is $\text{FeS}$. Taking iron as $56$ and sulphur as $32$:

$$\text{ratio by mass}=56:32=7:4$$

So $88\text{g}$ of iron sulphide always contains $56\text{g}$ of iron and $32\text{g}$ of sulphur. Supply extra sulphur and it simply stays behind unreacted — the compound will not take more than its share.

A mixture obeys no such rule. Stir in twice as much sulphur and you have a sulphur-rich mixture, which is still a perfectly good mixture.

More examples of properties changing completely:

- Water is a liquid that puts out fires. Hydrogen burns explosively and oxygen supports burning.
- Common salt is a safe, edible solid. Sodium is a reactive metal and chlorine a poisonous gas.
- Sugar is sweet and white. Carbon is black, and hydrogen and oxygen are colourless gases.

And the energy tells you a compound formed. Making iron sulphide gives out heat and light; making the mixture gives out nothing. An energy change during formation is one of the surest signs that the elements have chemically combined rather than merely mingled.

A homogeneous mixture is uniform throughout — every part looks and behaves identically, and the constituents cannot be told apart even under a microscope. A heterogeneous mixture is non-uniform, with parts that can be distinguished.

Homogeneous mixtures — salt dissolved in water, sugar solution, air, brass (copper and zinc), bronze, steel, vinegar in water, soda water.

Heterogeneous mixtures — sand in water, oil in water, soil, muddy water, chalk powder in water, a mixture of iron filings and sulphur, a mixture of sand and salt, smoke in air.

Applying it to the four the syllabus names:

- Air — homogeneous. Nitrogen, oxygen, carbon dioxide and water vapour are evenly blended and cannot be distinguished by eye. It is still a mixture, though, because the proportions vary from place to place and the gases can be separated physically.
- Brass — homogeneous. Copper and zinc are fused into a uniform alloy with no visible patches of either.
- Sand in water — heterogeneous. The sand is plainly visible and settles to the bottom on standing.
- Oil in water — heterogeneous. Two distinct layers form, with the oil floating on top.

The useful test. Let the sample stand undisturbed, then look for a boundary — a layer, a sediment, or visible specks. A homogeneous mixture never separates on standing; a heterogeneous one usually does.

Why homogeneous mixtures are the confusing ones. A salt solution looks exactly like a pure liquid. It is nonetheless a mixture, because you can make it weaker or stronger at will and recover the salt by simple evaporation. Uniform appearance proves nothing about purity — only fixed composition does.

The link back to compounds. Every compound is homogeneous by necessity, since it is one kind of particle throughout. But a homogeneous substance need not be a compound, as air and brass show. That one-way relationship is exactly what the next section makes precise.

Five, and each gives you a practical test you could actually carry out.

1. Composition. A compound has a fixed ratio by mass — iron sulphide is always $7:4$, water always $1:8$. A mixture has any ratio the maker chose.

2. Properties of the constituents. In a compound the constituents lose their own properties entirely, and the compound has new ones. In a mixture each constituent keeps its properties — the iron in the mixture is still magnetic, still grey, still reacts with acid the same way.

3. Method of separation. A compound can be split only by chemical means — heating to decompose it, electrolysis, or reaction with something else. A mixture separates by physical means:

- A magnet — iron filings from sulphur
- Filtration — sand from water
- Evaporation — salt from salt water
- Distillation — alcohol from water
- Decanting — oil from water, or clear water from settled mud
- Sublimation — camphor or ammonium chloride from salt
- Winnowing and sieving — grain from husk, flour from bran

4. Energy change on formation. Forming a compound absorbs or releases energy as heat or light — the iron and sulphur glow. Making a mixture involves little or no energy change.

5. Melting and boiling point. A compound has a sharp, fixed melting and boiling point — pure water boils at $100{}^{\circ }\text{C}$. A mixture melts or boils over a range, and the value shifts with composition, which is why salt water boils above $100{}^{\circ }\text{C}$ and by an amount depending on how much salt is present.

A sixth point: nature. A compound is always homogeneous; a mixture may be either.

How to use this in a question. You rarely need all five. One decisive test is enough, and the fastest are usually separation and properties: if a magnet, a filter paper or an evaporating dish gets the original substances back, it was a mixture.

The misconception to clear once and for all. A compound is a mixture of elements is wrong, and the words matter. In a mixture the substances are together; in a compound they are combined. Iron filings and sulphur sitting side by side are together; iron sulphide is combined, and there is no iron left in it to find.