Elements and Compounds

A piece of copper wire, a lump of sulphur, a crystal of table salt — every pure substance you can name belongs to exactly one of two families: elements or compounds. Elements are the simplest pure substances, the alphabet from which everything else is built; compounds are what you get when two or more elements combine chemically in a fixed recipe. This article stays inside the CBSE Class 9 chapter "Is Matter Around Us Pure?" and asks three questions. What actually separates an element from a compound? How are elements themselves sorted into metals, non-metals and metalloids? And why does a compound always contain its elements in the exact same mass ratio, however and wherever it is made — the Law of Constant Proportions that lets you predict one mass from another with nothing more than a ratio.

The core insight

Pure substances are either elements (which cannot be broken into anything simpler) or compounds (two or more elements united in a fixed mass ratio, the Law of Constant Proportions), and elements themselves fall into metals, non-metals and metalloids by their physical behaviour.

The core idea

A pure substance has the same composition and the same properties throughout its bulk — no matter which spoonful you sample. Every pure substance is either an element or a compound; there is no third option.

Elements are the simplest pure substances. No ordinary physical or chemical process can split an element into anything simpler — hydrogen stays hydrogen, iron stays iron. Examples: hydrogen, oxygen, iron, sulphur, carbon.

Compounds form when two or more elements combine chemically in a fixed ratio by mass. Two things distinguish a compound sharply from its parent elements. First, you cannot separate a compound back into its elements by physical methods (filtering, evaporating, sieving) — only a chemical reaction will do it. Second, the compound's properties are usually nothing like the properties of the elements that built it: hydrogen and oxygen are both gases, but water, the compound they form, is a liquid, and it is neither flammable like hydrogen nor a strong oxidiser like oxygen. Elements themselves split into three physical groups:

Metals — lustrous (shiny), malleable (can be beaten into sheets), ductile (can be drawn into wires), good conductors of heat and electricity, usually solid at room temperature. Examples: iron, copper, aluminium, gold.
Non-metals — usually dull (except iodine, graphite), brittle if solid, poor conductors. Examples: sulphur, oxygen, nitrogen, chlorine.
Metalloids — intermediate behaviour, sharing some properties of each side. Examples: boron, silicon, germanium.

The single quantitative rule that governs every compound is the Law of Constant (Definite) Proportions: in any sample of a given compound, its constituent elements are present in a fixed ratio by mass, regardless of the compound's source or method of preparation.

\frac{\text{mass of element A}}{\text{mass of element B}} = k \quad \text{(a fixed constant for that compound)}

where: the masses of element A and element B are measured in any sample of the same pure compound, and $k$ is a constant that depends only on which compound it is — never on how much of it you have or where it came from. For pure water, whether drawn from a river, melted from ice, or synthesised fresh in a laboratory, hydrogen and oxygen are always combined in the mass ratio $1:8$ .

Figure

Two balance-scale panels showing water samples from different sources both splitting into hydrogen and oxygen in the same one to eight mass ratio

Why it works

Water's fixed $1:8$ hydrogen-to-oxygen ratio is not a coincidence of one particular experiment — it follows from the chemistry the CBSE syllabus asks you to trust and apply, and this reasoning generalises to any compound you are handed.

Step 1. A compound is formed when atoms of its elements combine in a fixed, whole-number pattern (for water, always two hydrogen atoms for every one oxygen atom). Because every molecule of the compound has the same fixed atomic recipe, every molecule also contributes the same fixed mass ratio of its elements — the atomic masses of hydrogen and oxygen never change, so two hydrogen atoms always weigh the same fraction of one oxygen atom's mass.

Step 2. Scale that single-molecule ratio up to a bulk sample. Whether a sample contains a few molecules or a river's worth, it is built from the same repeating recipe, so the total mass of hydrogen divided by the total mass of oxygen in the sample equals the same constant $k$ as in a single molecule. Doubling the sample doubles both masses and leaves the ratio unchanged.

Step 3. This is exactly why source and method of preparation do not matter. A river sample, a melted-ice sample, and a laboratory-synthesised sample of water are all built from the identical two-hydrogen-to-one-oxygen recipe at the atomic level, so all three must show the identical $1:8$ mass ratio when decomposed — even though their total masses of water differ wildly. The law is therefore not an empirical accident but a direct consequence of a compound having one fixed atomic composition.

Step 4. This lets you predict one mass from another. If you know the ratio $k$ from one sample and are told the mass of just one element in a second sample of the same compound, the other element's mass follows immediately by multiplying or dividing by $k$ — no new experiment needed.

Worked example

Question: A sample of pure water is decomposed in the laboratory and found to yield 2 g of hydrogen and 16 g of oxygen. A second, independent sample of water — this time collected from a river — is decomposed and yields 3 g of hydrogen. How much oxygen should this second sample yield, if the Law of Constant Proportions holds?

Step 1. Find the mass ratio from the first sample: mass of hydrogen to mass of oxygen $= 2 : 16 = 1 : 8$ .

Step 2. Apply the same ratio to the second sample. Since 1 part by mass of hydrogen always corresponds to 8 parts by mass of oxygen in water, for 3 g of hydrogen:

\text{mass of oxygen} = 3 \times 8 = 24\ \text{g}

Step 3. Interpret the result. The river sample should yield 24 g of oxygen. Both samples, despite completely different origins, give the identical $1:8$ mass ratio — the signature of the Law of Constant Proportions.

Competitive stretch

Step 1. Suppose a third sample of water yields a total mass of 45 g on complete decomposition. Since hydrogen and oxygen are in the ratio $1:8$ , the 45 g splits into $1+8=9$ equal parts, each part $= 45/9 = 5$ g.

Step 2. Hydrogen gets 1 part $= 5$ g, and oxygen gets 8 parts $= 40$ g. Check: $5 + 40 = 45$ g and $5:40 = 1:8$ — consistent with the law.

Common traps

Thinking "pure" means simple-looking or single-coloured. Purity is about fixed composition, not appearance — pure water and pure milk are both liquid, but only water is a pure substance.
Confusing "cannot be separated" with "cannot be broken down." A compound cannot be split into its elements by physical methods, but it can be split by chemical methods; a mixture, by contrast, separates by physical methods alone.
Assuming a metalloid is a mixture of a metal and a non-metal. A metalloid is a single element with its own intermediate properties — never a blend of two different elements.
Believing a compound's properties sit somewhere between its elements' properties. They are usually completely different, as with water versus hydrogen and oxygen — never assume an "average."

Ask the tutor: Want to practice classifying more unknown samples as elements, compounds, metals, non-metals or metalloids, or work through a few more Law of Constant Proportions calculations?

Why does water always have the same 1 to 8 ratio no matter where it comes from?

Because water's atoms always combine in the same fixed pattern — two hydrogen atoms for every one oxygen atom — and the atomic masses of hydrogen and oxygen never change. That fixed atomic recipe forces the same mass ratio in every sample, whether it is river water, melted ice, or water made fresh in a lab. Source and method of preparation cannot change the underlying atomic recipe, so they cannot change the ratio either.

Is a metalloid just a fifty-fifty mix of a metal and a non-metal?

No. A metalloid, such as silicon or boron, is a single element in its own right, not a physical or chemical blend of two other elements. It simply shows intermediate physical behaviour — for example, partially conducting electricity rather than conducting well like a metal or poorly like a non-metal. Do not picture it as "half metal atoms, half non-metal atoms."

If I cannot separate a compound with a filter or by evaporation, does that mean it is an element?

Not necessarily — it means the substance is a pure substance, but it could still be a compound. The test for element versus compound is not "can it be separated by physical methods" but "can it be broken into simpler substances at all." A compound resists physical separation but can still be broken apart by a chemical reaction; an element resists both physical and chemical breakdown into anything simpler.