Compounds

1. Fixed ratio, chemical combination. A compound is not "some hydrogen plus some oxygen" — it is exactly two parts hydrogen to one part oxygen by atom count, every single time, anywhere on Earth. The electrolysis-of-water activity makes this visible directly: pass electricity through water and two gas jars fill up, one with hydrogen and one with oxygen, and the hydrogen jar always ends up with double the volume of the oxygen jar. That fixed 2:1 relationship, stated in words rather than chemical formulae at this level, is written as

where: the ratio is a count of atoms, not a count of grams or a count of test tubes of gas — it is fixed by how the elements bond, not by how much water you happen to make.

2. Sodium chloride's ratio. Common salt is built the same way, from sodium and chlorine combined 1:1. That single number is enough to answer most syllabus questions about salt's composition, and it is worth memorising alongside water's 2:1 the way you'd memorise a formula.

3. New properties, not blended properties. A compound's properties are not an average of its elements' properties — they are usually completely unlike either parent element. Sodium alone is a soft, dangerously reactive metal that bursts into flame in water; chlorine alone is a choking, toxic gas. Combined chemically as sodium chloride, the result is safe, stable table salt. This "new substance" behaviour is one of the two tests (along with fixed ratio) that mark something as a compound rather than a mixture.

4. Only chemical processes can separate a compound. Because the elements inside a compound are chemically bonded, no physical method — evaporation, filtration, dissolving, sieving, magnetism — can pull them apart. Undoing the bond needs a chemical change: passing electricity through water (electrolysis) splits it back into hydrogen and oxygen; strongly heating sugar chars it, decomposing it into carbon (left as a black residue) and water vapour (seen as droplets on the cooler glass). Both are chemical, not physical, changes — which is exactly why they succeed where stirring or heating gently would not.

This topic has no numeric formula to derive — the underlying reasoning is why any physical method fails to split a compound apart, and it is worth making airtight, because it is the single most tested idea in this chapter and the one most often confused with the very similar-sounding rules for mixtures.

Step 1. Start from what a physical process can actually do. Evaporation, filtration, dissolving, and using a magnet all work by exploiting physical differences between substances that are simply mixed together — different boiling points, different particle sizes, different solubilities, different magnetic behaviour. None of these processes changes what a substance chemically is; they only sort substances that were never chemically joined in the first place.

Step 2. Now look at what holds a compound together. In a compound, the constituent elements are joined by a chemical bond — an interaction between the elements themselves, not merely a physical arrangement of separate particles sitting side by side. Sodium and chlorine inside salt are not "sodium particles next to chlorine particles" the way sand grains sit next to salt grains in a dry mixture; they are combined at the level of the substance's identity, in the fixed 1:1 ratio, forming one new material.

Step 3. Because a physical process only ever exploits physical differences, and a chemical bond is not a physical difference in that sense, no physical process has any "handle" to grab onto to separate the sodium from the chlorine. Evaporating salt water removes water from around the salt — it never approaches the salt's own internal sodium-chlorine bond. This is why the sugar-heating and water-electrolysis activities in the chapter both use heat or electricity strongly enough to drive a chemical reaction, not just a physical change — decomposition (breaking a compound down) always requires a chemical process, which is precisely the boundary that separates compounds from mixtures.

Step 4. The same logic runs in reverse for formation: compounds form only through a chemical change (like the reaction of hydrogen and oxygen, or the combination of iron and sulfur on heating), never by simply stirring or mixing elements together at room temperature. Formation and separation of a compound are both chemical events; formation and separation of a mixture are both physical events. Keeping that formation/separation pairing straight is what this whole section is really testing.

Question. A jar contains salty water. (a) Explain what physical process can recover the salt from the water. (b) Explain why that same process cannot separate the sodium from the chlorine within the salt itself.

Step 1. Identify what kind of pure substance is present at each level. Salty water is a mixture: salt has simply been dissolved in water, and each substance — salt and water — keeps its own separate identity even while mixed. Salt itself, however, is a compound: sodium and chlorine combined chemically in a fixed 1:1 ratio.

Step 2. Answer part (a). Because salty water is a mixture, a physical process is enough to separate it: heating the salty water drives off the water as vapour (a physical change of state), leaving solid salt crystals behind in the container. This is evaporation, and it works because water and salt were never chemically bonded to each other — they were only mixed.

Step 3. Answer part (b). Evaporation only removes the water that surrounds the salt; it does nothing to the salt's own internal structure. Inside the salt, sodium and chlorine are chemically combined, not just physically mixed, so separating them needs a chemical process (in fact, molten salt has to be electrolysed to recover sodium and chlorine separately — well beyond this chapter, but it confirms the point). No physical method — not evaporation, not filtration, not repeated dissolving and drying — can break that chemical combination.

Step 4. State the conclusion clearly, distinguishing the two levels. Evaporation successfully separates the mixture (salt from water) because mixing is a physical process reversed by a physical process. Evaporation cannot touch the compound (sodium from chlorine) because chemical combination can only be reversed by a chemical process. Salt is recovered from salty water by evaporation, but sodium and chlorine cannot be separated from each other within the salt by any physical method.

Competitive stretch

The same two-level reasoning applies to sugar. Dissolve sugar in water and you can recover it by evaporation (mixture, physical separation). But heat solid sugar strongly and dry, and it chars into carbon plus water vapour — evidence that sugar itself is a compound of carbon, hydrogen and oxygen, breakable only by the chemical process of decomposition under heat, never by dissolving or filtering.

• Thinking a compound's properties are "somewhere in between" its elements' properties — they are usually completely different: sodium (reactive metal) and chlorine (toxic gas) give ordinary, safe table salt.
• Believing that because salt dissolves easily in water, the sodium and chlorine inside it must also separate easily — dissolving only reverses the salt-plus-water mixture; it never touches the sodium-chlorine chemical bond.
• Confusing a compound's fixed ratio with a mixture's variable proportions — water is always 2:1 hydrogen to oxygen by atom count, but a mixture like salty water can be made weak or strong, with any amount of salt in any amount of water.
• Trying to use chemical symbols or formulae for these compounds — this chapter states ratios in words only (for example, "two hydrogen atoms to one oxygen atom"); symbolic formula-writing belongs to Class 9 and beyond.

Ask the tutor: Want to see how the iron-and-sulfur heating activity proves the same mixture-versus-compound distinction from the opposite direction — starting from two elements and ending at one compound?