Decomposition of compounds
A compound cannot be pulled apart into its elements by any physical trick — no filtering, dissolving, or evaporating will do it. But a compound can be broken down by a chemical process, and when it is, the substances that come out are things that were never separately visible inside it. This is decomposition, and it is one of the cleanest ways chemists prove that something is a compound rather than an element or a mixture. This article builds up decomposition using the two activities every Class 8 student meets first: passing electric current through acidified water, and heating sugar strongly in a test tube. Both look, on the surface, like "separating things out" — the same way evaporating salty water separates salt from water. But one is decomposition and the other is not, and the difference between them is the real idea worth mastering here.
A compound decomposes into simpler substances only under a chemical trigger — electric current splits water into hydrogen and oxygen gas in a fixed 2 to 1 volume ratio, and heat chars sugar into carbon plus water vapour — and this chemical (not physical) breakdown is exactly what proves a substance was a compound.
The core idea
Compounds resist physical separation. Salt dissolved in water can be recovered by evaporation because dissolving was only mixing — no new chemical bonds were made or broken. A compound is different: its elements are chemically joined, and no amount of filtering, sieving, or evaporating will pull them apart. Splitting a compound needs an external chemical push.
Two decompositions, two different triggers. Electrolysis of water uses electric current as the trigger. Passing current through acidified water breaks it apart at two electrodes:
where: the reaction proceeds only while current flows — switch it off and the decomposition stops, unlike a physical process that can run on its own once started (like evaporation continuing in open air). Heating sugar uses heat as the trigger. Strongly heated sugar chars, releases water vapour that condenses on the cooler walls of the test tube, and leaves behind a black solid — carbon (charcoal). Neither electrolysis nor heating of sugar can be reversed by simply mixing the products back together; new substances have been created that did not exist as free substances before.
A ratio worth remembering. In the electrolysis of water, hydrogen gas collects in exactly double the volume of oxygen gas at the two electrodes. This 2:1 volume ratio is not a coincidence — it mirrors the 2:1 ratio of hydrogen to oxygen atoms in a water molecule, so the volumes of gas collected are direct experimental evidence for water's composition.
Why it works
Decomposition is a purely descriptive idea in this chapter, so instead of a formula, the goal here is to reason carefully about why a chemical process — and only a chemical process — can break a compound apart, and why that breakdown is proof of the compound's identity: a substance qualifies as a compound precisely because it can be decomposed by a chemical process into two or more simpler substances that behave completely differently from the original and from each other, and it fails to qualify as a compound (being instead an element, which cannot be decomposed further, or a mixture, whose components can be recovered by physical means alone) whenever no such chemical breakdown is possible.
Step 1. Start with the defining test of a compound: its constituent elements are chemically bonded together, not merely mixed. Physical processes — filtering, sieving, dissolving, evaporating, using a magnet — only separate things that were never chemically joined, so none of them can pull a compound's elements apart.
Step 2. A chemical process supplies energy (electrical, in electrolysis; thermal, in heating sugar) that is large enough to break the chemical bonds holding the compound's atoms together. Once those bonds break, the atoms regroup into new, simpler substances — hydrogen and oxygen gas from water; carbon, water vapour, and other volatile products from sugar.
Step 3. These new substances are genuinely different from the original compound in every property. Water is a liquid that puts out fires; hydrogen is a flammable gas and oxygen supports burning. Sugar is a sweet, white, water-soluble solid; carbon is a black, tasteless, insoluble solid. This sharp change in properties is the signature of a chemical process, distinguishing decomposition from any physical separation, where the recovered substances look exactly as they did before mixing.
Step 4. Because these new substances did not exist as free, separate substances inside the original compound — they only existed as chemically combined atoms — recovering them by decomposition demonstrates that the original was indeed a compound of those elements, not an element itself (which cannot be decomposed further) and not a mixture (whose parts could have been recovered physically, without any chemical change).
Worked example
Question: In the electrolysis-of-water apparatus, 30 cm³ of gas collects at one electrode while 60 cm³ collects at the other, after which a burning candle is tested at each. Explain which gas is which, and how the results confirm water is a compound.
Step 1. Note the volumes: one electrode has collected 60 cm³ of gas, the other 30 cm³ — a ratio of 60:30, which simplifies to 2:1.
Step 2. Recall the identification tests: the gas that produces a "pop" sound with a burning candle is hydrogen; the gas that makes the candle's flame glow brighter is oxygen.
Step 3. Match the volumes to the known ratio. Since hydrogen is known to be produced in double the volume of oxygen during electrolysis of water, the 60 cm³ sample is hydrogen (it should give the "pop" test) and the 30 cm³ sample is oxygen (it should make the flame glow brighter).
Step 4. Because acidified water, subjected to a chemical process (electric current), has broken down into two elements — hydrogen and oxygen — in a fixed 2:1 volume ratio, this confirms water is a compound of hydrogen and oxygen, and that the ratio of hydrogen to oxygen atoms in a water molecule is 2:1.
Competitive stretch
If a student instead collected 45 cm³ at one electrode and 90 cm³ at the other, the same reasoning applies unchanged: the 90 cm³ sample must be hydrogen and the 45 cm³ sample must be oxygen, because the 2:1 ratio is fixed by water's composition and does not depend on the total volume of gas collected.
Common traps
- Thinking dissolving, filtering, or evaporating can decompose a compound — only a chemical process (heat, electric current, or a chemical reaction) can break the bonds inside a compound.
- Swapping the two candle tests — hydrogen gives the "pop" sound; oxygen makes a flame glow brighter. These are opposite results and easy to confuse under exam pressure.
- Believing the black residue from heated sugar is a brand-new element "created" from nothing — decomposition only reveals carbon, hydrogen, and oxygen that were already chemically present inside the sugar.
- Forgetting that the 2:1 volume ratio of hydrogen to oxygen gas is the direct experimental evidence for the 2:1 atom ratio in a water molecule — these two numbers are the same fact seen two ways.
Ask the tutor: Want to see how this compares with the iron-and-sulfur activity, where mixing and heating give very different results?
If evaporating salty water and electrolysing water both end with substances I did not see separately before, why is only one called decomposition?
Evaporation only reverses a physical mixing step. The salt and water were never chemically joined — no bonds were made when they mixed, so none need to be broken to separate them, and each substance is exactly what it was before. Electrolysis breaks real chemical bonds inside the water molecule, producing hydrogen and oxygen that did not exist as free elements inside the liquid. That bond-breaking, driven by a chemical process, is what makes electrolysis a decomposition and evaporation merely a separation.
Does the 2:1 volume ratio of hydrogen to oxygen always come out exactly right in a real classroom experiment?
In a carefully run electrolysis, the volumes collected are very close to 2:1, and this ratio is what the chapter expects you to know and reason with. In practice, small experimental factors (a little oxygen dissolving in the water, unequal electrode surfaces) can make the observed ratio slightly less than exactly 2:1, but the underlying chemistry — and the ratio you should quote in an answer — is always 2:1, matching the 2:1 atom ratio in water's formula.
Is charring sugar the same kind of decomposition as electrolysing water, just using heat instead of electricity?
Yes, in the sense that matters for this chapter: both need a chemical process to proceed, both produce substances with entirely different properties from the starting compound, and both cannot be reversed by simply mixing the products back together. The only difference is the trigger — electric current for water, strong heat for sugar — and the products differ because the compounds themselves differ (water yields two gaseous elements; sugar yields carbon and water vapour, since sugar itself is a compound of carbon, hydrogen, and oxygen).