Odour and Other Clues

Introduction

Your senses often notice a chemical change before any instrument does. Milk that has gone off smells sour. Toast that is burning gives a sharp, acrid smell. A firework bangs. Bread dough turns into a springy loaf. The main signs of reaction are colour change, gas, precipitate and energy change, but smell, sound and texture can also help. This page explores these extra clues and why they must be used with care.

Core explanation

New odours. You smell a substance when some of its molecules escape into the air and reach sensory cells in your nose. If a reaction produces a new volatile substance, a new smell appears. Examples include:

- Sour milk: bacteria convert milk sugar (lactose) into lactic acid, giving a sour smell and taste. - Rotting eggs: proteins break down to release hydrogen sulfide, which has a strong "bad egg" smell. - Fruity smells in chemistry: reacting an alcohol with an acid can produce esters, which smell of fruits such as pear or banana. - Cooking: browning meat, toast and onions produces hundreds of new flavour and aroma molecules.

Sounds. Some reactions are noisy. A lighted splint gives a squeaky pop in hydrogen, because hydrogen and oxygen react very rapidly. Fireworks bang because a fast reaction produces hot gases that expand suddenly. Sizzling can accompany vigorous fizzing. However, sounds are usually caused by gases expanding or moving, and that can happen in physical changes too, such as a kettle whistling or a balloon bursting.

Texture and appearance. When a cake bakes, runny batter becomes a firm, spongy solid. When an egg is cooked, the clear, runny white becomes opaque and rubbery as the proteins change permanently. When iron rusts, a smooth, hard metal becomes a flaky, crumbly brown solid. A change in texture that cannot be reversed often points to new substances.

The limits of these clues. Smell, sound and texture are weaker evidence than a gas test or a precipitate, for several reasons:

Clue Why it can mislead --- --- New smell Warming a substance can make it smell stronger without changing it; smell is subjective, and people differ in what they can detect Sound Physical changes such as boiling, bursting or cracking also make noise Texture Melting chocolate or wet sand change texture without any new substance forming

Safety. Many gases made in reactions are toxic or irritating. In the laboratory, never put your nose directly over a container. If a teacher asks you to note a smell, waft the air gently towards you with your hand, and only when told it is safe. Work in a well-ventilated room or fume cupboard.

Step-by-step reasoning

To judge a smell, sound or texture change:

1. Note exactly what you observe, for example "a sharp vinegar-like smell". 2. Ask whether a physical change, such as warming, melting or mixing, could explain it. 3. Ask whether the change is permanent. 4. Look for stronger evidence, such as a gas test, precipitate or colour change. 5. Only then decide whether a new substance has formed.

Visual explanation

Picture a closed box of fresh milk and a box of sour milk, each with small arrows showing molecules drifting out. From the fresh milk, only a few mild-smelling molecules escape. From the sour milk, new lactic acid molecules, drawn in a different colour, spread through the air towards a nose.

Real-world analogy

Using smell alone to detect a reaction is like identifying a visitor by footsteps in the corridor. Sometimes you are right, but many people walk the same way. Seeing the visitor's face, like doing a gas test, gives much more reliable evidence.

Real-world example

Natural gas (mainly methane) has no smell. Gas companies deliberately add tiny amounts of strong-smelling sulfur compounds called thiols, so that a leak can be detected by its "rotten cabbage" odour. Here, smell is used as a warning, not as evidence of a reaction.

Why?

Why are some reaction products easy to smell while others are not? A substance can only be smelled if enough of its molecules escape into the air. Small, light molecules with weak attractions between them evaporate easily and reach the nose. Salts and most metals have almost no smell because their particles are held strongly together.

Common misconception

"If I can smell it more strongly, it must have reacted." Warming coffee or perfume makes it smell stronger simply because more molecules evaporate. The substance is unchanged; only the rate of evaporation has increased.

Worked example

Question: A student heats sugar in a pan. It melts, turns brown, smells of caramel, and on cooling forms a hard, brittle solid that tastes different from sugar. Is this a physical or chemical change?

Reasoning: Melting alone would be physical, and the solid would taste like sugar again when cooled. Here there is a colour change, a new smell and a new taste and texture that remain after cooling. Several clues point together to new substances.

Answer: A chemical change (caramelisation), supported by several pieces of evidence, not just the smell.

Quick check

1. Why should you waft a smell towards you instead of sniffing a test tube directly? Answer: The gas could be toxic or irritating, and wafting reduces the amount you breathe in.

Exam focus

If a question asks for evidence of a chemical change, the strongest answers are specific and permanent: "a new sour smell that remains", "a white precipitate formed". Examiners also like to test whether you can explain why a single clue, such as a smell or a sound, is not enough on its own.

Advanced insight

The human nose can detect some compounds at extraordinarily low levels. Hydrogen sulfide can be smelled at concentrations below one part per million in air. Dangerously, at high concentrations it paralyses the sense of smell, so a person may stop noticing it just when the risk is greatest. This is one reason chemists rely on instruments, not noses.

Summary

New smells, sounds and changes in texture can suggest a chemical change, as in sour milk, popping hydrogen, baking and rusting. They are weaker evidence than gas tests, precipitates or colour changes, because physical changes can also produce them and smell is subjective. Always waft smells safely and look for supporting evidence before concluding that a reaction has happened.

Practice questions

1. Explain why sour milk smells different from fresh milk. Answer: Bacteria convert lactose into lactic acid, a new substance with a sour smell. 2. Give an example of a physical change that makes a sound. Answer: Water boiling in a kettle, or a balloon bursting; no new substance is formed. 3. Why is a change in texture alone not proof of a chemical change? Give an example. Answer: Physical changes can also change texture; for example, chocolate becomes runny when it melts but is still chocolate. 4. Why is a smelly gas added to natural gas supplies? Answer: Methane has no smell, so the added compound lets people detect a leak.