Odour, Taste and Safe Observation

Introduction

Smell and taste are powerful senses. Many substances can be recognised instantly by their odour — vinegar, perfume, gas leaks — and people once identified chemicals by taste. Modern chemistry treats these senses with great caution. This page explains which observations are safe, why tasting is never allowed in a laboratory, and how a trained chemist notes an odour without harm.

Core explanation

Odour is a physical property. A substance has an odour when some of its particles escape into the air (evaporate) and reach receptors in the nose. Substances that evaporate easily, such as ethanol or ethanoic acid (vinegar), have noticeable odours; many solids, such as salt, have almost none. Odour can help identify substances — ammonia and hydrogen sulfide ("rotten eggs") are unmistakable — but it is a qualitative observation and can be dangerous.

Never taste chemicals. In a laboratory, tasting is never permitted, even for substances that seem harmless. Reasons include: - many chemicals are toxic or corrosive in small amounts; - apparatus and benches may be contaminated with other substances; - two substances that look the same (such as salt and sodium nitrite, or sugar and some poisons) can have very different effects.

Historical chemists who tasted their products sometimes poisoned themselves; modern safety rules exist because of such experiences.

Never sniff directly. Some gases are toxic, some damage the lining of the nose and lungs, and a strong smell can make a person jerk back and spill. To note an odour safely, chemists use wafting : hold the container upright, well away from the face, and gently fan the air above its mouth towards the nose with the other hand. Only a small, diluted amount of vapour reaches the nose.

Fume cupboards. Substances with harmful vapours are handled only in a fume cupboard, which draws air away from the user. Many odours should not be observed at all; the safe choice is to rely on other properties.

Safe observations first. Colour, state, lustre, transparency, density, melting point, solubility and conductivity can all be observed or measured without putting the chemical near the mouth or nose, and they are more reliable for identification.

Step-by-step reasoning

To describe an unknown liquid safely:

1. Wear eye protection and follow the teacher's instructions. 2. Observe colour, clarity and state from a distance. 3. Measure properties such as density or boiling point using suitable apparatus. 4. Only if instructed, note any odour by wafting — never by sniffing directly. 5. Never taste, even a tiny drop.

Visual explanation

A two-panel illustration: on the left, a crossed-out figure leaning over a beaker and sniffing; on the right, a figure holding the beaker at arm's length at chest height and fanning the vapour gently towards the face with an open hand. A small hazard pictogram (exclamation mark) reminds the viewer that vapours can be harmful.

Real-world analogy

Wafting is like testing the temperature of bath water with your hand before stepping in. You gather a small, safe sample of information before committing. Sniffing directly is like jumping straight into water you have not checked.

Real-world example

Natural gas used for cooking has almost no smell of its own. Gas companies add a tiny amount of a strong-smelling sulfur compound so that leaks can be detected quickly by smell. Here odour is used as a safety warning — but people are advised to leave the building and call for help, not to investigate the smell more closely.

Why?

Why is it dangerous to rely on smell to judge safety? Some very toxic gases, such as carbon monoxide, have no smell at all. Others, such as hydrogen sulfide, can paralyse the sense of smell at high concentrations, so a person may think the gas has gone when it is actually still present. Smell is therefore an unreliable safety test.

Common misconception

"A pleasant smell means a substance is safe." Some sweet-smelling chemicals are harmful, and some foul-smelling ones are harmless in small amounts. Odour tells you nothing reliable about toxicity; hazard labels and data sheets do.

Worked example

Question: A student wants to identify two colourless liquids and suggests tasting them. Suggest a safer plan.

Reasoning: Tasting is never permitted. Safe, measurable properties can distinguish liquids: boiling point, density, and whether each mixes with water.

Answer: Measure the boiling point and density of each liquid and compare them with data tables; if the teacher allows, note the odour by wafting.

Quick check

1. Describe how to note the odour of a substance safely. Answer: Hold the container away from the face and gently waft the vapour towards the nose with the hand; never sniff directly.

Exam focus

Safety questions often ask why chemicals must not be tasted, or how to smell a substance safely. Give specific reasons (toxicity, contamination, corrosiveness) and describe wafting clearly. Prefer measurable properties for identification.

Advanced insight

The human nose can detect some compounds at incredibly low concentrations — certain sulfur compounds at parts per billion. Chemists exploit this in perfume and flavour research using instruments (gas chromatographs) combined with trained human "sniff ports", under strictly controlled conditions and only with substances known to be safe at those levels.

Summary

Odour and taste are sensory properties, but in the laboratory tasting is never allowed and odours are observed only by wafting, if at all. Many chemicals are toxic, corrosive or contaminated, and some dangerous gases have no smell. Safe, measurable properties such as colour, density and melting point are preferred for identifying substances.

Practice questions

1. Give two reasons why chemicals must never be tasted in a laboratory. Answer: They may be toxic or corrosive, and the apparatus may be contaminated with other chemicals. 2. Why is carbon monoxide especially dangerous? Answer: It is toxic but has no smell or colour, so it cannot be detected by the senses. 3. What piece of equipment is used for substances with harmful vapours? Answer: A fume cupboard. 4. Why is odour a less useful property than boiling point for identifying substances? Answer: Odour is qualitative, varies between people and can be dangerous to test, whereas boiling point is a measured number that can be compared with data safely.