Hazard Pictograms: Physical Hazards

Introduction

Chemical containers carry small diamond-shaped symbols with a red border. These are hazard pictograms, and they give an instant warning that works across languages. Countries around the world use the same set of nine pictograms, defined by the Globally Harmonized System, usually shortened to GHS. On this page we look at the pictograms for physical hazards — dangers such as explosions and fires. The next page covers health and environmental hazards.

Core explanation

Each GHS pictogram is a black symbol on a white background inside a red diamond. Four pictograms mainly warn about physical hazards, and a fifth (corrosion) covers both physical and health hazards.

Exploding bomb. This warns that a substance may explode, for example because it reacts violently when heated, struck or rubbed. It is used for explosives, self-reactive substances and some organic peroxides. Precautions include keeping the substance away from heat and shocks and storing only small quantities under strict control. School laboratories rarely contain such substances.

Flame. This warns that a substance is flammable — it catches fire easily. It covers flammable gases, liquids, solids and aerosols, substances that can ignite spontaneously in air, and substances that release flammable gases when they touch water. Ethanol, propanone (acetone) and petrol all carry this symbol. Precautions: keep away from naked flames, sparks and hot surfaces; use small volumes; keep containers closed; store in cool, well-ventilated places.

Flame over circle. This warns that a substance is an oxidiser . Oxidisers may not burn themselves, but they can supply oxygen or otherwise make other materials burn much more fiercely, and may cause fires when mixed with combustible materials. Examples include potassium nitrate, potassium permanganate and concentrated hydrogen peroxide solutions. Precaution: keep oxidisers away from flammable and combustible materials such as paper, wood and fuels.

Gas cylinder. This warns of gases stored under pressure. The cylinder may explode if heated, and very cold liquefied gases can cause cold burns. Precautions: secure cylinders so they cannot fall, keep them away from heat and handle them only with training.

Corrosion. Among physical hazards, this pictogram warns that a substance can corrode metals. It also warns of serious damage to skin and eyes, which is discussed on the next page.

Pictograms are not the only information on a label. The label also carries a signal word, "Danger" for more severe hazards or "Warning" for less severe ones, and written hazard and precautionary statements, which are explained later in this unit.

Step-by-step reasoning

When you see a pictogram:

1. Identify the symbol (bomb, flame, flame over circle, gas cylinder, corrosion). 2. Recall the type of hazard it represents. 3. Read the signal word to judge severity (Danger or Warning). 4. Choose precautions that match: for flames, remove ignition sources; for oxidisers, separate from fuels; for gas cylinders, secure and keep cool.

Visual explanation

Pictogram Hazard Example Key precaution --- --- --- --- Exploding bomb May explode Some organic peroxides Avoid heat, friction and shock Flame Flammable Ethanol, acetone No naked flames nearby Flame over circle Oxidiser Potassium permanganate Keep away from combustibles Gas cylinder Gas under pressure Oxygen cylinder Secure upright; keep cool Corrosion Corrodes metals (and skin) Concentrated acids Suitable containers; eye protection

Real-world analogy

Hazard pictograms are like road signs. A driver does not need to read a long explanation to understand a sign warning of a sharp bend; the picture gives an immediate message, and the driver adjusts speed. In the same way, a flame pictogram tells you at a glance to keep flames away before you even read the details.

Real-world example

Petrol stations display flammable warnings and forbid smoking and the use of open flames, because petrol vapour can ignite easily. Transport vehicles carrying fuel and gases also display hazard diamonds so that emergency services know immediately what risks they face if there is an accident.

Why?

Why is an oxidiser dangerous even though it may not burn by itself? Burning is a reaction between a fuel and oxygen. An oxidiser can provide the oxygen, or play a similar role, directly at the surface of a fuel, so a small flame can become an intense fire, even in places where air cannot easily reach. This is why oxidisers must be stored separately from flammable materials.

Common misconception

Some students think the flame-over-circle symbol means the substance is "extra flammable". It actually marks an oxidiser — a substance that helps other things burn. Another misconception is that a substance without a pictogram is completely safe; many substances still need sensible handling.

Worked example

Question: A bottle of propanone carries a flame pictogram and the signal word "Danger". A student plans to use it near a lit Bunsen burner. What should they do?

Reasoning: The flame pictogram means the liquid and its vapour are flammable, and "Danger" indicates a severe hazard.

Answer: Turn off the Bunsen burner and remove all naked flames before using propanone, use only a small volume, keep the bottle closed when not pouring, and work in a well-ventilated area.

Quick check

1. Which pictogram warns that a substance is an oxidiser? Answer: The flame over circle.

Exam focus

You may be shown a pictogram and asked to name the hazard and give a precaution. Remember the difference between the plain flame (flammable) and the flame over circle (oxidising), and link each precaution directly to the hazard.

Advanced insight

The GHS divides each hazard type into categories based on measured properties. For flammable liquids, the category depends on the flash point (the lowest temperature at which the vapour can ignite) and the boiling point. This is why two flammable liquids can carry the same pictogram but different signal words.

Summary

GHS hazard pictograms are red-bordered diamonds used worldwide. The physical-hazard pictograms are the exploding bomb (explosive), flame (flammable), flame over circle (oxidiser), gas cylinder (gas under pressure) and corrosion (corrosive to metals). Labels also carry a signal word — Danger or Warning — and written statements. Each hazard calls for matching precautions.

Practice questions

1. What does the gas cylinder pictogram warn about? Answer: Gases stored under pressure, which may explode if heated or cause cold burns. 2. Why should oxidisers be stored away from paper and fuels? Answer: Oxidisers can make combustible materials burn much more fiercely or start fires when mixed with them. 3. Give an example of a substance that carries the flame pictogram. Answer: Ethanol, propanone (acetone), petrol or another flammable liquid or gas. 4. What does the signal word "Danger" indicate compared with "Warning"? Answer: "Danger" indicates a more severe hazard; "Warning" is used for less severe hazards.