Topic 4 · page 1 of 17

Hazard and risk

A hazard is what a substance could do to you. Risk is how likely it is to happen. Safe working means finding the hazards, judging the risk and cutting it down before you start.

12 min read4.14.24.34.3(i)4.3(ii)4.3(iii)

Hazard and risk are different things 4.1

People use the two words as if they meant the same. In chemistry they do not, and a mark scheme will penalise you for mixing them up.

Hexane is highly flammable. That is its hazard, and it is the same wherever the hexane is.

Situation Hazard Risk
500 cm³ of hexane heated over a Bunsen burner flammable high: large volume, vapour, a naked flame
5 cm³ of hexane warmed in a water bath, no flames in the room flammable low: small volume, no source of ignition
A sealed bottle of hexane on a shelf flammable very low: nothing is exposed

Hazards of organic compounds 4.2

Many organic compounds are volatile, which means they give off vapour easily at room temperature. Vapour is easy to breathe in and easy to set alight. That is why the same few hazards come up again and again.

Hazard Why it is common Example
Flammable volatile liquids that burn easily hexane, ethanol
Toxic or harmful vapours are breathed in; some pass through the skin methanol, trichloromethane
Irritant vapours and liquids irritate eyes, skin and lungs many halogenoalkanes
Carcinogenic some cause cancer after repeated exposure benzene

The hazard of a reagent is printed on its bottle as a pictogram and in the supplier's safety data. You read these first, before you plan the experiment.

Why a risk assessment is needed

A risk assessment is a plan written before the practical. It does not just list the hazards. It follows four steps:

  1. Identify every hazard in the method (the chemicals, the products, the apparatus, the heating).
  2. Judge the risk from each hazard: how likely is harm, and how serious would it be?
  3. Decide control measures that reduce each risk.
  4. Plan for things going wrong: spills, fire, first aid.

Reducing the risk 4.34.3(i)4.3(ii)4.3(iii)

There are three kinds of measure that you are expected to suggest.

Work on a smaller scale 4.3(i)

Less substance means a smaller fire, less vapour in the air and less waste if something goes wrong. Use 2 cm³ instead of 50 cm³, or micro-scale apparatus.

Take precautions specific to the hazard 4.3(ii)

The precaution must match the hazard.

Hazard Precaution How it reduces the risk
Flammable no naked flames; heat with a water bath or an electric heater removes the source of ignition
Toxic or volatile work in a fume cupboard vapour is drawn away, so it is not breathed in
Irritant or corrosive wear gloves and eye protection stops the substance touching skin or eyes

Use a less hazardous method 4.3(iii)

Change the method so that it uses less hazardous substances. For example, use dilute bromine water instead of pure liquid bromine, which is toxic and corrosive. The reaction can still be done, with much less risk.

Worked exampleReducing the risk when heating cyclohexane

Cyclohexane is flammable and its vapour is harmful. Suggest one way of each kind to make a reaction in which it is heated safer.

  1. Smaller scale: use 5 cm³ rather than 100 cm³, so less vapour is produced and any fire would be small.
  2. Specific precaution: the hazard is flammability, so heat with a water bath or electric heater, not a Bunsen burner, to remove the source of ignition. Work in a fume cupboard because the vapour is harmful.
  3. Less hazardous method: if a solvent is needed, choose one that is less flammable or less toxic than the one first considered.

Check yourself

  1. Explain the difference between a hazard and a risk.
  2. Ethanol is flammable. State one way in which the risk is higher and one way in which it is lower: (a) 200 cm³ heated on a gauze over a flame; (b) 2 cm³ warmed in a water bath at 50 °C.
  3. State two hazards that are common in organic compounds and give an example of each.
  4. Why is a risk assessment done before a practical, and not after it?
  5. A student must add liquid bromine to hexane. Suggest two ways to make this safer, and say which of the three kinds of measure each one is.
  6. A student writes "wear a lab coat" as the precaution for heating a flammable liquid. Explain why this does not earn the mark, and write a better answer.

Answers cover these until you have tried the questions

  1. A hazard is something with the potential to cause harm. A risk is the chance that harm will actually happen (and how serious it would be). The hazard belongs to the substance; the risk depends on how it is used.
  2. (a) The risk is higher: a large volume gives a lot of vapour and a naked flame is a source of ignition. (b) The risk is lower: the volume is small, the water bath at 50 °C is well below any flame, and there is no source of ignition.
  3. Flammable, for example hexane or ethanol. Toxic or harmful, for example methanol or trichloromethane. (Also acceptable: irritant, for example many halogenoalkanes; carcinogenic, for example benzene.)
  4. A risk assessment lets you identify the hazards and judge the risks so that you can plan control measures first. After the practical it is too late to prevent harm.
  5. Use a fume cupboard and wear gloves and eye protection (precautions specific to the hazard, because bromine is toxic and corrosive). Use dilute bromine water instead (a less hazardous method). Using a smaller volume would be a third acceptable answer (smaller scale).
  6. A lab coat does not reduce the risk from the flammability. A better answer is: "the liquid is flammable, so heat it with a water bath or an electric heater rather than a naked flame, because this removes the source of ignition".