Did you know?

Joseph Black observed that water which had first been boiled froze more quickly than unboiled water. It also produces clearer ice – as you will see if you try it for yourself.


Joseph Black (1728-1799)

Discoveries

Joseph Black realised that air was not the only gas, and discovered carbon dioxide. He also made discoveries about heat and temperature and latent heat.

Carbon dioxide

A chemical balance

Strangely enough, Black observed the absence of the gas, before he knew of its existence.

Because he measured and weighed so carefully during experiments, he noticed that the magnesium compound weighed significantly less after being heated. He then set out to discover what had been 'lost'.

Black established this was a form of air, but that it was different from 'common elastic air'. He reasoned that this air must have been 'fixed' within the magnesium compound, so called it 'fixed air'.

He noted that fixed air:

  • Was heavier than common air
  • Could extinguish a flame
  • Would suffocate an animal.

Heat and temperature

Chemical equipment

Joseph Black was the first to illustrate that heat and temperature are different things.

One theory of the time was that liquids contained an inner fire, released upon solidification. Another was the caloric theory, where heat was believed to be a chemical substance which flowed from hot to cold bodies.

Although thermometers existed, there were several different ideas as to what they were measuring.

Black showed that within a quantity of a substance such as water:

  • Heat is energy, which can be transported as molecules vibrate, move and collide into each other
  • Temperature is a measurement of the average motion or kinetic energy of the molecules.

One way Black demonstrated the distinction was to take a bucket of ice, and monitor its temperature constantly. The ice gradually melted, yet the temperature stayed the same.

Latent heat

Black is credited with the discovery of latent heat.

This is the heat required to induce a change of state, where a solid becomes a liquid or a liquid becomes a gas. Latent heat can either be:

  • The heat absorbed if the change involves solid to liquid or liquid to gas
  • The heat released if the change involves gas to liquid or liquid to solid.

From this Black developed the idea of 'specific heat'. This measured the amount of heat required to raise the temperature of a substance by a specified number of degrees.


Chemical balance detail Glass flask detail
Portrait of Joseph Black © Royal Medical Society, Edinburgh