Robert Bunsen: The man who lit a fire under chemical science

German professor’s burner laid the basis for development of spectrum analysis

The Bunsen burner is still ‘one of the most valuable inventions ever made’ as an obituary of its inventor claimed in 1899. File photograph: Ian Logan/Stone/Getty

The Bunsen burner is still ‘one of the most valuable inventions ever made’ as an obituary of its inventor claimed in 1899. File photograph: Ian Logan/Stone/Getty

 

My eldest son started secondary school this year and that means his first science labs (in addition to all the other changes of course). I loved school science labs with the scratchy oversized lab coats, the clinking glass test tubes and the notepads filled with graph paper.

But nothing says school science lab as much as the Bunsen burner. I couldn’t believe the teacher was going to let all the jokers in my class light fires at their desks. Every day thousands of chemistry teachers across the world take the same leap of faith and introduce their students to the Bunsen burner.

The Bunsen burner is still “one of the most valuable inventions ever made” as an obituary of its inventor claimed in 1899. Modern gas stove tops and barbecues operate on the same general principle as Bunsen’s 19th-century burner: a flammable gas is mixed with variable amounts of air to produce a controllable flame for heating.

The scientist who made many a school science experiment possible probably doesn’t get much discussion in the classroom. Robert Bunsen, a professor of chemistry in Heidelberg, invented the burner around 1854 to use with his university students. He was at the forefront of German chemical science where laboratories became a central component of teaching (as opposed to lectures alone).

His laboratory trained not only a generation of German chemists but scientists from across Europe and America such as Irish man John Tyndall (he was a student in 1848). One former student recalled that Bunsen used to wander the laboratory with a cigar that he lit on any available Bunsen burner.

Bunsen and Kirchhoff recognised that burning a substance could be a test for which elements it contained

Bunsen made important contributions to chemistry and had early interests in geology and physics. He invented a battery, investigated photo-chemistry and studied volcanoes in Italy and Iceland. Flame had an important role in his experimental science as well as his training of students. He and his collaborator Gustav Kirchhoff discovered two new chemical elements through spectrum analysis.

Bunsen and Kirchhoff built on the observation that different substances produced different colours of flame when they burned. Every science student sees this in action when they first light their Bunsen burner. When you burn only the methane gas without introducing any air to the burner (done by turning a little barrel at the bottom) you get an orange flame.

This is because combustion of methane is incomplete and the orange you see is soot being produced. When you add air to the burner you will see a blue flame, the colour that methane burns when combustion is complete.

Chemical elements also produce different colours when they burn. Sodium, for example, burns yellow while lithium burns red. In fact, each element creates a unique spectrum of colours that are a kind of signature of its presence within a mixture. When the light from the burning element is passed through a prism this signature can be seen. Bunsen and Kirchhoff recognised that burning a substance could be a test for which elements it contained.

The British Medical Journal explained the new science in 1862 with reference to cigars: if you burn cigar ash you would see a spectrum including yellow for sodium, pale red for potassium, bright red for lithium and orange and green for calcium. Bunsen and Kirchhoff also realised that the test was extremely sensitive to even the tiniest fraction of an element. When the pair introduced a very small amount of sodium into the air of a laboratory room, away from the spectroscope, they soon saw the characteristic yellow appear in the flame.

Bunsen and Kirchhoff eventually discovered two new chemical elements (rubidium and caesium) by spectrum analysis. The elements appeared in spa waters from Durkheim.

This discovery was significant on its own terms, but spectrum analysis also had application to astronomy. The spectrum of light emitted by stars, for example, would now be a guide to the elements that they contained.

Bunsen died 120 years ago but the humble Bunsen burner still brings a bit of excitement to school chemistry and a tangible connection to some of the most important chemical discoveries of the 19th century.

Dr Juliana Adelman lectures in history at Dublin City University

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