Why Do Chemicals Have to Be Heated in the Flame First Before the Colored Light Is Emitted?

Chemicals emit heat over a flame due to energy loss from their atoms.

Heating certain chemicals over a burner causes them to produce light, and is an experiment often conducted in student chemical labs. The process is actually very similar to the way a neon sign or a fluorescent light bulb works, and is directly related to the actions of the atoms in the compounds being tested. As in all chemical reactions, the effect is created by the transfer of energy, in this case the transfer of energy to and away from atoms.

Atomic State of Chemicals

Heat and cold are simple atomic states. When a substance becomes hot, its atoms begin to move more quickly and undergo certain changes. When a substances is cold, its atoms move more slowly and do not change as easily. The changes in atomic movement are mirrored as changes in the electrons of the atoms, which can jump energy levels or fall energy levels based on their heat.

Surrounding Temperatures

A chemical mixture at room temperature is fairly cool. Its atoms tend to move slowly, and electron levels are well balanced so that the mixture is benign and neither radiates light nor undergoes any type of change. By raising the heat of the mixture, scientists add energy to the chemicals, causing them to react in specific ways. Different types of atoms in different compounds will react in a variety of ways, some dangerous and some benign.

Heating Process

When a chemical that can be made to emit light is held over a burner, the energy of the burner spreads quickly through the chemical as heat. The atoms begin moving faster and faster as more energy is given to their electrons. Eventually the electrons gain so much energy that they bump up an energy level. This does not happen to all the atoms, and the electrons can only increase their energy level to a certain point, but the effects are widespread throughout the chemical.

Cooling Process

When the mixture is taken off the burner, the energy is sapped from the chemical mixture. The atoms slow down. The electrons are unable to continue operating at their higher energy levels and drop down to their original starting levels. But as the electrons move down, they lose some of their energy. This energy escapes the atoms, and in some atoms it escapes as photons that are emitted from the chemical as light.

Purposes

While this makes for an entertaining science experiment, it can also show a lot about the nature of the atoms. In fact, scientists use a more advanced method of the same process to identify atoms by measuring what spectrum of light waves they emit under these conditions.

References

• University of Illinois: Light from Heat
• Doc Stoc: Atomic Theory
• NASA: The Flame Test Student Worksheet