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Q How do fireworks get their color and shape?

— TJ Nissen, Omaha, Neb.

Rodney Schreiner, emeritus scientist, chemistry department at the University of Wisconsin-Madison:

Fireworks, as you can tell from the name, involve fire. To get a fire you need fuel, oxygen and heat, and that’s true in fireworks also.

The fuels used in fireworks are solids, most commonly carbon and sulfur.

In fireworks you need the carbon and sulfur to burn rapidly and need to provide much more oxygen to accomplish that. In making fireworks, you add a compound, like potassium chlorate, that contains a lot of oxygen that will be released when it gets hot.

When the potassium chlorate is mixed with the sulfur and carbon and heated, the chlorate begins to decompose. It gives off oxygen and reacts with the sulfur and carbon to give a nice, bright, really hot fire.

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The fireworks’ colors come from another ingredient in the mixture, a salt. Different salts produce different colors.

A solution of ordinary table salt sprayed into a flame produces a brilliant yellow color, while a strontium salt solution gives off an orange hue. A solution of a copper salt produces green, while a lithium salt gives off a red. Other elements will give other colors, and mixtures will produce an even wider variety of potential colors.

The big show fireworks that have many stars going off into different shapes — like hearts and bow ties and faces — are made of a bunch of little firework explosions that are put together in just the right order. The specific way they’re put together is a bit of a trade secret.

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Blue Sky Science is a collaboration of the Wisconsin State Journal and the Morgridge Institute for Research.

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