Preparation and reactions of acetylene

Acetylene (ethyne) is formed when water reacts with calcium carbide, CaC2:

CaC2(s) + H2O(l) →

Ca(OH)2(aq) + HC≡CH(g)

This apparatus allows us to produce a flow of acetylene that can be collected in test tubes.

—water

—calcium carbide

Turn on the tap and adjust it to slowly drip water into the reaction flask.

Bubbles become visible on the sand-like calcium carbide.

Collect the gas by downward displacement of water.

Add a little bromine water to a tubes of gas.

Shake well.

The addition reaction between an alkyne and bromine is much slower than with an alkene.

After about a minute, the bromine water has turned colourless.

Withdraw a little of the liquid from the reaction flask.

Add a little universal indicator solution to water, then add the liquid from the reaction vessel.

The colour turns blue: the liquid is alkaline.

Combustion of ethyne

It can be dangerous to light the gas directly from the delivery tube.

If the mixture of air and acetylene within the system is exactly right there will be an explosion.

The method which follows is not only safer, it’s more fun.

Sprinkle a little calcium carbide into a beaker containing a little water.

Light a wooden splint and use it to light the gas produced.

Notice how smoky the flame is.

The reaction is over now, but you can see a little of the gas trapped in bubbles.

The final solution is cloudy because calcium hydroxide is only slightly soluble.

Around 1900 carbide lamps like this one were used for bicycles and even down gold mines.

Even today, underground cavers often take carbide lamps rather than torches. It’s easier to carry a small volume of calcium carbide than a large number of batteries.