Is it limestone?

We had an L-themed day recently, and to liven up our permitted daily exercise walk, I gave my son a list of things beginning with the letter L to read and find on the way.  One of these was limestone, and we talked a little about how he was looking for a light-coloured rock.  He picked up a few such stones, and wanted to know how we would know if he'd actually found limestone.

Six rock samples to test

I explained that limestone is made of something called calcium carbonate, and in acids like vinegar this reacts to form carbon dioxide gas.  We've done something similar before with shells from the beach; both are made of calcium carbonate and actually this is a hint about how limestone was made - it's a sedimentary rock, formed as small fragments of marine creatures' shells are squashed together over time.  We had a chat about how rocks were made, and that it is different for different types of rock e.g. some others come out of a volcano (igneous) and others are formed when sedimentary rocks are heated under pressure (metamorphic).

Experiment in progress

He suggested trying a reaction with vinegar when we got home. I washed his six rock samples (some extracted from muddy puddles!) and set him up with these test tubes (affiliate link), although yoghurt pots or jars would work too.  I also put some slightly diluted spirit vinegar (about 1 part water to 3 parts vinegar) in a separate container.

Gas bubbles forming as the limestone reacts

He put one rock sample in each tube, and added some vinegar then had a look at each one.  Some looked cloudy and others clear - on closer inspection we could see lots of little bubbles in the cloudy ones.  As we watched we could see them forming on the rock and rising to the surface.  He counted and found there were three with bubbles and three without, and concluded that three of the rocks must be limestone, and three must be something different!  

For older children, you could use this experiment as a way to demonstrate the effect that acid rain (resulting when fossil fuels are burnt and sulfur dioxide/nitrogen oxides enter the atmosphere and react to form sulfuric/nitric acid) has on limestone.