Why is copper used for coins?

Answer 1

In modern coins, its not. Up until 1991 all 1p coins were struck in bronze (mostly copper) but from 1992-present they are copper plated steel.

The reason why they are is because its a continuation of an older practice when a coin's metal was worth about what the coin was worth.

For smaller sums, a base metal (usually copper but tin has historically been used for British Coins) is needed because a silver (or gold!) coin would be too small to really use effectively. And small coins are needed for change and small purchases.

For slightly larger sums, silver was used because it had a much better weight to value ratio. For example, back in the 1790s a threepence would need to be three ounces of copper to have 3d worth of copper! Silver coins were much smaller (about a gram or two for a 3d).

For large sums, gold was used because it had a better value to weight ratio than even silver.

Platinum (and Palladium) historically was not used for circulating coins because they weren't valued as much back then. Today platinum is usually more expensive than gold (though not at the moment!) but back in Victorian times, Platinum was used by counterfeiters to put in fake gold coins with a little bit of gold because platinum was much cheaper than gold.