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Crack is the freebase form of cocaine, as opposed to the hydrochloride salt. Reacting cocaine hydrochloride with a weak base (usually sodium bicarbonate (NaHCO3)) removes the hydrogen and chlorine ions, giving the freebase cocaine alkaloid, water, carbon dioxide and a salt (sodium chloride in the case of a reaction with sodium bicarbonate).
The freebase form has a much lower melting point, allowing crack to be smoked - cocaine HCl would just burn. The effects are essentially the same, but smoked crack enters the bloodstream much more quickly than snorted cocaine (due to the much greater surface area of the lungs vs the mucous membranes) giving a more intense and immediate hit. Crack cocaine isn't water soluble, so can't be effectively snorted.
Heroin offers an interesting parallel. The overwhelming majority of heroin sold in Europe is in the freebase form, which can be readily smoked but needs to be buffered with an acid (usually citric or Vit C) in order to be dissolved and injected. In the US, the market is split - on the east coast, most heroin is sold as a salt, whereas on the west coast it is usually sold as a freebase.
Also, that minge haunts my dreams, like a stubbly Eye of Sauron.