The glass with the alcohol will contain a mixture. This is because the melted ice will then be incorporated into the alcohol, creating a mixture of water and alcohol. However when the ice melts int he water, since ice is just frozen water, it the glass is still a pure substance because water is a pure substance and no other different substances have been added to it. <span />
Why is the active site specific for only one type of substrate? Enzymes have a lock and key type fit so only a specific substrate fits in the activation site.