How do you take your tea?
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Why use hot water?
cold water tea The substances in tealeaves that dissolve to give tea its characteristic flavours are not particularly soluble in cold water. If you stir dried tealeaves into cold water you will find that the water takes up very little colour.
warm water tea With lukewarm water some colouration takes place.
With hot water the familiar tea colour is produced. 
hot water tea
On standing, the water becomes cool again and some of the substances that were soluble at the higher temperature can no longer stay dissolved and come out of the solution as shiny deposits of solid.