It doesn't really taste sweet because the temperature is lower hence the sugar does not dissolve properly.
A solution is composed of a solute and a solvent. The solute dissolves in the solvent. In this case, the sugar is the solute in this case. We have to also note that temperature has a huge effect on solubility of a solute.
With the iced coffee, the temperature is lower hence the sugar does not dissolve properly therefore the coffee does not taste really sweet.
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