It is generally more difficult to dissolve sugar in a cold drink compared to a hot drink. The solubility of sugar increases with temperature. This means that more sugar can dissolve in a hot liquid than in a cold one.

When sugar is added to a cold drink, some of it may dissolve, but likely not as much as if the drink were hot. You might need to stir the drink for a longer time to help the sugar dissolve, and some sugar might remain undissolved at the bottom of the glass.