Solids dissolve faster in hot water than in cold water because the added heat increases the kinetic energy of the molecules, allowing them to react more quickly with the solid and speeding the chemical reaction. However, certain solvents, such as ionic solutes, have increased polarity at high temperatures, slowing the rate at which the solid dissolves.
When the temperature increases the kinetic energy, the quicker movement of the particles causes less inter-particle attraction, meaning that the molecules of the solvent are less attracted to one another and more likely to be attracted to the molecules in the solute. This change in attraction levels makes it more likely that the molecules of the solvent and the molecules of the solute form a bond, accelerating the dissolving process.
Depending upon the chemical structure of the solute, the increased temperature may also allow the solid to reach and surpass its melting point. At the melting point, the kinetic energy of the molecules increases and the attraction between molecules changes to enable a phase change in the state of matter. As molecules in a liquid state have a lower level of attraction to one another than those in a solid, the melting process can also expedite the dissolving process.