Does sugar affect the freezing of water? Why?
The presence of sugar (or salt, or any other dissolved substance in water) does indeed lower the freezing point of water. This phenomenon, called freezing point depression, occurs because the particles of the dissolved substance interfere with the formation of the crystalline bonds between water molecules that give ice its solidity.
Interestingly, the addition of a solute (dissolved substance) to water not only lowers the freezing point but also raises its boiling point - a phenomenon referred to as boiling point elevation. It does this by lowering the vapor pressure of water (a tendency that describes a substance's tendency to turn into a gas).
The vapor pressure, boiling point, and freezing point of water (or a solution containing water and another substance) are referred to as colligative properties - properties that depend on the amount of the solute in a solution. For more information on colligative properties, check out their encyclopedia entry.