Here, we are talking about three colligative properties of a solution. We are talking about the effect adding a solute (sugar) to a pure liquid (water) and how this affects the vapour pressure, freezing point, and boiling point of pure liquid.
Vapour Pressure Lowering and Boiling Point Elavation :
Here we need to consider the effect of adding sugar to water on the water's surface. That is, no longer do we have a surface of just water molecules alone. Instead, we now have sugar molecules are replacing some of those water molecules on the surface. Therefore, the surface has less water molecules, so that means less water molecules are available to evaporate -> less molecules to be in equilibrium with the vapour state. The solution's vapor pressure goes down (less vapor), and boiling point goes up (because there's less vapor, it takes longer to get the solution to boil).
Freezing Point Depression
So, how does adding sugar to water effect the temperature needed to freeze it? Well, this one is easy; water, and all pure liquids, form a solid fastest when it only has to freeze with itself. So, when other molecules (the solute molecules) are in the liquid, they simply mess up the nice crystal latice the liquid wants to form and then the process of forming the lattice takes longer. The result is that the a colder temperature is needed to freeze the solution, leading to the term freezing point depression.
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