how does adding salt water affect the way water changes tempeture? snd how does it affect the freezing point?

Question

GO Stonecold

Answer 1

This ques is about the annomolous behaviour of water. this behaviour is shown between 0-4 degrees . water expands when cooled from 4 to 0 degress instead of contracting.
if salt is added then it makes the water more dense and hence its volume decreases bcoz density=mass/volume.
thus an increase in temp. brings about an decrease in volume.
the freezing point is lowered due to the addition of impurities (salt)

Answer 2

I will explain both the "what" and the "why" of this. What refers to the equation for calculating boiling point elevation and freezing point depression. Why explains just why this occurs.

The "what" first.

The addition of a solute like reguar table salt can increase the boiling point by:

BP change =Kb*molality of solute

The Kb refers to a boiling point constant that is specific to the solvent (like water, whose Kb is 0.515C/molal) and molality is the moles of solute divided by the kg (not liters) of the solvent. Make sure you understand that molality depends on how many ions are produced in water. For example:
.10 molals C6H12O6 (glucose) yield exactly the same molals in water
BUT
.10 molals of NaCl (salt)--->2*.10=.20 molals, because for each NaCl molecule--->Na+ and Cl-, two ions are formed. The rule of thumb is that if a compound is not ionic, the molality of the solid solute is the same as the molality in solvent, but any ionic compound dissociates into its constituent ions. This might be a good place to use trick questions, say you are given a .10 molality NaCl in water, they ask for boiling point elevation and you do:

BP elevation= Kb*molality, and you forget that NaCl gives two ions, you will underestimate the boiling point elevation.

Another caveat: molality is the moles of the SOLUTE in KG of SOLVENT. Notice here that the moles of solute don't affect the volume or mass of the solvent, so you simply calculate the kg of the water separately. If you calculate that you have .2moles of NaCl in 1 L of water (1L water is about 1KG), you have a molality in solution of
.2moles/1KG=.2 molals, but we multiply by 2=.4molals of NaCl

The freezing point depression is calculated in the same manner, except that now we reduce the freezing point by:

FP depression=Kf(freezing point constant of solvent)*molality of solute.

Same idea, just that now the change in Temperature(freezing) is negative.

Now as to "why".

First, notice that when you calculate the Boiling Point or Freezing Point elevation/depression, you only care about two things: the molality of the solute and the kilograms of your solvent. You aren't concerned much with the chemical characteristics of the solute. Any property that depends only on the quantity- and not the characteristic- of solute and solvent is called a colligative property.

Why does a solute raise the BP and lower the FP. The reason is because the molecules (or in the case of salt, the ions) of the solute interfere with the bonding in the solvent like water. They nudge themselves in-between molecules of H20. As far as the boiling point, the additional salt solute reduces the vapor pressure of the water, it bumps up against it and makes it hard for water molecules to escape the liquid. If you recall, the boiling point is the point at which the pressure of water molecules is just enough to overcome atmospheric pressure and escape. But the salt reduces that vapor pressure, so water has to be heated to a higher temperature to make up for that difference.

The salt also interferes with the water molecules when they want to form a solid crystal (that is, when they want to freeze). It makes it harder for water molecules to align properly and form a solid.