Question

What happens when salt mixes in water? Do the properties of the liquid change? How?

Answer 1

Let's try a simple experience: take a glass of water, pure, clean tap water and put a teaspoon of sugar in it. Mix well. You will be able to see that the water remains clear, and you have now a solution of sugar in water, if you taste the water now is slightly sweet... the sugar dissolved in the water.

Now keep putting again one teaspoon after another teaspoon of sugar in the water. After 2 or 3 teaspoons the water will no more be clear, but turbid. That happens because every liquid (we call them solvents) has a definite capacity to dissolve some amount of each substance. Less than that amount we have a solution and the liquid remains clear; more than that amount the excess don't dissolve and remain floating to begin with making the liquid cloudy. Now if you put more sugar again the excess will go to the botton of the cup.

If you evaporate the liquid, the solid will not evaporate and will remain all at the botton of the cup. (do not try to heat the solution to hasten the evaporation because the sugar changes upon increasing of temperature and you will turn it into a syrup)

You can do that with salt and many other substances because the water dissolves a lot of them. The experiment will be slower because the salt is more soluble in water than the sugar is. But after putting some amount of salt in water you will find that it not solving anymore and it stays at the botton of the glass. The sea water tastes salty just because there are a large amount of salts dissolved in it. Now look, when a substance (as the sugar, or the salt) dissolves in water making a solution that doesn't changes their properties and the substance can be recovered, there still are the sugar (or salt) and the water, or as the chemists say solute and solvent.

When one makes a solution what happens is a physical fact, because there are no chemical changes. But if for example you heat the sugar solution making a syrup, now you have done a chemical change and it is not possible to recover the sugar.

With the salt solution what happens is different because the salt is not affected by the heating, and you can recover it.

Answer 2

When you add a salt to water and the container feels cold, yes, an endothermic reaction is taking place. The solution is grabbing heat from the surroundings to get the salt to dissolve. When you add a salt to a pure solvent (say, water), the freezing point will go down and the boiling point will go up. That's because the vapor pressure of the pure solvent is lowered (this would be easier to explain with a phase diagram).
Remember, the definition of BP is the temp at which the solution's vapor pressure equals the external pressure. By lowering the vapor pressure, the whole phase diagram shifts...BP to the right (higher), FP to the left (lower).
You may want to check out a intro level chemistry text or at least draw out the phase diagram to see more clearly what gets lost is just words here. Did you ever try boiling water without heating it? If you have the means, hook up a vacuum to a flask of water. You don't change the vapor pressure of the water, but you reduce the external pressure enough so that the water does boil. Good question!