Chemistry Lab Help Please!

Question

Use a thermometer to measure the temperature of boiling water. Now add salt to the boiling water. Wait until the water boils again then record your new temperature. What happened to the boiling point of the water? Why do you think this is the case?

Answer 1

Well,

 

You should have noticed that the boiling point of the salt water is a bit higher than the boiling point of pure water.

 

This is due to the fact that the ions of Na⁺ and Cl^– present in the salt solution interact strongly with the some of the water molecules.

 

Since some of the water molecules are strongly attracted to the ions, more energy will be needed to "kick" these water molecules out of the liquid into the gas phase (vapor). This higher energy need will require the molecules of water to move more violently (i.e. higher kinetic energy). This increase in kinetic energy is what we record as an increase in temperature.

 

Here is a somewhat helpful, but imperfect analogy.

 

Imagine a cup full of small iron spheres (e.g. metal shots)... the metal shots represent the molecules of water.

 

Now gently shake the cup of metal shots... this represents the motion of the water molecules in the liquid.

 

Now shake the cup more and more strongly... this represents the temperature increasing... the molecules are moving faster and faster.

 

If you keep up the strong shaking, you will see that some of the metal shots will "jump" out of the cup... this represents the molecules of water that are escaping the liquid and going into the gas phase (evaporation).

 

Now, imagine that you are adding a lot of little magnets to the cup full of metal shots... the little magnets represent the salt ions dissolving in water.

 

You now have a cup where the metal shots tend to stick around the magnets, and... if you want to kick out a metal shot by shaking the cup, you will need to shake the cup harder this time... the stronger shaking represents a higher temperature requirement to separate the shots from the magnets (or the water molecules from the ions in the salt)

 

I hope this helped.