MadSci Network: Chemistry

Re: How does salt affect water?

Date: Mon Feb 26 18:56:50 2001
Posted By: Steve Williams, Staff, Science Demonstrator, Pacific Science Center
Area of science: Chemistry
ID: 983050563.Ch

Hi Jessie! 

First, I would like to say you have done some great experiments! You have 
also covered some interesting ideas in chemistry. Let's cover each of your 
experiments and discover why you got the results you did.

Number One: Boiling
In your reported results, you mentioned that the pure water boiled faster 
than the salt water. While I am not arguing that your results are wrong, I 
would like to suggest you try the experiment again, but this time, put even 
more salt in the water. Maybe so much salt that some doesn't quite dissolve. 
Here's why:
When you boil water, you are causing a phase change to happen. A phase 
change is when a substance goes from one state of matter to another. In this 
case, you are changing water from a liquid to a gas. This phase change 
happens at a certain temperature. This temperature represents the point in 
which you are adding enough energy to allow the liquid to change. When you 
add a significant amount of salt to water (like 20% of it's volume according 
to this website:)

To sum up the idea in that website though, while salt water has a higher 
boiling point than regular water (which your experimental result suggests), 
the salt water heats up faster than the regular water, so it gets hotter 
Try this experiment again and see if what you get meshes with what I am 
saying here. But for now, let's talk about:

Number Two: Freezing
Regular water froze and the salty water didn't, and that is exactly right. 
You have just discovered the opposite side of the boiling point problem 
above. When you add an impurity to a substance, you tend to raise its 
boiling point, and lower its freezing point. Salt is an impurity to the 
water, and it prevents it from doing a phase change from liquid to solid as 
easily as when it is pure. Pure water freezes at 32 F or 0 C. Salt water 
freezes at a lower temperature though, about three or four degrees lower.
The reason your salty water never froze in your freezer is your freezer 
never got below this temperature! It is kept at a temperature just below 
freezing for pure water. Not quite cold enough for your salty water.

Number Three: Soap, Water and Bubbles
I love bubbles. One of the things I like to talk about is how a bubble (like 
a soap bubble) is formed. Molecules of pure water are attracted to other 
molecules of pure water. You can think of these molecules like being tiny 
magnets. Now these magnets only work in pure water, as the sodium and 
chloride ions (the stuff that make up salt) block the magnets from linking 
up in salty water. This feature of water, by the way, is called polarity. 
This polarity gives pure water another cool feature (and another cool word): 
elasticity. Elasticity makes liquid, pure water squeeze together into the 
smallest possible surface area. The smallest possible surface area is a 
sphere. When water is filled with air, it's elasticity forces it into a 
sphere shape. That's why bubbles have the shape they do.
Now, if you add salt to the water, you prevent the tiny water magnets from 
linking up. The water loses it's polarity and thus it's elasticity. No 
bubbles can form, and thus your salt water makes no bubbles.

Number Four: Sinking
Your last experiment has to do with density. Density is the property of 
things that tells you how many of something is packed into one place. 
Consider a chunk of lead and an equally sized chunk of cork. The lead is 
more dense than the cork. There is more stuff in the same amount of space 
with the lead.
Humans are pretty dense (haha), especially compared to water. Water can be 
made more dense by, you guessed it, adding an impurity like salt. When you 
added the salt you raised the density of the water slightly. 
Now, what does this have to do with something sinking? Well, more dense 
things sink in less dense things. A piece of cork will float on water 
because it is less dense. If you raise the density of the water, though, you 
will make it easier for things to float. Also, if you raise the density of 
the water, you will make it more resistant to something of the same density 
sinking in water.
Think of this: If I drop a rock in fifty feet of water and fifty feet of 
air, which would hit the bottom first? The air! Even though the rock was 
more dense than both the water and the air, the water resisted the sinking 
rock, and made it sink slower.
Now we can see why the object sunk more slowly in the salty water - it was 
more dense. Incidentally, they tell me that in really salty bodies of water 
like the Dead Sea people can float without any effort, and that it can be 
difficult to stay underwater!

I hope this answers your question. If you would like to know more about 
salty water and the phase changes you observed in boiling and freezing, I 
suggest you look up the following ideas in a chemistry book: colligative 
properties and solutions.

Take Care and Be Safe,
Steve E. Williams
Rock Star and Science Demonstrator
Pacific Science Center, Seattle, Washington, USA 

Current Queue | Current Queue for Chemistry | Chemistry archives

Try the links in the MadSci Library for more information on Chemistry.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2001. All rights reserved.