|MadSci Network: Chemistry|
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:) http://www.swri.com/ 10light/water.htm 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 faster. 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
Try the links in the MadSci Library for more information on Chemistry.