Why does water boil quicker when you add salt?

Salt should actually delay boiling in water by raising the water’s boiling temperature. The water simply won’t boil until its temperature reaches a certain value and that requires heat. There isn’t any trick that will shorten the heating time, except to add chemicals that boil at lower temperatures than water (e.g. alcohol). But sprinkling salt into very hot water may trigger boiling by providing nucleation sites on which the bubbles can begin to form.

What is the function/purpose of water in your body? What happens if your body doesn’t get sufficient water or gets too much water? What are the best sources of water?

Water plays so many roles in your body that I’ll have to choose which one to describe. The role that strikes me as most important is water’s capacity to dissolve chemicals and carry them about. Our bodies deal with thousands or perhaps millions of different molecules, each of which must migrate from place to place to perform its required tasks. For example, the molecules that are responsible for energy—carbohydrates, fats, oxygen, and carbon dioxide—must all move through us to keep us alive. Water acts as the carrier for all of these molecules. Many of the molecules dissolve easily in water—meaning that water molecules surround these molecules and carry them about one molecule at a time. Carbohydrates, carbon dioxide, salts, and many other organic compounds dissolve easily in water and are carried about by it. Other chemicals, particularly fats and oxygen, don’t dissolve easily in water and need help. Hemoglobin in red blood cells is the molecular structure that carries oxygen through our blood. So my answer to the first question is that water is the principal solvent for all the chemicals in our body.

If your body gets too much or too little water, the dissolved chemicals become either too dilute or too concentrated. When you have too much water in your body, the water tends to flow out of you through any membranes that are permeable to water. Among other things, this process tends to make you excrete extra water as urine. When you drink too much water, it goes right through you. When you have too little water in your body, water tends to flow into you through any permeable membranes, or at least tends not to flow out of you. When you eat lots of salt and are overconcentrated, you don’t excrete much water and feel extremely thirsty—your body is looking for more water.

Finally, the best sources of water are those that simply don’t have many dissolved chemicals; or at least none that cause trouble for your body. That means that your water shouldn’t have much lead or arsenic dissolved in it or any of a number of noxious organic chemicals. The purest waters are distilled water, rain water (assuming minimal air pollution), and water that has been chemically filtered (via ion exchange, reverse osmosis, and/or activated carbon). Spring and well waters tend to contain substantial amounts of dissolved calcium and magnesium salts, which make the water less pure but probably don’t affect its healthfulness. One special case to look out for is water that was very hard but that has been passed through a water softener. The dissolved minerals that made the water hard will have been replaced by sodium compounds during the softening process and excessive sodium consumption may be a problem for some people.

Is water, at say 35° F, more dense than water at 80° F?

Yes, water at 35° F is more dense than water at 80° F, so that the hotter water will float on the colder water. But water is special (and almost unique) in that it does expand slightly as it cools to its freezing temperature. Water’s density reaches a maximum at 3.98° C (about 39° F) and then actually becomes less dense as you cool it toward 0° C (32° F). That means that 33° F water will float on 39° F water! This bizarre behavior allows ice and very cold water to float above slightly warmer water and keeps ponds from freezing solid. Without it, many animals would perish during the winter.

Does salt/sugar raise the boiling point or lower it? I thought you added salt to water so that it could form bubbles faster, but if the boiling point goes up isn’t this kind of pointless?

When salt or sugar are dissolved in water, they raise the boiling temperature. It reduces the fraction of water molecules at the surface of the water, so that fewer leave each second. Because of this reduced leaving rate, the water has to get hotter before enough water molecules will leave each second to allow evaporation to occur inside the liquid (i.e. for the water to boil). As for the value of adding salt, sugar, or any other material to water to encourage boiling, that is a very different matter. Adding anything that can serve as a site for bubble formation will help the water to boil. Nucleating the tiny bubbles that eventually grow into the large bubbles we associate with boiling isn’t easy. Often it occurs at a hot spot in the pot, or near a defect on the pot’s inner surface. If there aren’t any hot spots or defects, then adding sharp objects will aid bubble formation. That’s why sprinkling sugar or salt into extremely hot water can help it boil. This boiling occurs before the sugar or salt dissolve. They are just acting as nucleation sites. You’d do just as well to add sand, which doesn’t dissolve at all.