. In steam generation, wouldn't it be more economical to heat a small boiler and feed it just enough water for it to maintain its optimal steam generating temperature than to heat a huge boiler as is normally done? — MF, Gillette, WY
Not really. Once you have heated the water to its steam generating temperature, all of the heat you add goes into converting water into steam. The presence of more or less water just doesn't make any difference. The extra water requires no extra heat while the boiler is making steam. And having that extra water does act as a buffer in case you add too much or too little heat for a short while. That's probably why most boilers have a bit more water than they need over any short period of time. Furthermore, it's not always easy to add water to a boiler when the boiler's pressure is very high.
. Can I soften small quantities of tap water by merely adding table salt to it? Any idea how much salt to add for tape water that is medium to very hard? I want enough to use in a steam iron regularly? — HD, Kintnersville, PA
There are two issues here. First, hard water is water that contains dissolved calcium, magnesium, and iron salts. The metal ions in these salts interfere with soaps and detergents, causing soaps to form soap scum and preventing detergents from effectively carrying away fats and oils. The standard way to soften water is to exchange
sodium ions for the calcium, magnesium, and iron ions because sodium ions don't have such bad effects on soaps and detergents. Adding salt to hard water, as you propose to do, won't exchange sodium ions for the other ions. It will only add more metal ions to the water and the water will remain hard.
Second, a steam iron shouldn't use hard water because when hard water boils away as steam, it leaves behind all the calcium, magnesium, and iron salts as unsightly scale. Again, adding salt to your hard water will simply leave more scale on the insides of your iron or on your clothes. You need demineralized water, not soft water, for your iron. The best way to demineralize water is to distill it.
. How does desiccant absorb and hold water? — JP, Houston, TX
Water molecules from the air are continuously colliding with surfaces and sometimes one of those water molecules will stay attached to a surface for some amount of time. That water molecule forms a weak chemical bond with the surface and remains there until thermal energy knocks it back into the air. As a result of this occasional sticking, most surfaces have a thin layer of water molecules on them. Desiccants are materials that tend to keep those water molecules for a relatively long time and that have lots of surface area on which those water molecules can stick. However, the strongest desiccants react chemically with water molecules so that those water molecules essentially never leave.
. What effect does ice have on potholes? - AH
Water and ice are major contributors to potholes. When water flows into cracks in the road and then freezes, it tears the roadway apart. That's because ice takes up more room than the water from which it's formed—ice is less dense than water. Since the water expands as it freezes, it enlarges the cracks that contain it and gradually breaks up the roadway.
. How fast can glaciers move? — SF, Burton, OH
Glaciers move at a variety of rates, ranging from inches per year to many feet per day. Currently there are very few "galloping" glaciers (those that move many feet per day) and most are either stagnant or retreating.
. What happens when salt is added to water? If I mix 1 cup of salt with 1 cup of water, will I end up with 2 cups of solution? - RT
As a crystalline solid, salt consists of a beautiful cubic lattice of sodium atoms that have lost one electron to become sodium positive ions and chlorine atoms that have gained one electron to become chlorine negative ions. The crystal is held together by the attractive forces between these oppositely charged atomic ions. When a salt crystal dissolves in water, it decomposes into individual sodium positive ions and chlorine negative ions that are then carried about by shells of water molecules. Water molecules are electrically polar, meaning that they have positively charged ends and negatively charged ends. The water molecules line up around a positively charged sodium ion with their negatively charged ends inward and carry that ion about. Similarly, water molecules line up around a negatively charged chlorine ion with their positively charged ends inward and carry that ion about. Whether you will end up with 2 cups of solution after mixing 1 cup of salt and 1 cup of water depends on how tightly the atoms and molecules pack together in each case. Remember that your 1-cup of salt contains a fair amount of air between the salt grains. You'll have to try it to find out the answer—I'm not sure what the answer will be.
. Why does popcorn pop? - AB
Inside the hard, dry hull of a popcorn kernel is a portion of moist starch. When you heat the kernel well above the boiling temperature of water, the water in the starch converts to hot, high-pressure steam. The hotter this steam gets, the higher its pressure rises and the stronger the outward forces it exerts on the hull. Eventually, the hull rips open under the stress and exposes the starch to the low-pressure air around it. The pressurized steam then pushes the starch outward, expanding it to many times its original size. The kernel "pops."
. How does steam work? — SS, Nairobi, Kenya
Steam is the gaseous form of water. When the water molecules in liquid or solid water have enough thermal energy, they can break free of one another and become independent particles. Even at room temperature, the air you are breathing is several percent water molecules. But at higher temperatures, the rate at which water molecules leave the surface of solid or liquid water increases so much that these water molecules can form a dense, high-pressure gas. This gas is called steam.
. Why does regular water freeze faster than salt water? — CD, Crown Point, IN
When salt dissolves in water, its individual sodium positive ions and chlorine negative ions are carried about by the water molecules. Each of these ions is wrapped in a solvation shell of water molecules. These solvation shells and the salt ions themselves interfere with the water's ability to crystallize into ice. The ice crystals that form when salt water freezes rarely include the salt ions so the water molecules must abandon the salt ions in order to crystallize. Because of the attraction between the salt ions and the water molecules, and because of the loss of randomness that comes with forming pure ice crystals in the midst of salty water, you must lower the temperature of salt water below the freezing temperature of pure water before that salt water will begin to freeze into ice. When ice does begin to form, it will be relatively pure water crystals and the remaining water will become increasingly saltier. If you're ever lost in the winter without a supply of fresh water, look for sea ice—even though it forms from salt water, it contains very little salt.
. I know that adding salt to water will raise its boiling point, which would seem to imply that it would take longer to come to a boil. But does it take longer? As a cook I've always been told to add a little salt to the water to bring it to a boil faster. It seems to work or is that just the power of suggestion? If it does boil faster, why does it? — ND, Ashland, OR
I think that power of suggestion is at work here. Salt water boils at a higher temperature than pure water. Thus if you set two identical pots of water, one salty and one pure, on burners and heat them at equal rates, the pure water will reach its boiling temperature first.
However, water boils more vigorously when it contains impurities that can nucleate bubbles of water vapor. Just before the water in a pot reaches a full boil, its temperature is often nonuniform and there are some regions that are boiling while others aren't. The edges and corners of crystals are particularly good at nucleating bubbles, so that tossing salt grains into such nearly boiling water will encourage its hot regions to boil more vigorously, at least until those salt grains dissolve away. The appearance of bubbles makes you think the water is at a full boil when it really isn't.