Tag: water steam and ice

How does reverse osmosis work? – MC

Normal osmosis in water is a process in which pure water flows through a semi-permeable membrane to dilute a concentrated solution on the other side. It is driven by statistics—it’s much more likely for a water molecule on the fresh water side to pass through the membrane than it is for a water molecule on the concentrated solution side to pass through the membrane. There are simply more water molecules trying to cross the membrane from the fresh water side! In fact, water molecules will continue to flow from the fresh water side to the concentrated solution side until the solution has been highly diluted or an accumulation of pressure on the solution side slows the passage of water and brings it to a halt.

Reverse osmosis occurs when the pressure on the solution side is raised so high that the movement of water reverses directions. If you squeeze the concentrated solution hard enough, you can drive additional water molecules from that solution through the semi-permeable membrane and into the fresh water on the other side. The raised pressure on the solution changes the statistics, making it more likely for water molecules to go from the solution side to the fresh water side. This technique is used to purify water in homes and to desalinate water in desert countries.

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.

Is it true that you can get lead poisoning in your home more easily from hot water than from cold water? — WH, Erial, NJ

Yes, assuming that your home has either lead pipes or copper pipes that were joined with lead-containing solders. That’s because lead compounds are more soluble in hot water than they are in cold water. The amount of lead that was permitted in pipe solders has diminished over the years until now, when pipe solders can’t contain any lead at all. While very little lead actually leaches out of the solder joints and enters the water, the effect is slightly more significant in hot water pipes than in cold water pipes. That’s why it’s recommended that you not use water from hot water pipes in cooking.

How does a dehumidifier know when to turn on and off? The one I bought from Sears doesn’t use the “wet-bulb/dry-bulb” method (of which I could use a better understanding, too). How does its on-off switch work? — JS, Amherst, NY

Most humidity sensing switches or “humidistats” use the expansion or contraction of certain materials to measure humidity. The more humid the air is, the more water molecules there will be in those materials and their shapes and sizes will be affected. For example, human hair becomes longer when wet and it makes an excellent humidity sensor. On a dry day, a hair will contain relatively few water molecules and its length will be shorter. On a humid day, the hair will contain more water molecules and its length will be longer.

A wet-bulb/dry-bulb system measures humidity by looking at the temperature drop that occurs when water evaporates. As water evaporates from the bulb of the wet thermometer and the bulb’s temperature drop, the rate at which water molecules leave the bulb’s surface decreases. The bulb temperature drops until the rate at which water molecules leave the bulb is equal to the rate at which water molecules return to the bulb from the air. At that point, there is no net evaporation going on. In humid air, water molecules return to the bulb more often so that this balance is reached at a higher temperature than in dry air. The wet bulb temperature is thus warmer on a humid day than it is on a dry day.

Is there any way to make a homemade fog machine, like they use in clubs? — JW, Westport, CT

While it’s pretty clear that fog machines fill the air with tiny water droplets, I’m not sure how all of them work. Some probably use high-frequency sound waves to break up water into tiny droplets and then blow these droplets into the room with a fan. That technique is used in some room humidifiers and you can see a stream of fog emerging from them as they operate. An easier way to make fog is to mix water and liquid nitrogen. While liquid nitrogen is harder to find, all you have to do is put them together and they’ll start making fog. The boiling nitrogen shatters the water into tiny droplets, which flow out of the mixture in a layer of cold nitrogen gas.

When ice placed in water melts, does the overall volume of water increase, stay the same, or decrease? — AB, Riverside, CA

The volume decreases. That’s because ice at 32° F (0° C) is less dense than water at that same temperature. As the ice melts to form water, the density of its molecules increases and the overall volume of material decreases. This situation, in which the solid form of a material is less dense than the liquid form of that material, is virtually unique in nature and explains why ice floats on water.

How do Eskimos burn fires in their igloos without melting the snow and/or ice that the igloos are built out of? I know they use holes in the top to vent the smoke and some heat, but what about the ambient heat? — AK, Bridgeport, CT

To avoid melting the ice, the Eskimos must keep the ice below its melting temperature. That means that they can’t add heat to ice indefinitely. But while a central fire will always deliver some heat to the ice of the igloo, the ice of the igloo will also tend to lose heat to colder air outside. As long as the ice loses heat at least as fast as the fire delivers heat to it, the ice won’t become any warmer and it won’t melt. If heat loss to the outside is fast enough, it may be possible to have the air inside the igloo warmer than 32° F (0° F) and still have the ice remain colder and frozen. However, I’m sure that the average air temperature in the igloos can’t be made much warmer than freezing without causing trouble. Still, the air right around the fire can be quite warm without threatening the walls. The area under the fire must be carefully insulated to avoid melting the underlying ice—which must continue to lose heat as rapidly as it arrives from the fire.

What happens to water in space? — DZ, Illinois

That depends on the water’s temperature. At extremely low temperatures, ice remains stable indefinitely. That’s why comets that are as old as the solar system have been able to hold on to their water despite having almost no gravity. But at more moderate temperatures, ice and water both slowly lose water molecules. These water molecules evaporate (or sublime, in the case of ice) and drift off into space. Because there’s no air pressure in space to prevent evaporation from occurring inside the body of water, water will actually boil at any temperature. That’s what boiling is: evaporation into steam bubbles located inside the water. Atmospheric pressure normally smashes these bubbles as long as the water temperature is below 212° F (100° C), but in empty space the bubbles form without opposition at any temperature.

If you have a glass of water that is real cold but not frozen, can the addition of one normal ice cube make it all freeze? Can I do this in the kitchen? – D

The answer to both questions is yes. If you begin with very pure, dust-free water in a very clean glass, you should be able to supercool it below its normal freezing temperature of 32° F (0° C). That’s because water has difficulty forming the initial seed crystals upon which ice can grow. If you then add an ice crystal to the supercooled water, it should begin to freeze rapidly. While I have never done this myself, it shouldn’t be too hard. You should probably use distillated and filtered water and a brand new glass that you’ve cleaned thoroughly. Cover the water to keep out dust. Cool it carefully through 32° F in the freezer and then add a tiny ice chip. The water should begin to crystallize around that ice chip. A simpler example of this sudden freezing phenomenon is a heat pack—one containing sodium acetate. At room temperature, it contains a supercooled solution of sodium acetate that is unable to freeze spontaneously. When you press a button in the pack, you trigger the crystal formation and the whole pack freezes in seconds. The crystallization process releases enough thermal energy to keep the pack hot for hours. Incidentally, ski resorts regularly seed the water they use to make artificial snow with molecules that initiate crystal growth to avoid forming supercooled liquid. Doing so greatly enhances the amount of snow they make.

What happens to the temperature of water when ice is added? — OS, Havana, FL

Whenever a glass contains a mixture of both ice and water, the temperature of its contents will be 32° F (0° C). That’s because liquid water isn’t stable below this temperature and solid ice isn’t stable above this temperature. The two can coexist stably only at 32° F.

When you first mix the water and ice, the water is likely to hotter than 32° F and the ice is likely to be colder than 32° F. Heat flows from the water to the ice—from hotter to colder—and soon one of them reaches 32° F. While heat may then continue to flow from the water to the ice, the one that’s at 32° F won’t change temperature any more. Instead, it will begin to turn into the other form. For example, if the ice reaches 32° F first, it will begin to melt as heat flows into it and turn into water at 32° F. Or if the water reaches 32° F first, it will begin to freeze as heat flows out of it and turn into ice at 32° F. Pretty soon, both the ice and the water will be at 32° F and there they will remain so long as both are still present in the glass.

From a molecular standpoint, ice is an orderly crystalline solid in which the water molecules are neatly arranged in rows, columns, and stacks. Liquid water is a disorganized soup of water molecules that are regularly changing neighbors and moving about, though always in contact with one another. As you add heat to cold ice, its molecules jiggle more and more vigorously against one another until they finally begin to move about as liquid water, a process we call “melting.” As you remove heat from warm water, its molecules move about less and less rapidly until they finally begin to cling permanently to one another as solid ice, a process we call “freezing.”