When ice placed in water melts, does the overall volume of water increase, stay …

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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.

What happens to water in space?

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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.

How do Eskimos burn fires in their igloos without melting the snow and/or ice th…

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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.

How does an air pump work and how does the air pocket in a Nike Air or Reebok pu…

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How does an air pump work and how does the air pocket in a Nike Air or Reebok pump shoe keep its form? — MD, Toronto, CA

A typical bicycle pump uses a piston to squeeze air that it has trapped inside a cylinder. As you push the piston into the cylinder, the trapped air molecules are packed more tightly together and their pressure rises. Moreover, because you are transferring energy to the air by doing mechanical work on it, the air’s temperature also rises. Air always accelerates toward regions of lower pressure, so this pressurized air will tend to flow through any opening that leads to lower pressure—such as the inside of an underinflated bicycle tire. A one-way valve at the base of the cylinder allows this pressurized air to flow out of the cylinder through a pipe and enter the bicycle tire. Thus each time you push down on the piston, you pressurize the air inside the cylinder and it accelerates and flows toward the lower pressure inside the bicycle tire. As you pull the piston out of the cylinder, a second one-way valve allows new air to enter the cylinder from outside so that you can repeat this process.

In a pumped air athletic shoe, squeezing a rubber bulb packs together air molecules and increases their pressure. When the pressure is high enough, a one-way valve allows this pressurized air to flow into the underinflated air pocket of the shoe. A second one-way valve allows the bulb to refill with outside air when you stop squeezing the bulb. Once the air pocket has been filled with large numbers of air molecules, these molecules exert substantial outward forces on the inner surfaces of that air pocket. The more molecules there are inside the pocket, the more often they collide with the surfaces and the more force they exert on those surfaces. These outward forces from the air molecules allow the air pocket keeps its shape.

Why does a can of Diet Coke float on water while a can of regular Coke sinks? Do…

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Why does a can of Diet Coke float on water while a can of regular Coke sinks? Does that have to do with density and Archimedes’ law? —AB, Riverside, CA

Because regular coke contains large amounts of dissolved sugar, it is much denser than water or than Diet Coke (which has far less dissolved material). Although even Diet Coke is denser than water, as is the aluminum in the can, a can of Diet Coke contains enough gas bubbles to lower its average density to just below that of water. According to Archimedes’ law, an object with an average density less than that of the liquid in which it’s submerged will float upward. A can of regular Coke has an average density that’s greater than that of water, so it sinks.

Without gravity in space, what would happen to the recoil if a gun were shot off…

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Without gravity in space, what would happen to the recoil if a gun were shot off? — DZ, Illinois

Even in the depths of space, so far from any planet that gravity is virtually absent, a gun will have its normal recoil. When you push on something, it pushes back on you just as hard as you push on it. That rule, known as Newton’s third law of motion, is as true in empty space as it is on earth. Thus when the gun pushes the bullet forward, the bullet pushes the gun backward equally hard and you feel the gun itself jump backward as result. This recoil effect is the very basis for rocket propulsion—the rocket pushes its exhaust backward and the exhaust pushes the rocket forward. That’s why rockets can work outside the earth’s atmosphere and away from any celestial objects—the rocket only has to push on its exhaust in order to obtain a push forward.

How is chlorine gas used to disinfect water at treatment plants? – KM

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How is chlorine gas used to disinfect water at treatment plants? – KM

Chlorine molecules (Cl2) dissolve easily in water, where they react with water molecules to form hypochlorous acid (HOCl), chlorine ions (Cl–) and hydrogen ions (H+). Hypochlorous acid is a weak acid that partially dissociates into hydrogen ions (H+) and hypochlorite ions (OCl–). Studies have shown that it’s predominantly the hypochlorous acid molecules and the hypochlorite ions that disable and kill microorganisms. These molecules and ions diffuse onto and into the microorganisms and oxidize important biological components, such as the protein coats of some viruses, key enzymes in many bacteria, and the genetic materials in both bacteria and viruses. — Thanks to J. Symons for pointing out this mechanism to me and providing me with detailed reference materials.

How does a discotheque laser work and how could I build one?

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How does a discotheque laser work and how could I build one? — JPD, Spiennes, Belgium

If you are referring to a system that displays illuminated line drawings on a wall that move with the music, then building one is easy. You need a small isolated speaker—just the electronic device, not a whole speaker unit—that you can connect to the music amplifier. Place an elastic membrane over that speaker—a stretched sheet of thin rubber from a latex glove should work well. Then glue a tiny, front-surface mirror to that rubber membrane, choosing a point that is about midway between the middle of the speaker and its edge. A front surface mirror is one that is shiny on its top, so that light doesn’t have to go through glass before reflecting. A broken fragment of mirror, about 3 mm on a side, should work. Finally, shine the beam of a laser pointer onto the mirror and begin to play music through the speaker. The mirror will move with the music and the reflected laser beam will form pretty patterns on the wall.

Why is it bad to put metal in a microwave oven? – OR

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Why is it bad to put metal in a microwave oven? – OR

It isn’t necessarily bad to put metal in a microwave oven, but it can cause cooking problems or other trouble. Microwaves cause currents to flow in metals. In a thick piece of metal, these currents won’t cause problems for the metal. However, in thin pieces of metal, the currents may heat the metal hot enough to cause a fire. Metallic decorations on fine porcelain tend to become hot enough to damage the porcelain. But even thick pieces of metal can cause problems because they tend to reflect the microwaves. That may cause cooking problems for the food nearby. For example, a potato wrapped in aluminum foil won’t cook at all in a microwave oven because the foil will reflect the microwaves. The currents flowing in the metal can also produce sparks, particularly at sharp points, and these sparks can cause fires. In general, smooth and thick metallic objects such as spoons aren’t a problem, but sharp or thin metallic objects such as pins or metal twist-ties are.

What is the black holey stuff on the doors of microwave ovens? Is it for looks, …

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What is the black holey stuff on the doors of microwave ovens? Is it for looks, protection, or what? – K

The black holey stuff on the window of a microwave oven is a metal shield that keeps the microwaves inside the cooking chamber. Because the holes in this metal sheeting are so much smaller than the wavelengths of the microwaves (about 12 cm), the microwaves respond to the sheeting as though it were solid metal and they reflect almost perfectly. By keeping the microwaves inside the oven, this sheeting speeds cooking and protects you from the microwaves.