. How can you speed up the process of dissolution? — T
Dissolution occurs when a solid material is disassembled into its constituent atoms, molecules, or ions and these particles are carried around in a solvent liquid. Since the disassembly is a statistical process, with thermal energy allowing particles to leave the solid and chance allowing them occasionally to return to the solid again, anything you can do to accelerate the leaving process and impede the returning process will speed dissolution. Heating the solid and liquid will speed dissolution by making it easier for particles to leave the solid and harder for them to stick when they try to return. Keeping fresh, pure solvent in contact with the solid will also prevent molecules from returning.
. What is surface tension? — C
The molecules in a gas are independent and only collide with one another briefly before separating again. In contrast, the molecules in a liquid cling to one another so that they always remain in contact. While their mutual attachments aren't as strong as normal chemical bonds, these molecules have reduced their overall potential energies by moving as close as possible to one another. However, the molecules at the surface of a liquid have no neighbors on one side and don't benefit from the full energy-lowering effects of moving as close as possible to other molecules on all sides. Molecules at the surface of the liquid thus have higher potential energies than molecules within the liquid.
Because physical systems tend toward arrangements that minimize their overall potential energies, a liquid tends to minimize its surface area in order to minimize the number of high-energy molecules it has at its surface. This tendency to minimize surface area is the origin of surface tension in a liquid. The liquid behaves as though its surface were a taut elastic membrane. If you poke at a liquid, you can deform its surface but as soon as you stop pushing on it, it will spring back to its original flat or smoothly curved shape. That springiness is the result of surface tension.
. How does electricity work?
I'll assume that you are asking about moving or dynamic electricity, the type that lights the bulb in a flashlight (as opposed to static or stationary electricity). In that case, you are referring to a flow of electric charges that is generally called an electric current. This movement of electrically charged particles carries with it energy, both as kinetic energy (energy of motion) in the charged particles and as potential energy in the electrostatic attractions and repulsions of these particles. The particles typically acquire this energy from a battery. The battery pulls opposite charges away from one another and pushes like charges together. These actions increase the energy of those charges. The charges then rush through electrically conducting materials, generally metals, in order to bring opposite charges closer together. This flow of charges releases the energy given them by the battery.
In a flashlight, the batteries provide the charges with power and the light bulb makes use of the power. The charges first flow through the battery (which gives them energy), then through wires to the light bulb, then through the light bulb (where they give up their energy), and finally back through wires to the battery. The charges move in a loop—a circuit—so that they don't accumulate anywhere. They travel endlessly between battery and bulb, shuttling energy from the battery to the bulb. As is always the case in electric circuits, two wires connect the battery and bulb—one wire to carry charges to the bulb and one wire to return them to the battery to begin their trip over again.
. Does food coloring change the color of food?
Food coloring is a solution of dye molecules—molecules that absorb light of certain wavelengths extremely efficiently. When a particle of light—a photon—of the right wavelength encounters one of these dye molecules, an electron in the molecule uses the photon's energy to shift from one quantum level to another. The photon vanishes and the molecule is placed in an electronically excited state. The dye molecule's electron quickly returns to its original quantum level by releasing this extra energy as thermal energy within the molecule and its surroundings. Overall, the photon has vanished and the dye has become warmer. When you add these dye molecules to food, the dye gives the food a color by preventing that food from transmitting or reflecting certain colors of light. The dye simply absorbs those colors.
. How dangerous are plastics for storing and reheating food? I remember hearing that plastic containers can release carcinogenic materials when reheating food in the microwave. I also heard that plastics can release "plasticizers" into food even when cold. What studies exist about these dangers? — CVL, Fairfax, VA
While I'm not up to date on actual studies, I would think that most food storage plastics introduce very little contamination into the foods stored in them. We have become so concerned as a society about toxic chemicals in recent years that we tend to overreact much of the time. While the actual polymer molecules in most plastics are relatively inert and harmless, plastics inevitably contain some small molecules, either by accident or by design, that work their way into food. Even if some of these molecules are toxic or carcinogenic, the quantities involved are almost certainly insignificant. Modern chemical testing can detect incredibly small quantities of various chemicals and we panic every time we find them in our environment. But the societal cost of banning or avoiding all contact with or use of these chemicals may have hidden costs that are worse than the problem we're trying to solve. Moreover, I'll bet that many of the foods put in plastic containers are greater health hazards than the containers themselves.
. How does one prove that the earth is round?
There are many possibilities, so I'll suggest an intriguing method that is familiar to surveyors. While the overly simple technique I suggest isn't particularly practical, it is closely related to surveying techniques that are practical.
Take a very long string, say about 20 miles long, and attach one end of the string to a post. Now draw the string taut and walk all the way around the post while holding on to the other end of the string. If you measure the distance you walked while completing one full trip around the post, you would expect it to be related to the length of the string by a factor of 2 times pi because you learn in grade school that the circumference of a circle is 2 times pi times the radius of that circle. However, that relationship is only true if you're working on a flat surface. Since the earth is curved, the circumference of the circle around which you walk will be somewhat less than 2 times pi times the radius of the circle. That result is enough to prove that you're on a curved surface.
You can see this effect by performing the experiment I just suggested on the surface of a basketball. Take a short length of string and use it, together with a pin and a pencil, to draw a circle on the surface of the ball. If you measure the circumference of that circle and compare it to 2 times pi times the length of the string, the circle's circumference will be a bit shorter than expected. As with the earth, the basketball is a curved surface. The larger the circle you try to draw in this manner, the greater the discrepancy between 2 times pi times the radius and the actual circumference of the circle.
. How do bicycle shocks and suspension affect the performance of a bicycle? — D
When the wheels of a bicycle are attached directly to the frame of a bicycle, the wheels and frame must move together. When one of the wheels hits a bump, both that wheel and the frame must accelerate upward together. When this happens, the bump exerts a huge upward force on the wheel and everything, including the unfortunate rider, experiences a sudden upward acceleration. A sudden jolt of this sort is unpleasant—the seat of the bicycle pushes upward violently on the rider and the rider feels large forces throughout his or her body. Each body part pushes upward on the body part above it so that everything leaps upward.
To reduce the upward acceleration that the rider experiences, the direct connection between the bicycle wheels and the frame can be replaced by a spring suspension. When the wheel of a bicycle with a spring suspension encounters a bump, the springs compress and the force on the frame and rider is much smaller. The rider still accelerates upward, but not as rapidly as the wheel and without the abrupt jolt of a suspensionless bicycle. In fact, by the time the rider has begun to rise much, the wheel will probably have rolled back off the bump and the spring will return to its original shape. Overall, the rider will barely move at all and will hardly notice the bump.
But a spring suspension isn't perfect by itself. Suppose that the bicycle rolled over a curb and onto a sidewalk. This bump doesn't end—the pavement level rises permanently. When the wheel hits the curb, it rises suddenly and compresses the spring. But since the wheel never drops back to its original height, the only way for the spring to decompress back to its original shape is for the frame and rider to rise. And that's what happens. But the frame and rider don't stop moving once the spring has reached its original shape. They have upward momentum and they continuing rising. The spring begins to stretch upward now. Eventually the frame and rider stop rising and begin to descend again, but they continue to bounce up and down as though they were on a pogo stick. In effect, they are on a pogo stick. When a spring is compressed or stretch, it stores energy. If there is nothing to get rid of the energy stored in the bicycle's compressed or stretched spring, the frame and rider will continue to bounce up and down indefinitely.
To stop the bouncing (and prevent most of it in the first place), a bicycle with a spring suspension also has shock absorbers. These devices waste energy whenever the wheel and frame move relative to one another. Whether the spring is compressing or stretching, the shock absorber extracts energy from the wheel, frame, and spring, and turns that energy into thermal energy. As a result, the frame and rider don't bounce significantly after the wheel rides up and onto the curb. Similar issues occur in cars, where shock absorbers damp out the bouncing that can occur because the car body is suspended above the wheels on springs.
. Could you describe the process of an ice cube melting only from ambient (room) temperature? — JAS, Malta, NY
An ice cube is a crystal of water molecules. It is only stable up to a temperature of 32° F (0° C). When you place it in ambient temperature, it gradually warms until it reaches 32° F and then its surface begins to melt. As heat from the room flows into the ice cube, its molecules begin to separate briefly from one another and to exchange neighbors. These molecules lose their crystalline rigidity and structure and to become liquid. The liquid that forms is still at 32° F, but it has less order than the crystalline ice had.
As more heat flows into the mixture of ice and water, the ratio of solid ice to liquid water gradually changes and the fraction of liquid water increases. But only after all the ice has converted to water does the temperature of the water begin to rise significantly above 32° F.
. What makes a soap bubble round? — MZ, Massachusetts
The molecules in a liquid are touching one another and this touching reduces the molecules' potential energies. Separating the molecules and reducing the extent of their touching requires energy and is something that the liquid won't normally do on its own. The molecules at the surface of a liquid have fewer neighbors than they would have if they were in the body of the liquid. Those molecules thus have higher potential energies than they would have in the body of the liquid. To minimize the overall potential energy of a liquid, it naturally tends to minimize its surface area.
When a soap solution has trapped some air to form a bubble, that solution can no longer shrink into a tiny droplet. The air keeps bubble large and it can't avoid having lots of molecules on its surfaces, where they have higher than normal energies. But what the soap solution can do to minimize its total potential energy is to minimize the number of its molecules that are on the surface. The soap solution experiences what is called "surface tension"—an elastic tightening of its surface. This surface tension tends to minimize the surface area of the soap solution to minimize its potential energy. The soap solution minimizes its surface area around the trapped air by forming a spherical shape. A spherical shell makes the most efficient use of its surface area in enclosing a volume.
. Why is it that the times at which the moon sets and rises change so much from day to day?
The moon orbits the earth about every 27.3 days, so its position relative to the sun changes from day to day. Because of the moon's movement around the earth, the moon rises and sets about 1 hour later every day. When the moon is on the sun side of the earth, it rises at sunrise and sets at sunset. Fourteen days later, when the moon is on the side opposite the sun, it rises at sunset and sets at sunrise.