Читать книгу The Handy Chemistry Answer Book - Justin P. Lomont - Страница 20

We commonly use the word vacuum to refer to any area that is of relatively low pressure. The air in areas of higher pressure will spontaneously move into areas of lower pressure, and this is the principle upon which a vacuum cleaner operates. A fan is used to push air out of the vacuum cleaner, creating an area of lower pressure behind the fan. Air from the outside then comes rushing in to reduce the gradient in pressure, carrying dust and dirt along with it. Since the fan is running continuously, the gradient in pressure is constantly being maintained, so the vacuum cleaner is able to keep running even though air is always flowing in.

Can light move through a vacuum?

Yes. Unlike sound, light is an electromagnetic wave, so no molecules are needed to propagate the wave. But you knew this—the Sun’s light crosses the vacuum of space to reach Earth each and every day.

What are some chemical reactions that we can observe with our naked eye?

There are lots of chemical reactions that we can easily detect by sight. Some examples are the formation of rust on metal, wood burning, fireworks exploding, silver becoming tarnished, or baking soda and vinegar reacting with one another.

What makes paper towels absorbent?

Paper towels are composed of cellulose fibers, which are polymers that contain many sugar monomers. These sugar monomers can interact strongly with water molecules, which is what makes paper towels so useful for cleaning up when something gets spilled.

What is an electric current?

Electric currents are the flow of electrons through a material. The electricity that comes out of the wall in your home or office is just the flow of electrons through a wire.

An explosion is a dramatic example of a chemical reaction we can see with our naked eyes.

What makes a material a good conductor of electricity?

Materials that are good conductors of electricity have many “free” electrons. Here we mean free in the sense that they are not bound strongly to a given atom or molecule. Metals are often good conductors of electricity. Whether or not a material has freely conducting electrons is related to the detailed electronic structure of the material, and a thorough description would be beyond the scope of this book. Basically, the more electrons that are free to move around in a material, the more easily it can carry current, and the higher its conductivity.

What makes a rubber band stretchy?

Rubber bands are made of long polymer molecules. These are all tangled together, and you could think of them as being similar to a bunch of interwoven springs. The polymers can be stretched to a more extended state, which is what allows a rubber band to stretch without breaking. More accessible configurations exist, however, when they are in a more contracted state, which means that the more contracted states have higher entropy (see “Physical and Theoretical Chemistry”) and for this reason are more favorable. This is what makes the rubber band want to contract and is what gives rise to its elasticity. (Check out “Polymer Chemistry” for more questions on polymers.)

How are soft drinks carbonated?

Soft drinks are carbonated using pressurized CO₂ gas and a siphon that introduces the pressurized gas into the water or soda. The CO₂ gas is forced into the liquid at a concentration beyond that which could exist under atmospheric conditions and then the container is sealed, preventing it from being released. That’s why if you leave a glass of soda sitting out, it will go flat; the CO₂ escapes into the air.

Why might a soft drink freeze when you open it?

Remember that the freezing point of a solution is decreased by having anything dissolved in it. When a soft drink is opened and CO₂ rushes out, less CO₂ is present in the solution, causing the freezing point of the solution to increase. So at a constant temperature, the increase in freezing point causes the soda to freeze. If you haven’t seen this happen, you can try to make it happen by placing a soda in the freezer for a while, but be careful not to leave it in too long or it might explode!

Helium balloons float because the gas inside the balloon is much less dense than the surrounding atmosphere.

Why do helium balloons float?

Helium balloons float because helium is less dense than air, so gravity pulls down on air more than it does on the helium balloon. The difference in density is enough that the displaced air supports the weight of the balloon and makes it float…up, up, and away.

What is dry ice, and why does it “evaporate” from the solid phase?

Dry ice is solid carbon dioxide (CO₂). At constant atmospheric pressure we see that an increase in temperature will bring it from the solid phase directly to the gas phase. This process is called sublimation, and it takes place at −78.5 °C.

How far away can one molecule “feel” another?

Molecules “feel” one another through the intermolecular forces they exert on one another. These typically span lengths that are a little longer than the length of chemical bonds, or something in the neighborhood of 5 × 10⁻¹⁰ meters.