"Matter, though divisible in an extreme degree, is nevertheless not infinitely divisible. That is, there must be some point beyond which we cannot go in the division of matter. The existence of these ultimate particles of matter can scarcely be doubted, though they are probably much too small ever to be exhibited by microscopic improvements. I have chosen the word atom to signify these ultimate particles."-John Dalton (1766-1844)
The nature of all the matter around you depends on the atoms of which it is made. Solid materials are solid due to strong interactions between atoms of the material. Hot materials are hot because the atoms are moving very rapidly. Some metals carry an electric current easily because their atoms lose electrons easily. Every property of matter can be explained through an understanding of the small particles of which all matter is made.
Why does spreading salt keep a road free of ice?
If you live in a place with freezing winters, at some point you have probably followed a truck spreading salt on the road. Shortly after the truck passes, ice on the road melts, making it safer to drive. Why would putting salt on top of ice make it melt?
Actually, the salt does not really melt the ice. Forces between molecules of water cause them to be attracted to one another. As a substance, such as water, cools, its molecules move more slowly because they have less energy. If the energy of the molecules is less than the forces that cause them to be attracted to one another, they cling together, forming a solid. Ice forms at 32°F because that is the temperature at which the water molecules form a solid crystal. However, not every molecule of water is tightly linked. Some are constantly losing energy and freezing while others are gaining energy and melting. This process occurs continually at the surface of a piece of ice. If water freezes and ice melts at the same rate, the ice appears to be unchanging.
When salt is added to the top of the ice, it dissolves and its particles don’t fit neatly into the crystal pattern at the surface. The salt tends to prevent the freezing of the water. However, the salt does not affect the melting of ice, which continues at the same rate. Because the melting rate is now faster than the freezing rate, the amount of ice decreases and the amount of water increases. It appears as though the salt caused the ice to melt.
Eventually, if the mixture of salt and water gets cold enough, it will freeze, so the effect of the salt is a lowering of the freezing point of water. This is an important consideration. On a really cold night, salting the roads does not help because the saltwater still loses enough energy to slow down and freeze. Below 2°F, even a solution with 20 percent salt will freeze. At very low temperatures, the trucks spread sand or gravel, which improves traction on top of the ice without melting it.
Salt is not the only substance that lowers the freezing point of water. In fact, anything that can be dissolved in water will have the same effect. When you add ethylene glycol to the radiator of your car, it lowers the freezing point of the water in the radiator, preventing the damage that would occur from running the engine without coolant if the water were all frozen in the radiator.
An ion is an atom that has an electric charge due to the gain or loss of an electron. If the atom gains an electron, its ion has a negative charge. If it loses an electron, its ion has a positive charge.
The freezing point change does not vary based on what material dissolves in water. Instead, it depends on the number of particles dissolved in a specific volume of water. Ordinary salt, sodium chloride, dissolves to form two particles, called ions, for each unit of salt. A different compound, calcium chloride, is often used on ice. When calcium chloride dissolves, it forms three ions, so a smaller amount is needed to obtain the same effect. In addition, calcium chloride is less likely to damage concrete surfaces and living plants.
Why does a rug feel warmer than a tile floor?
Walking barefoot across a bathroom on a cool night, you will notice a big difference in the feel of a tile floor and a rug on top of it. While the tile feels cold against your feet, the rug is quite comfortable. You know that the temperature of both surfaces should be the same as the temperature of the air around them, so why does one feel warm and the other cool?
A thermometer will show that rug and floor are indeed at the same temperature. However, our sensation of how hot or cold something is depends on more than just its temperature. Heat is actually a transfer of energy that always flows from a higher temperature to a lower temperature. An object feels cold when energy is transferred as heat from your skin to the object.
Although the difference in temperature between your skin and the floor is the same as the difference in temperature between your skin and the rug, the transfer of heat is not the same. The two materials have a different thermal conductivity.
The thermal conductivity of a material is its ability to transfer heat by the collision of atoms and molecules. Materials such as metals, in which the atoms are tightly packed, generally have a higher thermal conductivity than materials such as air, in which the particles are farther apart.
Every atom and every molecule is in motion all the time—moving around and bumping into nearby atoms. When heat flows into an object, its temperature increases, which means that these particles move faster. As these atoms and molecules move, they collide with others. Each collision transfers energy, again as heat. Materials in which the particles collide very frequently transfer heat throughout the object more efficiently than materials in which there are fewer collisions. A greater number of collisions means a higher thermal conductivity, because heat is rapidly carried away through the material.
In order to understand the importance of thermal conductivity in everyday things, think about the difference in the effect of air temperature and water temperature on your body. On an 80°F day, you jump into a swimming pool whose water is also at 80°F. The air feels warm but the water feels cool. Why? Air has a very low thermal conductivity, so very little heat is transferred from your skin to the air. Water has a much higher thermal conductivity, so you feel cold when you jump into the water.
The low thermal conductivity of air is one reason that the carpet does not feel cold. The fibers of the carpet have a lower thermal conductivity than the ceramic of the tile. The greatest obstruction to heat flow, however, is the air that is trapped within the pile of the carpet.
"If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, if you wish to call it that) that all things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another."
How can diamond and graphite both be pure carbon?
Most pure elements occur in one form that has particular properties that are always the same. Pure gold is always a shiny, yellow metal; nitrogen is a colorless, odorless gas; chlorine is a corrosive, greenish gas. How can diamond and graphite, which have such different properties, both be forms of pure carbon?
The difference in types of carbon is due to the arrangement of atoms in large crystals or sheets. Because a carbon atom is able to make as many as four chemical bonds with other atoms, carbon atoms can join together in large arrays.
Pencil "lead" does not contain the metal lead at all. It is a mixture of graphite and clay. Graphite was originally called lead because the people who first discovered natural deposits, in the 1500s, thought that they had discovered a lead deposit. The name graphite comes from the Greek word meaning "to write."
The difference in the properties of the forms of carbon is built into the way the atoms are arranged. Graphite consists of broad, flat sheets in which each atom forms bonds with three other carbon atoms. In a sample of graphite, these sheets are stacked, one on top of another. A piece of graphite that is 1 millimeter thick consists of about 30,000,000 layers. The forces that hold these layers together are very weak compared to chemical bonds, so the layers tend to slip and slide past one another. As a result, graphite has a greasy feel and makes a good lubricant.
In another form, carbon is a powdery black solid, known as lampblack, which is commonly used in inks or a chunky solid—charcoal. On an atomic scale, lampblack and charcoal resemble small sheets of graphite that are randomly arranged rather than layered.
Carbon is not the only element that occurs in several different forms. When two oxygen atoms are bound together as a single molecule, which we also call oxygen, they are an essential substance for humans. When three oxygen atoms are bound together to form ozone, they make a toxic gas.
In a diamond, on the other hand, carbon atoms form bonds with four other atoms in a tetrahedral pattern. The bonds between atoms are very strong and do not break easily. The strength of these bonds makes diamond the hardest known natural substance.
A fourth form of carbon was discovered in 1985. In this form, carbon is arranged in large molecules that look something like soccer balls. Because a model of the first such molecule to be discovered— a 60-carbon-atom ball—resembled a geodesic dome, the molecule was named buckminsterfullerene after the architect who designed that type of dome. A number of carbon molecules have been discovered since and, as a class, they are known as fullerenes. Scientists who study fullerenes hope to use some of their unusual magnetic and electrical properties and to trap other atoms or molecules inside the molecule in order to obtain new properties.
How do soaps and detergents work?
Try washing your children’s clothes after they’ve played outside all day and eaten peanut butter sandwiches—without using any detergent. It is reasonable to predict the shirts will come out of the washer in something less than perfectly clean condition. And what about washing those dirty hands without any soap? How do soaps and detergents make washing more effective? And what is the difference between soap and detergent, anyway?
To start, let’s take a look at why dirt sticks to us, our clothes, and everything else that gets dirty. Sometimes, particles of dust just sit on a surface. A good dust cloth will take them off. Then there is the stuff that sticks—won’t rub off, shake off, or rinse off with pure water. This dirt is the real challenge, because it is held in place by oil or grease—things that seem to cling tightly to skin, cloth, and even hard surfaces like dishes. Water rolls away from oil or grease with no effect.
Molecules of water are polar, which means that they have small electrical charges— positive on one end of the molecule and negative on the other end. Oil molecules, made of carbon and hydrogen, do not have electrically charged ends. Electrons in the bonds that make up the molecules are attracted just about equally by the two atoms involved in the bond. So, if you put water and oil (or grease, which is basically made of great big oil molecules) together, the water molecules congregate in one place and the oil molecules stick together elsewhere. You can remove oil with a nonpolar solvent, such as paint thinner, alcohol, or kerosene, but these are not usually compatible with things like skin and electrical appliances.
This is where soap comes in. Soap molecules have an interesting mix of polar and nonpolar parts. One end of the soap molecule has a bond between an oxygen atom and a metal atom, usually sodium or potassium. This bond is very polar, and when the soap molecule is placed in water it forms ions. The metal ion wanders off and interacts with water molecules while the negatively charged part of the soap molecule attracts the positive ends of water molecules and dissolves in the water. The rest of the molecule is made up of a long string of carbon atoms with hydrogen atoms attached to the chain. This sounds like the description of the oil molecules—and it is. Whenever one of these long nonpolar strands meets an oil molecule, they immediately latch onto one another.
The end result of these attractions is that the oil-like ends of the soap molecule are attracted to oil and grease. Soap molecules surround the nonpolar parts of dirt, with their polar ends sticking out in every direction. These polar ends are dissolved in the water, so the whole ball of dirt and soap leaves the fabric or your skin and, along with the (now grungy) water, heads down the drain.
Dry cleaning of clothing, because of its name, is often assumed to use a process without solvents. Actually, a dry cleaner removes greasy or oily dirt by tumbling clothes in a nonpolar liquid solvent which dissolves the grease and oil. It is called dry because no water is used. Because the solvent matches the dirt in polarity, detergents are not needed.
Soap is made by reacting animal or plant fats with an alkali, such as lye. Detergents are similar to soap in that they have a long nonpolar chain and a very polar end, but they are generally made from petroleum products. For some purposes, though, detergents work better than soap. Some water systems provide "hard" water, which means the water contains high levels of calcium or magnesium ions. When soap is added to hard water, these ions react with the soap to make a solid precipitate, which is then not able to dissolve dirt in water. This solid can be seen as a bathtub ring. Detergents do not form a precipitate, so they can clean in hard water as well as soft water.
A precipitate is an insoluble compound that forms when two or more substances in a solution react with one another. The precipitate does not remain in the solution.
How are different colors produced in fireworks?
Everyone loves a fireworks show on the Fourth of July. The loud booms of the exploding rockets accompany bright flashes of light. The colors are an important part of the display. You just know that at some point in the show you will see red, white,and blue flashes flying across the sky. How do fireworks manufacturers design their rockets to show the desired colors?
Each time one of the fireworks explodes, a carefully designed mixture of explosives, combustible fuel, and compounds containing metals provides the desired effect. The key to fireworks color lies inside the atoms of the metal atoms added to the rocket. The colors are produced by heating the metal atoms, which then emit light in a color that is characteristic of the metal.
Every atom consists of a nucleus surrounded by rapidly moving electrons. The energy of the electrons is not random, though. Every type of atom has a unique set of possible energy levels for its electrons. Under normal conditions, the electrons occupy the lowest allowed energy levels. The atom is then in its ground state. If an atom absorbs enough energy, in this case in the form of heat, the added energy moves one or more of its electrons to a higher level. When electrons are at higher energy levels than the ground state, the atom is in an excited state.
Atoms don’t remain excited for very long. The electron quickly sheds the extra energy to get back to its ground state. The move from a higher energy level to a lower energy level is always accompanied by the release of a unit of energy as light.
An emission spectrum is the electromagnetic radiation emitted by the atoms of an element when they are heated. Each element has a characteristic emission spectrum that can be used to identify that element.
For any given atom, there are many clearly defined energy changes that can occur when electrons lose energy. One specific wavelength (or color) of light is always emitted with each change. Using a prism, the colors produced by each element can be separated to make an emission spectrum. Emission spectra can be used to identify the elements in a chemical sample. While most of the changes release light in wavelengths that we cannot see, some energy changes produce light that falls into the visible spectrum of light.
Pure metals are used for a few effects. For example, burning magnesium metal produces a brilliant white light. For most colors, however, the fireworks designer uses a salt that includes the metal. What metals produce the colors of fireworks? Here are some of the common fireworks colors and the metals added to produce them:
|
Color |
Metal(s) |
|
red |
lithium or strontium |
|
orange |
calcium |
|
yellow |
sodium |
|
green |
barium, thallium, zinc |
|
blue |
copper, lead |
|
purple |
cesium, rubidium |
|
white |
magnesium |
|
silver |
aluminum or titanium |
|
gold |
iron |
You can investigate some of these effects in your own fireplace. Try tossing a few crystals of table salt (sodium chloride) into the flames. You will immediately see a brilliant yellow color in the flames as the sodium atoms become excited and then relax to the ground state. Or you can try using a sodium-free salt substitute, potassium chloride, which will make a reddish-purple colored flame.
The element helium was discovered in the sun before it was discovered on Earth. In 1868, astronomers looked at a spectrum of the sun during an eclipse. They noticed light at wavelengths which did not correspond to the emission spectrum of any element known at that time and concluded that they had observed a new element. The element was named helium based on the Greek name for the sun, helios.
What causes popcorn to pop?
You toss a bag with a few hard, yellow kernels into the microwave, turn on the power, and wait about three minutes. When you pull the bag out of the microwave, it is completely full of fluffy, white popcorn. Each popped kernel has expanded to about 50 times its original volume. How does something change so drastically with just a bit of added energy from the oven?
Like so many other things, you have to look at the tiny particles—the molecules of water and starch—to figure out what happens when popcorn changes. One clue to what is going on is that not just any corn will pop. There has to be just the right amount of moisture in the kernel or nothing happens. Another clue is in the kernels that remain hard and unpopped. A close look will reveal that many of them are cracked or otherwise damaged.
Fast Facts
Popping corn is not a recent idea. Archaeologists have discovered unpopped (and sometimes still viable) kernels in many Native American sites. The oldest known popcorn was harvested more than 5,000 years ago.
Three things are needed in a popcorn kernel to make it function correctly when heated. First, the kernel is filled with starch. Starch is made of long molecules that string together hundreds or thousands of small sugar molecules. The second component is a hard cellulose shell on the outside that is pretty much impervious even to small molecules like water. The third component is water in just the right amount.
Inside a good kernel of popcorn is water, accounting for about 14 percent of its total mass. When you turn on the heat, the water inside the kernel changes from its liquid form and becomes steam. The big starch molecules in the kernel surround thousands of tiny pockets of steam, miniature bubbles surrounded by strands of starch. As the starch cooks, it expands and as the steam gets hotter and hotter, it also expands. The cellulose shell around them does not change, though, so inside the shell the starch and steam build a big head of pressure. Finally, the pressure ruptures the outer shell with a loud pop. The little bubbles expand, the steam escapes, and left behind is cooked starch—its familiar crispy, fluffy, white blobs.
It sounds pretty simple, but a lot of things can go wrong. The water must heat to about 400°F very rapidly. If it heats too slowly the starch cooks without forming the bubbles and you get a small dense chunk of starch instead of a big puff. If there is too little water, the kernel never pops, but if there is too much, it breaks too soon without cooking the starch sufficiently, leaving a hard cracked kernel. Finally, the popcorn kernel needs a good, strong shell that has no flaws or cracks, which would let the steam escape without building pressure. If your supplier has been really careful, only about 4 percent of your kernels will fail. On the other hand, if the popcorn has not been processed correctly, you may find almost half of them staring at you from the bottom of the bowl.
"One of the wonders of this world is that objects so small can have such consequences: Any visible lump of matter—even the merest speck—contains more atoms than there are stars i n our galaxy."
How can carbon be used to find the age of an object?
Archaeologists frequently study the sites of ancient villages or encampments to learn more about the people who lived at that time. One major challenge, though, is to determine exactly when the village or camp was populated. If there are wooden tool handles, ashes from a campfire, or bones from an ancient meal, then scientists can analyze the carbon in the artifact to determine its age. How can the carbon in an object be used as a calendar?
When we discuss chemical reactions of atoms, we focus on the electrons that move around the nucleus. In order to find the age of an object, we must look past the electrons, into the nucleus itself. An atomic nucleus contains protons and neutrons. Although the number of protons is always the same for all the atoms of any given element, there is some variation in the number of neutrons. Some combinations of protons and neutrons are very stable, but other combinations change over time, a process known as radioactivity.
Radioactivity is the spontaneous decay of the nucleus of an atom. Radioactivity is a process that rearranges the protons and neutrons into a more stable configuration. Normally, it occurs by emitting an alpha particle—two protons and two neutrons—or a beta particle—one electron. Elimination of an alpha particle reduces the number of protons by two, forming an atom of an element whose atomic number is two less than that of the original atom. Elimination of a beta particle is accompanied by the change of one neutron into a proton, increasing the atomic number by one.
As the nuclei of radioactive atoms decay, particles are thrown out of the nucleus and the number of protons changes. Radioactive materials are dangerous because these particles can damage the tissues of living things.
Most carbon atoms have six protons and six neutrons. This type of carbon is called carbon-12 for the total number of particles in the nucleus. Some carbon atoms, however, are carbon-14, a type, or isotope, of carbon that has six protons and eight neutrons. Carbon-14 is a radioactive isotope of carbon, which decays by emitting a beta particle and changing to nitrogen-14.
The key attribute of radioactivity, for dating purposes, is that it occurs at a predictable rate. If you start with a given amount of carbon-14, half the atoms will change to nitrogen over a period of 5,730 years. Half of the remaining carbon-14 atoms will decay during the next 5,730 years. This period is known as the half-life of carbon-14. The half-life of an isotope is a characteristic of that isotope. Some half-lives are a fraction of a second, while others are billions of years, but for any given isotope the half-life is constant.
Carbon-14 is formed in the atmosphere. Radiation from space converts a small amount of nitrogen into carbon-14 through a nuclear reaction. Because this is a continual process and the breakdown of carbon-14 is a continual process, the ratio of carbon-12 to carbon-14 atoms is fairly constant. In chemical reactions, the two isotopes act the same way. This means that plants, which use carbon dioxide from the atmosphere, will therefore have the same ratio of carbon isotopes. Animals that eat the plants will also have the same ratio of carbon atoms. This ratio stays the same during life because living things are constantly receiving new carbon from the environment.
It might seem that radiocarbon dating could be used to find the age of any fossil— for instance, the fossils of dinosaurs. However, radiocarbon dating can be used only to find the age of objects up to about 50,000 years old. Beyond that age, there is not enough carbon-14 left for an accurate measurement.
Radiation dating, using isotopes of uranium with very long half-lives, has been used to find the age of rocks that are several billion years old.
When a plant or animal dies, however, it no longer takes in new carbon. The ratio of carbon-14 to carbon-12 begins to change as the carbon-14 decays and is not replaced by new carbon-14. In order to find out how long ago something was alive, scientists measure the ratio of the isotopes. If there is exactly half as much carbon-14 as expected, the object came from something that died 5,730 years ago, one half-life. If the ratio of carbon-14 is one fourth the expected amount, the object is two half-lives, or 11,460 years old.
