Chemistry—Matter (Science)

"You will die but the carbon will not; its career does not end with you. It will return to the soil, and there a plant may take it up again in time, sending it once more on a cycle of plant and animal life."-Jacob Bronowski (1908-1974)

Chemists study matter, which is defined as anything that has mass and volume. Everything around you is made of matter. Surprisingly, everything is composed of only about a hundred different basic types of matter, called elements. Some elements are quite familiar: iron, gold, oxygen, carbon, lead, and helium. You may have never heard of others: osmium, hafnium, and gadolinium. A few, such as the americium used in smoke detectors, do not occur naturally. These elements are produced in huge laboratories. It may seem strange that everything around you can be built with only 100 types of building blocks. Keep in mind, though, that the English language, with all its possibilities and nuance, is built of 26 letters.

Each element has a smallest particle, called an atom. Although atoms are themselves made of smaller particles, an atom is the smallest unit that has the properties of the element. The basic particles of atoms are protons,neutrons, and electrons. You have probably seen pictures showing an atom as a nucleus with electrons orbiting around it. Although this model does not exactly match our current understanding of atomic structure, it is close enough to be useful.

The nucleus of an atom has protons and neutrons, which make up most of its mass. Around the nucleus, electrons are in constant motion, making up most of the atom’s volume. Altogether, they build a structure that is unimaginably small. Ten million atoms could make a line across the period at the end of this sentence. Even so, the properties of all matter depend on the interaction of atoms, one at a time.

Why is it more important to paint steel than aluminum?

Rust is a serious concern to anyone who builds or uses objects made of iron or steel. Take a look at an old car that has been sitting in a junkyard for a couple of decades. Although the shell of the vehicle still has its original shape, you can push your hand through the rusted steel with very little effort. What happens to a metal object when it rusts?

Rust is a chemical compound that forms when electrons are transferred from iron atoms to oxygen atoms. The two elements combine in a ratio of two parts iron to three parts oxygen to form ferric oxide, or rust, which has properties that are very different from those of either element alone. In fact, the iron ore that we find in nature is essentially rust, which explains why water in an iron mine has a characteristic red color.

Generally, three things must come together to cause substantial rusting of steel—iron, oxygen, and water. Electrons are not transferred very efficiently from the iron atoms to oxygen as it occurs in air. However, water molecules very effectively aid in the movement of electrons between the two elements. That is one reason that cars tend to survive longer in the dry deserts of the Southwest than in places with humid climates.

A chemical compound is a substance formed by the combination of two or more elements in a fixed proportion of atoms. The atoms are bound together by transfer or sharing of electrons, making a substance whose properties can be very different from those of the elements of which it is composed.

Paint prevents rust by keeping oxygen and water away from the iron atoms. The modern paints used on automobiles essentially form a hard layer of plastic that bonds strongly to the metal surface. Oxygen cannot penetrate the paint unless it becomes scratched or chipped. A small break in the coating, however, can open the metal underneath to corrosion.

Although aluminum is often painted for decorative purposes, the coating is not necessary for surface protection. It’s not that aluminum does not react with oxygen. In fact, aluminum reacts with oxygen even more readily than iron does. The difference is that when iron rusts, the soft iron oxide flakes away, revealing more iron atoms, while aluminum oxide forms a tightly bound coating on the aluminum surface. The aluminum oxide forms a protective layer that cannot be penetrated by oxygen and water.

Stainless steel is an alloy of iron that does not rust readily. In addition to iron, stainless steel contains at least 12 percent chromium and often other metals as well. The chromium atoms on the surface react with oxygen to form a protective layer that prevents further reaction by atoms beneath the surface.

Why do snowflakes have six sides or points?

If you look closely at a snowflake, you will usually find that it has six sides or points. A snowflake is a crystal formed by the arrangement of molecules of water in its solid form. Snow crystals form when water begins to freeze around small particles of dust inside clouds. Snow forms in several shapes depending on the temperature: near 32°F, the crystals form small flat plates; a few degrees cooler and they take on a needle or pencil shape; large, lacy shapes form at about 5°F or colder. No matter which shape the crystals take, they have six sides.

Many solid materials in nature form crystals when liquid or gases cool to become solid. The smallest particles of the material—atoms or molecules—are constantly in motion. As these particles cool, they lose energy and move more slowly. Eventually their energy of motion drops to a level at which the forces that cause particles to attract one another are stronger than the forces that cause them to move apart. Then a solid crystal forms. The shape of the crystal is determined by the forces that cause particles to stick together.

Because drawings of snow-flakes generally show them with perfect six-sided symmetry, people often assume that that is how real snowflakes form. Because conditions are constantly changing as a snowflake tumbles and falls, snowflakes almost always have an irregular six-sided shape. Photographers who make pictures of snowflakes may spend hours looking for a beautiful, somewhat symmetrical pattern.

Molecules of water are made of two atoms of hydrogen bound to an atom of oxygen, forming the shape of a V. The oxygen and hydrogen atoms share electrons, forming a chemical bond, but the electrons are attracted to the oxygen atom a bit more than they are attracted to the hydrogen atom. As a result, the point of the V has a bit of a negative charge and the tips have a bit of a positive charge. Because of these charges, water molecules are attracted to one another. As the solid forms, the oxygen atom of one molecule lines up close to the hydrogen atoms of two other molecules. The most stable arrangement occurs when six water molecules form a ring of V shapes. This is the beginning of a six-sided crystal.

The crystal grows as more and more water molecules join the original group. The attractions between the charged parts of the molecules cause the crystal to grow in six equally spaced directions. Eventually, after many trillions of molecules have joined together, the crystal begins to fall. As it descends, the snowflake can gain or lose molecules, so the shape is constantly changing. Differences in temperature and humidity cause each pattern to change as it moves up and down through the atmosphere, so two snowflakes falling close to one another may have very different shapes. In general, though, the six sides remain.

Why does dry ice not melt like regular ice?

Imagine ordering a frozen-food item for shipment from a company across the country. If the item is packaged with a block of ice, it may stay cold until it reaches you, but even with good insulation, your product is likely to arrive in a pool of water. Instead of ice, cold products are generally cooled during shipping by dry ice—solid carbon dioxide. When the package arrives, the block of dry ice is much smaller than when it was shipped but there is no pool of liquid.

At atmospheric pressure, carbon dioxide is a solid at -109°F (-78.5°C), cold enough to keep water-based foods frozen solid. The major advantage of shipping with a block of dry ice is that it goes directly from solid to gas in a process called sublimation. This process is sometimes used to create fog for stage performances.

Sublimation is the process in which a material passes directly from the solid phase into the gas phase without becoming a liquid.

If dry ice is placed in warm water, it quickly sublimates and the gas flows away, carrying with it tiny droplets of water, making a fog. Because the gas is cold and carbon dioxide is heavier than air, the fog flows across the stage instead of rising above it. This is very useful for creating an eerie feel in a performance of Macbeth. Dry ice is also used in a process, similar to sandblasting, to clean surfaces. Solid pellets are sprayed at the surface, stripping contaminants and paint. The pellets sublimate into a gas and don’t leave behind piles of sand to be cleaned up.

Why doesn’t carbon dioxide form a liquid like most substances? In fact, it does, but only under pressure. The state—gas, liquid, or solid—of a substance depends on temperature and pressure. For example, because atmospheric pressure is lower at high altitudes, the boiling point of water is several degrees lower in Denver than in Houston. If you increase the pressure to about five times normal atmospheric pressure, carbon dioxide becomes a liquid at room temperature.

Carbon dioxide fire extinguishers hold liquid carbon dioxide under pressure. When it is released from the extinguisher, the carbon dioxide expands rapidly, which causes it to cool. A spray of carbon dioxide "snow" rapidly sublimates to form a gas that smothers the flame. Liquid carbon dioxide can also be used in some dry cleaning processes for clothing.

Under the right conditions, water sublimates. Below 32°F (0°C) water exists as a solid at atmospheric pressure. Although it cannot melt at colder temperatures, some ice passes directly to the atmosphere as a gas. You can see this process when frost disappears from the outside of a window on a very cold day. In parts of Antarctica, cold winds cause several centimeters of ice to sublimate from the surface each winter.

How do synthetic diamonds differ from real diamonds?

Diamond is the hardest naturally occurring substance, which makes it useful for industrial cutting tools. It also has an amazing ability to play with light, which makes it useful for adornment. However, diamonds are relatively rare and expensive to mine. That makes them expensive to buy.

A number of diamond substitutes or simulated diamond, including cubic zirconia and moissanite, fill a niche for low-cost diamond look-alikes, but they are not real diamonds. These materials have a different chemical composition and are easily identified by an expert, or frequently by the casual observer. For industrial applications, simulated diamond is not at all suitable.

Natural diamond can only form at the high temperatures and pressures found at least 100 miles beneath the surface of Earth, and they are carried to the crust by deep volcanoes. Most natural diamonds were formed between 1 billion and 3.3 billion years ago and carried to the surface much later.

Since the discovery in 1797 that diamonds are made of pure carbon, people have looked for ways to manufacture a real diamond. A hundred miles beneath Earth’s surface, extreme pressures and temperatures squeeze carbon atoms into a network in which they are tightly bound to one another. Until recently, there was no way to recreate these conditions on the surface (with the possible exception of Superman’s fist). The first synthetic diamond was made by General Electric in 1954. Recent developments have begun to challenge nature’s monopoly on diamond making, and not incidentally, the profits of DeBeers, the largest producer of mined diamonds.

Synthetic diamonds are real diamonds, having the same chemical composition as natural mined diamonds. They are created in hours rather than millions of years. Every year, manufacturers produce more than 100 tons of synthetic diamonds. In general, they have colors that make them less than ideal as gems but they are very important for industrial applications. Synthetic diamonds have been used in cutting tools, surgical instruments, and electronics applications.

What is the most abundant element?

When you think of elements, some, such as oxygen, carbon, and iron, seem to be very common and abundant. Other elements, such as gold, silver, and platinum, are much rarer and very expensive. Which of the 100 or so elements are the most abundant?

The answer to that question depends on where you look. If you are interested only in elements that are readily available here on the surface of Earth, then the most abundant element is oxygen. Most of the mass of the ocean’s water is made of oxygen and almost all of the minerals that make up the rocks and soil of Earth’s crust contain a large amount of oxygen. The following table shows the eight most abundant elements by mass in the crust of Earth, which make up 98 percent of the total.

Most Common Elements in Earth’s Crust


Weight Percent

















It may seem surprising that silicon is the second most abundant element. It is only with the introduction of solid-state electronics that most people have even become aware of silicon as a substance. It has been used for a long time, however. Glass—and the sand that glass is made from—is composed mainly of silicon dioxide. Also surprising is the absence of some well-known elements from the list. Carbon and nitrogen are found in a great many common compounds, but they are only a fraction of the remaining 2 percent of the matter around us. Hydrogen is also missing, which is something of a surprise because it is actually a very abundant element in the universe, as we will see shortly. If you look at Earth as a whole, not just the crust, then the most abundant element is iron, which makes up most of the core of the planet.

However, if you consider not just Earth, but the entire universe, the picture is very different. While eight elements combine to make up about 99 percent of Earth’s crust, only two elements make up 98 percent of the universe—and neither of them is on the previous list. The following table shows the relative abundances of the top eight elements in the universe. As you can see, it is mostly hydrogen, the simplest element.

Most Common Elements in the Universe


Weight Percent

















An interesting aspect of modern models of the universe is that all of the elements combined make up only 4 percent of the universe as a whole. The rest is called "dark energy" and "dark matter." However, neither of these has ever been detected, so no one knows their properties.

Is jet fuel more dangerous than gasoline?

When an airplane must land with malfunctioning landing gear, dozens of fire trucks and other emergency vehicles stand ready to move to its final destination. Sometimes the crashed plane is quickly enveloped in flames. Is jet fuel an inherently dangerous chemical, more hazardous to handle than the gasoline that you have in your shed for your lawnmower?

While jet fuel is a flammable compound, it is actually less likely to catch fire or explode than ordinary gasoline. Most jet fuel, like gasoline, is made of hydrocarbon compounds that are distilled from crude oil. These molecules consist of chains of carbon atoms linked to one another and to hydrogen atoms. When the hydrocarbon burns, it reacts with oxygen in the air to make carbon dioxide and water—and it releases a lot of energy. The fewer carbon atoms in the hydrocarbon, the easier it catches fire. Natural gas, or methane, has a single carbon atom in each molecule and burns very easily. Gasoline is a mixture of hydrocarbons with varying carbon chain lengths, averaging seven or eight carbons per molecule. Jet fuels, such as kerosene, have more carbons per molecule than gasoline so they do not ignite as easily.

One measure of flammability is the temperature at which a mixture of a vapor and air will ignite when exposed to an energy source, such as a spark. For gasoline, the flashpoint is approximately -40°F. For jet fuel, it is +100°F. An open container of gasoline is much more hazardous than an open container of jet fuel because the vapors above the jet fuel will not burn below 100°F. However, when a jet airplane undergoes a crash landing, there is still a significant risk of fire. The engines are hot and friction between the runway and the body of the plane also generates a lot of heat. That’s why the firefighters are standing by.

The low flashpoint of gasoline is an important reason to take care when filling the tank of a hot lawn mower engine. The heat of the engine rapidly vaporizes gasoline that is splashed on it. The vapor above the engine can ignite at any temperature above -40°F, which includes just about all the conditions in which you would be mowing your lawn. We are so used to using gasoline, it is easy to forget that it can be a dangerous chemical.

Watch a movie with a massive chase scene and you are almost certain to see at least one car explode when it crashes. Guess what—it doesn’t happen in real life. An explosion can only occur when a mixture of gasoline vapor and air is in a narrow range. Most cars have very sturdy gas tanks that do not rupture in a collision, and even if they did, an explosion could not occur until a substantial amount of gas had leaked.

Why do apple slices turn brown?

Slice an apple into pieces and, depending on the variety, the flesh you see is pure white to light yellow or pink. Within a few minutes, however, the apple slices have started to turn brown. The browning occurs because enzymes in the apples start a chain of chemical reactions. The enzyme in fruits that causes browning is called polyphenol oxidase (PPO). When the enzyme and oxygen are exposed to compounds called phenols that are present in the tissues of apples and other fruits, the phenols are converted to other compounds called quinones. These quinones rapidly react with proteins in the apple to make the brown-colored compounds that you see on the surface of the apple slices.

If the PPO is inside the apple, why doesn’t the flesh of the apple turn brown before it is sliced? The main reason is that the skin of the apple keeps oxygen outside. In addition, the PPO and the phenols are generally located in different cells inside the apple. As a knife ruptures the cells, the compounds mix. In fact, there are some cases where this mixing can occur without cutting the apple. Drop an apple on the floor and the impact ruptures cells beneath the surface. In a short time, a bruise develops under the skin. The brown bruise is caused by the same reaction, using oxygen that is available in the cells.

You can slow the browning of apple slices by keeping oxygen away from the surface if you cover the fresh slices with water, sugar, or syrup. A splash of lemon juice will also keep the slices fresh looking. Lemon juice, and the juice of other citrus fruits, contains compounds that are antioxidants. These compounds, including Vitamin C, react with oxygen very quickly, so the splash of juice keeps oxygen from reaching the PPO in the apple.

PPO is present in many plants and in many foods that turn brown rapidly, including bananas, mushrooms, peaches, and pears. The purpose of PPO in these plants is not completely understood. The PPO may be part of the cycle by which plants use oxygen in their cells, or the PPO may provide protection from certain pests, or both.

"About seven years later I was given a book about the periodic table of the elements. For the first time I saw the elegance of scientific theory and its predictive power."

How can lightweight body armor stop a bullet?

Whenever police or military personnel enter a situation that could involve a firefight, they put on body armor, sometimes called a "bullet-proof vest." This armor is a descendent of the suits of armor worn by knights in battle long ago, although its construction is very different. How can a vest made of woven cloth stop a bullet?

Think about what happens when a professional tennis player returns a 100 mph serve. The racquet consists of a mesh of interwoven filaments that form a net. When the ball and the face of the racquet connect, the force of the collision is divided among many separate strands. As each strand bends, it spreads the force over its whole length and transfers some of the force to each of the perpendicular strands of the weave.

If the entire force of the collision were concentrated on a single string, that string would likely break, but, by spreading the impact, the ball is stopped and then its motion is reversed.

Woven body armor works on the same principle. Because the bullet is smaller than a tennis ball and moving many times faster, the woven net must have a very tight weave. In addition, the strands must be made of a very strong, but flexible, material. The fabric of modern body armor is woven of synthetic fibers that are lightweight like cotton or silk, but have a tensile strength many times greater than steel fibers of the same size.

Tensile strength is a measure of the stress required to stretch a material until it breaks. It is calculated by measuring the maximum load that the material can support and dividing that load by the original cross-section of the material. It is commonly expressed in units of pound per square inch.

As each fiber in the vest bends, it absorbs energy and spreads it across the length of the fiber and across the length of the fibers connected to it. As a result, a dense net of fabric can absorb a lot of energy. Modern body armor consists of 20 to 40 layers of densely woven fibers, each layer absorbing and spreading part of the impact force.

These two functions—stopping the bullet and spreading the force of the impact—are the source of the armor’s protection. The person wearing the vest will still feel the energy of the bullet’s impact, but it is spread over a wide area of the body to prevent serious injury. Some of the newest types of armor have a fluid material between the woven layers. This fluid actually solidifies when it is under stress. In this way, it is like a mixture of cornstarch and water which can be rolled into a ball as long as pressure is continually applied but becomes a liquid as soon as the pressure is removed. As the fluid solidifies in the vest, it can absorb a lot of energy from the moving bullet very rapidly and then release the energy slowly as it again becomes a liquid.

The fiber with the strongest known tensile strength is spider silk. A spider’s web is built of strings of protein that bend but don’t break under extremely high forces. Researchers are working on ways to make artificial spider silk, which could be used to make body armor far superior to those made with materials available today.

"There is no better, there is no more open door by which you can enter the study of natural philosophy than by considering the physical phenomenon of a candle."

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