"If the history of technology tells us anything, it is that the future lies in the world of the very small."-Eric Cornell (1961-)
When you put a log on a fire, it burns and releases energy as heat and light, leaving behind only a small amount of ash. A steel pipe, on the other hand, gets hot but does not otherwise change in the fire. Mix some baking soda in water and nothing happens, but when you add some lemon juice to the mixture, bubbles form and it foams out of the container.
Every substance has different properties that are defined by its interactions with other substances and with energy. These interactions occur at the atomic level. Atoms of one element react with atoms of another element to form chemical bonds. Atoms absorb and release energy in various forms, including light, heat, or electric current. Every change that you see, hear, taste, or feel involves an interaction of atoms with matter or energy.
How do fireflies make flashes of light?
Light is a form of energy that can be produced by conversion of other forms of energy. We usually associate light with heat, electric current, or both. However, there are some lights that flash without either of these energy sources. How do fireflies produce their bright flashes of yellow-green light on a summer evening?
The process that fireflies use to make light is called bioluminescence. This process, which also occurs in many other organisms, especially in the deep sea, converts chemical energy to light. The chemical energy comes from molecules of adenosine tri phosphate (ATP), a chemical that living cells use to store and transfer energy.
An enzyme is a protein that speeds up reactions in living cells. Enzymes take part in the chemical reaction but are not changed by the reaction.
Fireflies have specialized cells that contain a chemical compound call luciferin and an enzyme called luciferase. Luciferase is a protein that speeds up the chemical reaction. The flash of light is controlled by the amount of oxygen available to the cell.
The reaction of Lucifer in with ATP and followed by reaction with oxygen makes a compound called oxyluciferin. Unlike most chemical compounds, the oxylucifer in has electrons that are in an excited state when the compound is formed. As the electrons move to a more stable energy state, they release energy in the form of light. When oxygen is available inside the cells, the firefly’s light organs glow yellow or green. When there is no oxygen, the light goes out.
Fast Facts
The cool glowing plastic sticks that children (and adults) love to play with use the same principle as firefly flashes. When the stick is bent, a glass cylinder inside is broken, allowing two solutions to mix and produce light from a chemical reaction. However, the chemical reactions used in glowsticks produce light with an efficiency of only about 30 percent compared to an efficiency of 88 percent for the bioluminescence of a firefly.
Living organisms use bioluminescence for several purposes. Fireflies use their flashes to attract mates and also to warn predators that the insect’s body contains some really foul-tasting stuff. Some marine animals glow to attract food, and there are a number of animals and bacteria that use bioluminescence for reasons that are not clear.
People have also found ways to use light-emitting chemical reactions based on these natural systems. Biochemists use luciferin as a detector to measure the amount and location of ATP in living cells. The gene that produces luciferase has been implanted in other organisms so that it can be used to follow chemical reactions in cells.
Why don’t batteries work well when they are cold?
You use batteries in just about everything—cell phone, iPod, flashlight, car, toys. Batteries provide a way to store energy until you need it and avoid the inconvenience of needing a place to plug everything in. But when it gets really cold, you may find that your batteries let you down, at least until they get warmer. Why would batteries quit working at low temperatures but come back to life when they get warmer?
An electrochemical reaction is a chemical reaction in which one atom or molecule loses electrons and another atom or electron gains electrons. Electrochemical reactions can be divided into two half-reactions: reduction, during which electrons are gained; and oxidation, during which electrons are lost. Electrochemical reactions are sometimes called oxidation-reduction reactions or redox reactions.
First, let’s take a look at what happens inside a battery. A battery is filled with chemicals that undergo reactions known as electrochemical reactions. Each electrochemical reaction has two parts: oxidation and reduction. During oxidation, the atoms of an element or molecule lose electrons; during reduction, the atoms or molecules gain electrons.
If the atoms are mixed together, the electrochemical reaction occurs spontaneously and energy is released, generally as heat. However, inside a battery, the two parts of the reaction are separated from one another. Nothing happens unless electrons have a path to follow between the two sections. A closed circuit provides a path that electrons can follow. Electrons flow from one terminal of the battery, produced by the oxidation reaction. They flow through the wire to the opposite terminal, allowing the reduction to take place. In effect, the battery stores chemical energy and then releases that energy as a flow of electrons.
If the terminals are connected by a wire, the electrons flow quickly and the reaction occurs rapidly. The battery and the wire become hot and the chemicals in the battery are consumed quickly. However, if a light bulb or the motor of a DVD player are connected to the circuit, some of the energy of the moving electrons is converted to light or motion. The electron flow is slowed as the energy is removed and the reactions in the battery occur at a slower rate.
It may appear that the current flow occurs only between terminals of the battery. However, it is necessary for charges to flow inside the battery as well. If this did not happen, a large negative charge would build at one side of the battery and a large positive charge would build at the other. The battery contains a solution or paste that allows ions to move, making a complete circuit, but which restricts the flow of electrons. Positively charged ions flow through the battery in the direction opposite the flow of electrons. For that reason, if you place a wire with no load across the battery terminals, the battery and the wire both get hot.
On a very cold day, the battery in a car may not have enough current to start the engine. This happens because chemical reactions proceed at a slower rate at lower temperatures. Starting an engine requires a lot of power in a short time. When the chemical reactions proceed too slowly to provide enough power, you get a weak turnover of the engine and it does not start. In this case, the battery operates normally as soon as it is warm because all of the chemical energy of the battery is still available. In very cold climates, battery warmers that plug into an outlet keep the battery warm and make it easier to start the car.
Why do bronze statues turn green over time?
Many cities and towns have parks that include a large bronze statue that has been standing for decades. Its color is generally some shade of green or brown. However, if you buy a new bronze statuette or vase, it usually has a shiny copper or gold-toned surface. Why is the appearance of the metal of the statues so different from that of a new bronze ornament?
An alloy is a mixture of two elements that has metallic properties that are different from those of either element by itself. An alloy contains at least one element that is a metal and it may contain many different elements. Examples of alloys include brass and bronze, alloys of copper; steel, an alloy of iron; pewter, an alloy of tin; and sterling silver.
Bronze and brass are alloys that contain copper along with other metals such as tin or zinc. When copper weathers, it reacts with oxygen and with other materials in the air to form compounds that have properties very different from those of the metal itself.
Most people know that the Statue of Liberty is green. The statue is actually made of shaped sheets of copper attached to a framework. In its pure form, copper is a shiny, reddish-gold metal. However, compounds of copper with other elements or combinations of elements tend to be strongly colored in shades of green, blue, or sometimes pink. When exposed to most air, copper atoms react with oxygen to form two kinds of oxide, one of which is pink and the other black. These oxides react with sulfur dioxide in the air. Copper sulfate is green and copper carbonates are green or blue, depending on the exact composition of the compound. Near the sea, copper chloride may be the predominate salt on the surface. It is copper chloride that gives the Statue of Liberty its color.
Unlike those made of iron and steel, tools made of copper alloys, such as bronze, do not make sparks when they strike other metal objects or concrete. Industrial facilities in which explosive mixtures can form in the air require the use of bronze tools.
The coating on the metal is called its patina. The patina is a surface coating that tends to protect the metal beneath it from further corrosion.
Although the Statue of Liberty has been standing for over 100 years, attacked by wind, rain, and sea salts, its patina extends only about one two-hundredth of an inch into the copper. If the layer of copper salts were to be removed, the copper beneath would have the shiny appearance of pure metal. However, within months, it would again be green because the surface reaction occurs fairly quickly. Ancient Greek and Roman bronze coins have been found covered with a patina that has protected the surface beneath them well enough to recover the images when the coins were cleaned.
How can a gecko walk across a ceiling?
Geckos are small lizards, recently showing up on television selling insurance, with an amazing ability to climb. A gecko can scurry up a wall, cross a glass window, and even walk along the ceiling. Try doing any of those things and you risk a nasty tumble, if you can even get far enough to fall. Do geckos have some special knowledge of climbing, or do they have tools that you lack?
Watching a gecko climb a wall, it is easy to assume that its feet would feel sticky. In fact, a gecko toe feels soft and smooth when you touch it. The ends of the fibers on the foot are made of a protein called keratin, which is similar to the proteins of your hair and fingernails.
People can climb what looks like a solid wall by wedging their hands and feet into tiny crevices. It would seem logical to assume that a gecko does the same thing, perhaps using crevices that are too small to be visible to our eyes. That cannot be the case, however, because the lizards can even clamber up a sheet of polished glass with no significant cracks to wedge into.
Several explanations can be ruled out—suction doesn’t work because a gecko’s feet don’t have a cup shape to push out air; friction might account for the wall but not the ceiling; gluelike sticky stuff would work except the feet don’t have any glands to produce it.
To find the answer, scientists have taken a really close look at gecko feet. They found that it’s all in the interactions between molecules. The key to being able to stick to the wall doesn’t even have that much to do with what gecko toes are made of. Instead it is the shape. Each toe has a network of many millions of tiny hairs, about one millionth of a centimeter long. At the end of each hair are thousands of tiny pads.
The ability to climb, and even hang by a single toe, comes from miniscule electronic interactions, called Van der Waals forces, which occur between molecules. Even molecules that are not polar have small, fluctuating variations in electric charges that occur as electrons move around. These fluctuations cause attractions between molecules. Normally these forces are too small to be noticeable on the scale of objects that we can see. They operate only at very small distances—about the diameter of an atom—so it is hard to get two surfaces close enough to take advantage of the force. The gecko’s foot, however, takes advantage of these forces by packing together lots of really tiny pads, each only a hundred or so atoms wide.
The attraction between each pad and the molecules of the surface beneath (or above) the foot is very small. When you have billions of tiny attractions, though, the force adds up. Researchers studying geckos have calculated that, if all the pads were touching the surface at one time, the gecko’s feet could hold almost 300 pounds. Because there are so many individual points of contact, the gecko can control the adhesion by varying the amount of contact with the surface. Each contact point only adheres when it is dragged along the surface. That allows it to dash up the wall and not get stuck.
Researchers have used this concept to design a tape that depends only on the shape of millions of tiny structures as an adhesive. They believe that gecko feet may provide the inspiration for new and improved adhesives that hold tight but come off easily.
"It is often stated that of all the theories proposed in this century, the silliest is quantum theory. In fact, some say that the only thing that quantum theory has going for it is that it is unquestionably correct."
What causes concrete to get hard?
When a truck of concrete arrives at a construction site, its tumbler is filled with a sloppy mixture that is easily poured into a wheelbarrow. The wet concrete is poured into place and smoothed with a trowel. A few hours later, the poured concrete is hard enough to support a person’s weight, and rather warm. How does concrete become a hard, stonelike substance?
Concrete is not a recent invention—concrete structures thousands of years old are still standing. It has the strength and durability of stone but can be easily formed into various shapes. Concrete is basically a mixture of cement and water and aggregate— sand, gravel, or other solid particles. The wet mixture can be poured and shaped before a chemical reaction changes its characteristics.
Cement is a mixture of calcium, silicon, and aluminum oxides, often made by heating limestone with clay and then grinding the mixture to make a fine powder. As the concrete hardens, the water reacts with the mineral components of the cement to form crystals. During this process, called hydration, the water becomes part of the crystal structure itself. The water is tightly bound into the cured concrete in a specific ratio to the mineral components, becoming part of the new crystal, and cannot be removed. The interlocking crystals which form around the aggregate convert the wet mixture to a dry, strong, rocklike substance.
It takes some time for the hydration process to completely occur. Depending on the use and amount of strength needed, the concrete must be kept under controlled conditions of temperature and humidity, a process known as curing, for several hours to several weeks. Once it has become solid, water can no longer enter the concrete. If the concrete becomes too dry during curing, it will lose substantial strength.
Concrete does not harden by "drying." When something dries it loses water by evaporation. In concrete the water becomes part of the final material, so loss of water by evaporation during the hardening process results in weaker concrete. Concrete will even set and cure when it is completely submerged in water.
If you touch concrete during the curing process, you are likely to notice that it is warm. The hydrated cement is a more stable chemical substance than the starting materials. As the chemical reaction proceeds, chemical energy is converted to heat. Removing this heat, or allowing it to dissipate, can be a problem for engineers designing very large concrete structures.
How do heat packs work?
Sitting in the bleachers during a November football game can be uncomfortable if you live in the north. Wouldn’t it be nice if you had something warm to keep your hands from freezing? Fortunately, there are heat packs you can buy that generate heat right there where you sit—no electricity, no flames, just warmth. Where does the heat come from?
There are two kinds of heat packs available that depend on changes in chemicals inside the pack. A change the releases heat is called an exothermic change. Both types of heat packs use exothermic changes to chemicals. One of them is reversible which allows the pack to be reused. The other involves a chemical reaction that changes the materials in the bag so it cannot be reused.
An exothermic change is a chemical or physical change that releases energy as heat. An endothermic change is one that only proceeds by absorbing energy from its environment.
Reusable heat packs are based on the fact that freezing a liquid material is an exothermic change. This may not be obvious, but if you think about what happens when you make ice cubes, it may be more clear. You place water in the freezer. As it loses energy to the air around it, the water gets colder and the moving molecules move a bit slower. As the water freezes, the moving molecules slow down even more and more energy is released. The solid form of water has less thermal energy than the liquid form, although the temperature remains at 32°F (0°C) during the freezing process, so the process is exothermic. If the freezer did not pump excess heat out into the kitchen, the compartment would become warmer.
If water is very pure, it can be super cooled. If you have very pure water in a very clean glass container, crystals do not form at 32°F. The water can be cooled several degrees below its normal freezing point. However, if you scratch the inside of the glass surface, or even cause shock by tapping the container, crystals will begin to form at the site of the disruption. All of the super cooled water will freeze almost instantaneously.
Citrus growers use the energy released by freezing water to protect crops from cold weather. When the temperature drops below 32°F, citrus trees are blanketed with a fine spray of water. As the water freezes, it releases energy and protects the fruit.
Reusable heat packs work on this principle. They contain sodium acetate tri hydrate, a chemical that super cools very easily, inside a very clean plastic pouch. The freezing point of the liquid is about 130°F (54°C). Inside the pouch is a bent, flexible metal disk. If you flex the disk, the shock causes a few tiny crystals to form. Because the liquid is super cooled, the crystals quickly grow to encompass the whole solution. The heat pad rapidly warms to 130°F as the molecules become more ordered and release heat.
Sodium acetate tri hydrate heat packs are reusable. If you place the pouch in a pot of boiling water, the crystals melt again to their liquid form. As the pouch full of liquid cools, the liquid again becomes super cooled, ready to freeze again.
The second type of heat pack is filled with a mixture of powdered iron, carbon, water, and salt. To activate the heat pack, an outer pouch is opened that allows air to reach the mixture. A chemical reaction, similar to rusting, begins. The reaction is exothermic, releasing heat over several hours as the iron is converted to iron oxide. Unlike the heat pack based on freezing, this pouch cannot be reactivated and reused.
Why don’t matches ignite in the box?
When you think about it, the specifications for a useful match are pretty demanding. It must ignite easily, keep burning long enough to start your fire, and (most importantly) not ignite in your kitchen cabinet until you want the flame. Why do matches light when you strike them but remain stable in the box?
As it happens, the first matches did not remain stable. These matches, invented in the early 1800s, were made using white phosphorus, potassium chlorate, and thickeners to hold it all together. Potassium chlorate is a strong oxidizing agent, which means it readily accepts electrons from other compounds or elements. Phosphorus is easily oxidized. If you mix the two materials, you get an explosive mixture. To light the match, you rubbed it along a rough surface. Friction provided enough heat to cause the volatile mixture to start burning. These early matches tended to ignite at the wrong time and to send sparks flying. They were easy to light, though. When you consider that white phosphorus is extremely toxic and that a single box of matches was likely to contain a lethal amount of it, you realize that these matches were a bit less practical than the matches we use today.
Later, matches used two separate sections on the match head to make a more stable match. Most of the tip was covered with a mixture of potassium chlorate and sulfur in a mixture of clay and glue—very flammable but more stable than white phosphorus. On the tip of the match was a small amount of phosphorus trisulfide, which will ignite when it is rubbed across a rough surface but burns at a fairly low temperature. When this tip ignited, the heat of its flame ignited the hotter-burning mixture behind it. This type of match is still sold, marketed as "strike-anywhere" matches, but they can be hard to find. Strike-anywhere matches can be identified by the two differently colored sections of the match head.
Most modern friction matches are of a type called safety matches. They cannot ignite accidentally because they must be rubbed on a specific surface in order to burn. This surface is generally placed on the outside of the box or on a strip at the bottom of a book of matches. The tip of the match, which is all one color, contains a mixture of antimony trisulfide, potassium chlorate, and binders such as glue. The mixture is flammable but will not ignite just anywhere.
The key to using safety matches lies in the special striking surface. There are two common forms of the element phosphorus—white phosphorus (the highly flammable, very poisonous form) and red phosphorus (less flammable, less toxic). Heat can convert red phosphorus to white phosphorus, which, you may remember, is not very stable in the presence of potassium chlorate. The striking surface has a mixture of red phosphorus and an abrasive material such as powdered glass.
When the match is struck, a very tiny amount of red phosphorus is converted, by the heat of friction, to white phosphorus. This little bit of white phosphorus mixes with the chemicals on the match head. The chemical reaction that results ignites the match head, which then ignites the wooden match.
"It is the object and chief business of chemistry to skillfully separate substances into their constituents, to discover their properties, and to compound them in different ways."
How can a grain silo explode?
You would think that working with flour—finely ground wheat—would be a safe job. The material is nontoxic and is a basic component of our food supply. How could it be hazardous? Well, there is a definite hazard for workers who handle large quantities of flour, or any other powdered grain. There is a risk of a devastating explosion. People have died in explosions that leveled grain mills, grain elevators, and silos. How can something as common and apparently harmless as flour be an explosion hazard?
Flour is made mostly of starch, which is a compound of carbon, hydrogen, and oxygen, and it would be expected to be about as flammable as wood. Although wood burns rapidly under the right conditions, no one expects it to explode. However, the danger of flour in a grain silo is not based on its flammability as much as the size of the particles of flour.
Combustion of a substance in air is a chemical reaction, which occurs when molecules of the substance—in this case flour—combine with oxygen to make new chemical substances—in this case again, carbon dioxide and water. Combustion is a highly exothermic process. That’s why a fire in the fireplace warms your living room. The rate of the combustion reaction depends on several factors, including the presence of oxygen and a material that can react with it, a source of ignition such as a spark or a flame, and the ability of the oxygen and the fuel molecules to come together so that they can react. That’s where the particle size comes in. The only place that oxygen can react with molecules of the fuel is at the surface of the particles of fuel.
If you’re burning a log, or a bag of flour, the surface area of the fuel is very small compared to its total volume. That means the reaction with oxygen is fairly slow. Consider the surface area of billions of small particles of flour dispersed in a large volume of oxygen-rich air. Now you have a lot of places where the two components of the reaction can come together. If a spark or heat source provides a source of energy that causes one of the grains to burn, you have a recipe for disaster. The tiny particle burns almost instantaneously, releasing a lot of heat energy. This energy causes nearby particles to ignite, beginning a chain reaction of burning flour grains. The products of the reaction are gases, which take up much more space than the flour. Hot gases expand rapidly in an explosion. If they are contained in a grain silo or elevator, the rapid expansion can level the structure within seconds of the first ignition.
Flour is not the only material whose dust can form an explosive mixture. Sugar, sawdust, and coal dust have all been involved in deadly explosions. If the particles are fine enough (less than 01 millimeter diameter), even metals such as aluminum and iron dusts can explode when they are suspended in air.
How does sunscreen work?
Anyone who has experienced a severe sunburn does not want to repeat the experience. Why does sunlight cause your skin to burn and how does sunscreen prevent sunburn?
The damage to skin is caused by ultraviolet light from the sun. Remember that the radiation from the sun extends far beyond the familiar visible spectrum. Wavelengths that are longer, infrared radiation, can be absorbed by molecules in your body and felt as heat. Wavelengths that are shorter than violet light, ultraviolet (UV) radiation,can pass through the outer layers of your skin. Although UV radiation is very energetic, you don’t feel it when it strikes your skin. That’s why you can get a sunburn without realizing it on a cloudy day. If the atmosphere absorbs or reflects the infrared light, you don’t feel like radiation is reaching you.
Many people think that sunscreen is only necessary on hot, sunny days. While clouds absorb and reflect visible light and infrared radiation, ultraviolet radiation passes through them and reaches you even on a cloudy day. Temperature is also not a good indicator of UV radiation. In fact, the danger of exposure increases when there is snow cover because ultraviolet radiation reflects from snow rather than being absorbed by the ground.
However, even though you don’t feel the UV radiation, it is still able to do damage. If your skin is dark, you may avoid the burn. The pigment that causes skin to appear dark, melanin, absorbs the radiation near the surface, before it penetrates to the cells beneath. If the UV rays are not absorbed by melanin, then they are absorbed by molecules inside the skin cells. The energy of the absorbed radiation changes some of the molecules and damages the skin several layers deep.
Why is it that the pain and redness of a sunburn do not appear for several hours after the exposure? The symptoms of sunburn are not caused by the damage itself. The pain that you feel comes from your body’s efforts to repair the damage. The heat and pain are symptoms of inflammation. The redness comes from enlarging the capillary blood vessels just under the skin so that the body can rush repair cells and materials to the site. The greatest danger of sunburn is the risk of irreparable damage to cells, leading to malignancy.
Sunscreens and sun blocks use chemicals to prevent the ultraviolet radiation from reaching your cells. There are two ways to do this: reflection and absorption.
Some ingredients, such as zinc oxide and titanium oxide, reflect the radiation that strikes them. These compounds are ground into a fine powder and suspended in lotion or cream. They have a white color because they reflect visible light as well as UV radiation. Because it does not allow any radiation to pass to the skin, a thick layer of one of these white creams provides excellent protection from sun damage.
Other compounds used in sunscreens and sun blocks absorb the radiation rather than reflecting it. These ingredients allow visible light to pass through so they are invisible on the skin. However, when ultraviolet light strikes a molecule, the radiation is absorbed, increasing the energy of the molecule. This energy is converted to heat, which does not damage the skin.
Like visible light, ultraviolet radiation consists of not just one wavelength, but an entire spectrum of wavelengths. Many of the compounds used in sunscreens absorb only part of the spectrum, so an effective sunscreen may need more than one component to provide complete protection. The labels for these products show that they block both UVA and UVB radiation, which means that they work against the entire range of harmful UV wavelengths.
A glass window can protect you from sunburn. Glass is transparent to visible light, but it absorbs ultraviolet radiation. You can get a sunburn working in the garden, but not the greenhouse—unless the greenhouse has plastic windows.
