Why Is Milk White? (19 page)

The whole world is made of atoms, and those atoms react with one another to create the molecules that make up everything we see. What things are made of and how they come to be made is the province of chemistry.

Some chemical reactions are interesting because of the hint of danger. I keep a Dewar of liquid nitrogen here in the lab, and Alexa loves to play with it when she comes over, freezing whipped cream so that she can chew it and make fog come out of her mouth like a dragon. She has heard of things like electrons and gluons and biochemistry, but until she asked the questions, she had no idea why they might be important.

Because salt dissolves easily in water. Water in the oceans evaporates into the air, leaving solids like salt behind. The water vapor then rises and cools until it falls as rain. If the nearly pure water in the rain falls on land, it dissolves some of the dirt it falls on. Rain
that has dissolved carbon dioxide in it is slightly acidic, and the acid helps dissolve the dirt. Dirt is mostly silica, so about 15 percent of the dissolved solids in river water turns out to be dissolved silica—silicon dioxide, what glass and quartz is made of.

Many things dissolve more easily than silica. Gypsum (calcium sulfate), for example, and chalk (calcium carbonate), each dissolve in slightly acidic water, adding calcium, sulfate, carbonate, and bicarbonate ions to the water. But sodium and chlorine ions are even more soluble. There is just much more of the other ions in dirt than there is salt. So the solids in river water are mostly bicarbonate ions (from the carbon dioxide in the air), calcium, silica, sulfate, chloride, sodium, and magnesium, in that order.

But when the river water gets to the sea, the organisms in the ocean start to remove ions from the water to build their shells. Diatoms in plankton remove silica. Other plankton and shellfish remove calcium and bicarbonate ions to make shells and coral reefs.

As the water evaporates and concentrates the ions, the less soluble ones precipitate out of the water and fall to the bottom of the ocean. Calcium carbonate, calcium sulfate, and magnesium sulfate form deposits on the sea floor. But no living organism builds its house out of salt, and very little salt gets locked up in the mud. Therefore, ocean water ends up being mostly salt water, with a number of other molecules in it, but in much smaller amounts.

Liquid nitrogen boils off into the air, where it came from, at room temperature.

Liquid nitrogen is made by compressing and cooling air. Air is mostly nitrogen. The oxygen in the air becomes a liquid at a higher temperature than nitrogen, so it liquefies first and boils off last, when air is liquefied. When the gases that liquefy first are removed, almost pure nitrogen is left.

Liquid nitrogen boils at -320° Fahrenheit. So having a bowl of liquid nitrogen in your kitchen is similar to having a bowl of water in an oven at 600° F. The water would boil away. But as long as there was water left in the bowl, the water itself would still only be at a temperature of 212° F—the boiling point.

The same thing happens with liquid nitrogen. As long as there is liquid in the bowl, it cannot get hotter than the boiling point. So anything we put in the liquid will be cooled to the boiling point of the liquid.

When a rose is put into liquid nitrogen, the liquid vigorously boils around the rose, since the rose is 400° F hotter than the liquid's boiling point. The rose transfers its heat to the nitrogen until they are both at the same temperature: -320° F.

At that temperature, the water in the rose petals is frozen solid. The petals become as brittle as thin sheets of glass. If the rose is then dropped onto a table before the air can melt the ice, the rose will shatter into hundreds of tiny shards. But you have to act quickly—the air is 400° F hotter than the rose petals and can melt the ice quickly.

Sometimes it is just because they are colored chemicals. If you rub your hands in food coloring, on grass, or on wet paint, the molecules that make these things absorb light will be left on your hands.

Some molecules are better at sticking to skin than others. You can rub your hands in dark yellow egg yolk, but the dark substance is easy to wipe off with a paper towel. It does not leave a stain. The molecules that make egg yolk yellow are large and don't attach themselves to the skin.

Food coloring, on the other hand, is made from very small molecules that can get into the very small crevices and pores in the skin, and they can even react with the skin itself, forming chemical bonds that make them stick tightly there. To wash them off, you
need lots of water to help them dissolve and be carried away, but you may also need to scrub off the top layer of dead skin cells to which the dye has bonded.

Some chemicals react with the skin and change its color. Skin can be burned or bleached with strong oxidizers. Some other chemicals react with the skin and themselves change color. Potassium permanganate (KMnO
₄
) will oxidize (burn) the skin and make manganese dioxide, which stains the skin brown. Silver nitrate will react with the skin and the salt on the skin to form silver chloride, which will then break down in strong light to form tiny particles of silver. These silver particles stay stuck in the skin and look black. As the skin grows and the cells die and are scrubbed off, the stain gradually goes away.

Nitric acid reacts with (burns) the skin, creating a yellow or brown stain. It is particularly dangerous because the nerves in the skin do not react to it, so you may not realize you are getting burned.

That will depend on the acid and on how much the acid is diluted. There are strong acids and weak acids. There are oxidizing acids and non-oxidizing acids. In general, skin reacts to strong acids and oxidizing acids.

People handle weak acids all the time. Carbonated water is a weak acid. So is the citric acid that makes orange juice taste sour. Some weak acids, like the acetic acid in vinegar, can attack skin if they are concentrated. In vinegar, the acetic acid is highly diluted, so you can even drink it. Some strong acids, like hydrochloric acid, don't react much with skin if they are diluted well. Your stomach produces hydrochloric acid, and the lining of the stomach is protected from it by a layer of mucus. Even so, the hydrochloric acid in the stomach is often less acidic than the typical carbonated beverage, because it is diluted with water.

Concentrated strong acids and diluted oxidizing acids can burn skin. The acids react with the proteins in skin and break them down, so they can no longer act as a barrier. The acid can then continue to react with tissues, killing cells. Living tissue can only function within a narrow range of acidity, and outside of that range the cells die.

Sulfuric acid is not only a strong acid, but it reacts with the water in your skin so strongly that it will create blisters. This is not because of its acidity but because of its dehydrating ability.

You can protect your hands from strong acids by wearing gloves made of materials the acid cannot attack. With many acids the fumes are also dangerous, so you should also make sure that you have plenty of ventilation. The fumes can attack the lining of your nose, throat, and lungs, as well as your eyes. Always wear eye protection when working with acids and bases.

That will depend on the acid. Leaves are protected by several barriers, such as a wax coating, and thick cell walls made of lignin, cellulose, and pectin, none of which react very much with most acids. But they do eventually react, if slowly. That is why special acid-free paper is used for artwork and archival documents. Paper is mostly cellulose, and acid will eventually make it brittle and yellow.

Strong acids like sulfuric and nitric acid will dissolve a leaf. The thinnest parts of the leaf will dissolve first, since there is less material there. The result is a lacy, delicate web of the ribs of the leaf that give it strength and structure. You could then neutralize the acid to prevent further corrosive action and preserve the lacy leaf.

Living leaves have pores in them (called
stomata)
that allow them to breathe. If acids get in these pores, it can kill the cells inside the leaf.

Dead leaves have less protective wax on them, and they can absorb water and acids more easily. The dried leaves are thus more
prone to attack by acid and will deteriorate faster than a freshly picked green leaf.

Electrons jump away from one another. That one fact explains most of electricity. If you push more electrons into a wire, they all push against one another and will move toward the place where there is the least pressure. We call that pressure
voltage.
A
current
is simply how many electrons are moving past a particular point in a second.
Electrical power
is how much pressure there is, multiplied by the amount of current.

In chemistry, electrons are what hold molecules together. Electrons are attracted to the positive charges at the center of atoms, but only specific numbers of electrons can fit in each energy-level shell around the nucleus.

If an atom has an empty spot in an energy level, and another atom has an electron in an otherwise empty energy level, that lone electron can fall into the empty space in the other atom's shell, and the two atoms will stick together, because both nuclei will be pulling on that one electron.

This also works if there is more than one empty slot or more than one extra electron. The electrons will fall into the energy level as close to a nucleus as possible, no matter which atom it is in. So an oxygen atom, which has two empty slots in its shell, can take the lone electron from two hydrogen atoms and make a molecule of water. By sharing electrons in this way, the electrons can fall into the lowest energy level, closest to the nucleus.

In metals, the outer energy shells of the atoms merge into one big, empty slot, and the outer electrons can move around freely. In atoms like chlorine, the empty slot is so close to the nucleus that the electron spends most of its time near the chlorine atom and little time around the atom it came from (for example, the sodium atom in a molecule of salt). This allows the molecule to dissolve easily in water, leaving a positive sodium ion and a negative chloride ion.

Electricity is mostly mechanically produced. Mechanical energy is turned into electrical energy in machines called
generators.

Electricity and magnetism are two sides of the same thing. Moving electrons create magnetic fields. Moving magnetic fields cause electrons to move. In a generator, a magnetic field is made to move near copper wires. The electrons in the wires begin to move, and the moving electrons are what we call electricity.

Moving electrons can heat up wires as they move through them. Electric stoves and incandescent lights work by heating up wires this way. Since moving electrons create magnetic fields, and magnets can attract one another, we can make electric motors to power our gadgets around the house.

The electricity that comes out of the plug in your house is made by a generator, but there are other ways to make electricity. It can be made directly from heat in a simple device called a
thermocouple.
Twist together two different kinds of wire, such as copper wire and iron wire, and when you get the twisted part hot, it makes electrons move.

Electricity can also be made from light using solar cells or from pressure by using a piezoelectric ceramic, such as those in electric lighters. Electricity can be made by moving electrons on an insulator past some sharp wires in a Van de Graaff generator or chemically by building a battery.

Metals are used to make a battery. Metals have a convenient property called
conduction
, in which the electrons in the metal are not bound to just one atom at a time as in other materials but are free to wander from atom to atom. This allows the metals to conduct electricity, because electricity is just charged particles moving through something like a wire.

Many other things can also conduct electricity. For example, hydrochloric acid is a good conductor. When hydrogen chloride (HCl) dissolves in water, it breaks up into hydrogen ions (H+) and
chloride ions (Cl
^-
). These electrically charged particles can move through the water. Moving charged particles is electricity.

If you put a strip of aluminum and a strip of copper into the same acid and then the two metal strips are connected to a meter, you can watch electricity being created. Copper holds on to electrons more tightly than aluminum does. Electrons start to move from the aluminum, through the meter, to the copper.

This leaves the aluminum with a positive charge. It attracts the negative chloride ions to it, through the water. The chloride ions attract positive aluminum ions away from the metal strip and into the water, where they form an aluminum chloride solution.