Why Is Milk White? (23 page)

This is why an iceberg has 91 percent of its volume underwater and only 9 percent above the water. You can see this when you look at an ice cube floating in a glass of lemonade.

Water generally has air dissolved in it. As the water molecules form nice crystals of ice, the air comes out of solution. This forms tiny bubbles of air in the ice, which is why most ice cubes look white instead of clear. The bubbles make the ice float even higher than it would without them.

You can make ice without bubbles. Boil some water, so that most of the air leaves the water. Then transfer the water to the ice cube tray and freeze it. If there are still some bubbles in the ice, you can thaw the ice again, letting the bubbles escape, and refreeze it. The result will be crystal-clear ice cubes.

Organic chemistry is the study of carbon compounds. Carbon is special. It is small, having only six electrons. Two of them are in the low energy inner cloud, leaving four in the outer cloud where
they can form bonds with other atoms. These two things are what make carbon special.

Being small, carbon can easily fit into molecules that would not have room for larger atoms. Being small also means that the electrons are close to the nucleus, so strong bonds can be formed.

Having four outer electrons means that carbon also has four empty slots for electrons from other atoms, since the second electron shell has room for eight electrons. Carbon can form lots of bonds with other atoms, forming long chains, loops, sheets, branching tree-like structures, and many other forms. This versatility is what leads to life. We call carbon chemistry
organic
because life is based on carbon compounds.

Organic chemistry is the study of carbon compounds whether or not they come from living things. We see carbon compounds in interstellar dust, inside meteorites, in coal and petroleum, and in the flames as carbon-based fuels burn. Organic chemistry is usually thought of as the chemistry of compounds that have a C-H bond (carbon bonded to hydrogen), although there are organic molecules that have no hydrogen, such as Teflon.

The study of chemical reactions in living things is a separate branch of chemistry called
biochemistry.
Of course, the two fields (organic chemistry and biochemistry) are closely related and overlap in many areas. Inorganic chemistry also overlaps with organic chemistry, as many simple carbon compounds such as chalk and carbon dioxide are considered inorganic, even though both are usually made by living things.

Living organisms have a very complex chemistry, producing proteins, carbohydrates, nucleic acids (including DNA and RNA), lipids (fats and oils), and other biomolecules such as hormones.

The study of these molecules, and especially of the processes involved in making these molecules, is called biochemistry. A closely related and overlapping field is called molecular biology. Genetics, which once dealt only with inheritance, is now mostly a branch of biochemistry or molecular biology, since the discovery of DNA and the ability to sequence an organism's genetic code.

Biochemistry links the functions of an organism to the proteins and other biomolecules involved in its makeup. Molecular biology relates the genes in an organism to the proteins and biomolecules that the genes code for. Genetics relates the genes of an organism to the functions of the organism. Together, the three disciplines form the foundation of modern biology and medicine.

There are groups of biomolecules that are of particular importance to biochemists. Four in particular are carbohydrates, lipids, proteins, and nucleic acids.

Carbohydrates are sugars and things made from sugar. They are called carbohydrates because they have a chemical formula in which there is one carbon for every water molecule. Simple sugars can be joined to form disaccharides (like table sugar) or longer chains of sugars like starch, cellulose, and pectin.

Lipids are long chains of carbons, attached at one end to a glycerin molecule by a fatty acid end. Short-chained lipids with kinked tails make oils. Longer chains, or chains that are flexible and unkinked, can lie flat against one another easily, forming more solid fats.

Proteins are long chains made up of molecules called amino acids. Your skin, hair, and muscles are made of protein, and proteins called enzymes control almost every chemical reaction that happens in your cells.

Nucleic acids are the building blocks of DNA and RNA, the long molecules that encode how proteins are made in cells. This is the code that determines whether an organism is a tree, a frog, or a human.

There are five main branches of chemistry, and then there are many subdivisions of chemistry that cover specialized fields.

The five main branches are inorganic chemistry, organic chemistry, analytical chemistry, physical chemistry, and biochemistry.

Biochemistry and organic chemistry have been discussed already. Inorganic chemistry is usually described as the chemistry organic chemistry does not deal with. Analytical chemistry is the study of the properties of matter and tools to discover those properties. Physical chemistry applies physics to the study of chemistry, including thermodynamics and quantum mechanics.

Sub-branches of chemistry include astrochemistry (the chemistry in stars and interstellar gas and dust), electrochemistry (what happens when electrical currents flow though chemicals), food chemistry, geochemistry (the study of the composition of the Earth), nuclear chemistry, polymer chemistry, spectroscopy, theoretical chemistry, and many more.

Because chemistry is too big a subject for any one person to fully understand, people specialize in one part of chemistry, usually the part that interests them the most. Someone who studies the chemistry of petroleum products might have little need to know about the chemistry that goes on in interstellar nebulae or the chemistry of snail mucus. A person studying DNA to find cures for cancer might not want to take time out to learn about how to make a better rubber band.

Specializing allows someone to concentrate on one area of chemistry and one set of chemical techniques that are important in that area. A molecular biologist has less need of a spectroscope than an astrochemist would, and an explosives chemist would have no use for a DNA sequencer.

Theoretical and quantum chemists might not need any equipment at all, other than paper, pencil, and perhaps (these days) a computer.
But industrial chemists might spend a lot of time designing chemical equipment to produce things more efficiently or more safely.

Chemical bonds are what hold things together. Electrons are attracted to protons. Protons are contained in the nucleus of the atom, and electrons crowd in as close as they can to the nucleus, attracted by the protons. But electrons repel one another, and only a few can occupy each energy level around the nucleus.

Electrons try to get as close as possible to a nucleus but are prevented from getting too close if the spots near the nucleus are already filled with other electrons. This means that if we have two atoms, and one has an electron far away from the nucleus and the other has an empty spot near the nucleus, the electron from the first atom can fall into the empty slot of the second atom.

Sometimes the electron actually leaves the first atom and joins the second. That leaves two ions. The first is positively charged, since it has one more proton than electron. The second is negatively charged, since it has an extra electron. These two ions are attracted to one another, because they have opposite charges.

In other cases, the electron doesn't fully leave the first atom. It falls into the empty slot in the second atom, but it still fills the outer slot in the first atom. The two atoms are held together by the attractions of their nuclei to the shared electron.

In the metallic elements, the outer electrons are very mobile, since they are far away from the nucleus and are attracted to all of the nearby nuclei by about the same amount. They move from nucleus to nucleus, always staying far out where the forces of attraction are fairly weak. The effect is that the positive nuclei are surrounded by a sea of negative electrons, flitting around from atom to atom.

The first type of bond discussed in the previous answer, where the electron leaves the first atom and joins the second, is called an
ionic
bond.
This kind of bond is found in molecules like sodium chloride (salt), where an atom that easily loses its outer electron (such as sodium) combines with an atom that has an empty slot very close to the nucleus (like chlorine) that is very good at attracting electrons.

The second type discussed, in which two nuclei share an electron, is called a
covalent bond.
These are very strong bonds, and they hold molecules together very well. Bonds between carbon atoms, or between carbon and hydrogen, are usually covalent bonds.

The third type of bond discussed is called a
metallic bond
, because it is characteristic of the bonds seen in metals.

All three of these bond types are bonds in which one or more electrons is involved. There are other kinds of bonds, usually much weaker than the first three, which form when, on average, less than a whole electron is involved.

We looked earlier at the
hydrogen bond
(in the discussion of water and chemistry,
page 153
) which forms between molecules rather than between individual atoms. Molecules are required because this kind of bond only happens when an electron spends more of its time around one atom in the molecule than around another atom. This makes one side of the molecule a little more negative and the other side a little more positive. The positive end of one molecule is then attracted to the negative end of another to make the hydrogen bond.

A similar set of effects, collectively called van der Waals forces (named after the Dutch scientist Johannes Diderik van der Waals) can happen even to single atoms. The electrons around two atoms can become correlated, so that when an electron in the first atom is on one side, so is an electron on the other atom, and when the electrons spin around the atom, they do so in synchrony, so that there is always a positive side of one atom facing the negative side of the other, creating an attraction.

About 379,000 years after the universe began in the Big Bang, it had cooled enough for electrons to slow down enough to stick around protons, and the first atoms of hydrogen formed. Since scientists only know when the universe began to within 110 million years, they can't say exactly when this happened, except to say it was between 13.64 and 13.86 billion years ago.

People first started using techniques of chemistry sometime around 3,000 years ago, when they began extracting metals from ores, making beer and wine, and making pottery and glazes. The first controlled chemical reaction may have been fire.

Early attempts at understanding chemical reactions were largely unsuccessful, although the practice, called
alchemy,
led to many discoveries, including the production of several important acids, the practice of distillation, and other techniques still in use.

The beginnings of modern chemistry are usually traced back to 1661, with the publication of
The Sceptical Chymist
by Robert Boyle. Later, when Antoine Lavoisier developed the law of the conservation of mass, chemistry became a science, in which careful measurements allowed mathematical interpretation of the results of experiments.

One of the earliest people to systematize the investigation of chemical reactions was the Persian scientist Abu Musa Jābir ibn Hayyān, known as Geber to Europeans. Born in 721, Geber is credited with the invention of distillation. Much of his writings were deliberately hard to decipher, so that only other alchemists could read it, and this may be the origin of the word
gibberish,
from the name Geber. His main contribution was his stressing of the importance of experimentation in chemistry.

I have never done this. But it can be done.

Although transforming base metals like lead into precious metals, specifically gold, was a goal of early alchemists, it is not actually
chemistry. Chemistry deals with combinations of atoms, not with changing one element into another.

Transmuting elements is part of a branch of physics that deals with elemental particles.
Transmutation
(converting one element, such as mercury, into another, such as gold) occurs naturally. All of the elements heavier than lithium were created in stars by transmutation.

Nitrogen in the upper atmosphere is transmuted into radioactive carbon-14 by neutrons created when cosmic rays strike the upper atmosphere. The carbon-14 later decays back into nitrogen.

Scientists first realized that transmutation was taking place in 1901, when Ernest Rutherford and Frederick Soddy found that thorium was decaying into radium. Later, in 1917, Rutherford was able to transmute nitrogen into oxygen by bombarding it with helium nuclei (called
alpha particles).

Making gold from other metals would be more expensive than buying gold that was mined from the ground. But when absolutely pure gold is needed, it may be cheaper to create gold from mercury (by bombarding it with gamma rays) than to try to extract the copper and silver impurities often found in natural gold.

Going in the other direction, making pure mercury from gold, is also useful, as the pure mercury can be used to make a kind of light source that is very pure.

Radiation
is anything that “radiates” away from something. In chemistry and physics, it refers to light, radio waves, X-rays, and gamma rays, as well as to particles such as neutrons, protons, helium nuclei (alpha particles), and electrons (beta particles).