The elements that together we call the halogens can all be found in the sea. The ancient Greeks knew where to find them because the word "halogen" comes from the Greek word meaning "salt-producing". Although you cannot see any trace of salt, taste sea water and you will find that it is salty, for dissolved in the water is sodium chloride, a compound of chlorine, one of the halogens, and sodium. Sodium chloride is commonly known as table salt.

Sodium chloride is not the only salt in sea water. You will also find salts containing fluorine (as in the fluoride put into your toothpaste), iodine (as in the substance used to iodise table salt) and bromine (as in the bromide used on photographic film).

The sea contains natural collectors of some of these valuable elements. In particular, iodine is gathered by seaweed and stored in its tissues.

Each of the halogen elements is so reactive you will never find a halogen alone. Halogens bind so well to other atoms that great energy has to be applied if they are to be separated from their compounds. This may be just as well, because all of the halogen elements are very dangerous. Chlorine, for example, has been used as a chemical weapon in wartime, and fluorine gas is so hazardous that laboratory demonstrations using it were not even attempted for this book!

As compounds, though, the halogens are harmless, even beneficial. Think of the halogens as health-givers. Iodine salts are essential to prevent thyroid gland problems, fluoride strengthens your teeth, and chlorine salts can be used to preserve food and disinfect water supplies. Halogens also make up many of the commonly used anaesthetics used in surgery. Chloroform (a compound containing chlorine) was one of the early anaesthetics; halothane (a compound containing fluorine, chlorine and bromine) is now used in its place.

Chlorine

Chlorine is a greenish-yellow gas (the name chlorine in Greek means greenish-yellow), which is more dense than air and which will dissolve to some extent in water. It forms an amber-coloured liquid at its boiling point.

Nearly one-fiftieth of sea water is made of chlorine, where it occurs combined with sodium as sodium chloride.

People become sensitive to the presence of chlorine in concentrations as little as three to five parts per million because it irritates the membranes of the nose. At concentrations of 30 parts per million its effects on the eyes and nose become severe. At only slightly higher concentrations, it causes difficulty in breathing.

Like other halogens, chlorine is a very reactive substance and it is never found as a free element in nature. It is particularly reactive in the presence of heat, and although it does not dissolve well in water, it does react in the presence of moisture. It is particularly reactive with organic substances, sometimes causing explosions. For these reasons chlorine needs to be kept in dry, cool conditions.

A flame will continue to burn in a vessel containing chlorine gas. Many products containing chlorine (such as PVC, polyvinyl chloride) will decompose when heated. If used extensively for home furnishings, for example, these materials can be a major hazard during a fire.

Chlorine is one of the ten most important industrial chemicals. The success of an industrial country is sometimes measured in the amount of chlorine its chemical factories use!

Laboratory preparation of chlorine

A gas jar of chlorine gas is prepared by reacting concentrated hydrochloric acid on potassium permanganate crystals. Chlorine is heavier than air, so it can be allowed to collect at the bottom of a gas jar, forcing the air out of the top.

Dangers of chlorine

Transporting chlorine can be a health hazard. If the transporting vessel is involved in an accident and the chlorine escapes to the atmosphere, many people might have their lungs damaged or even be killed. For this reason chlorine is not usually transported; instead other chemicals are brought to where the chlorine is made. This is the case, for example, with making the common plastic PVC. A material called ethene is brought to the chlorine plant to be reacted with the chlorine. The product is shipped off to be further manufactured elsewhere.

Also

When large amounts of waste hydrochloric acid are available ­
for example, when PVC is being made ­ the chlorine can often be recovered and recycled by oxidising the acid.
EQUATION: Preparation of chlorine

Concentrated hydrochloric acid + potassium permanganate = chlorine + water + manganese chloride + potassium chloride

16HCl(aq) + 2KMnO₄(s) = 5Cl₂(g) + 8H₂O(l) + 2MnCl₂(aq) + 2KCl(aq)

Properties of chlorine Pungent smell
Chloride ions make up 0.015% of the Earth's crust
Found in common salt
About two and a half times as dense as air
Slightly soluble in water
The foundation for hydrochloric acid
Chloride ions make up nearly 2% of sea water
Atomic number 17,
atomic weight about 35

Also
Why halogens are so reactive

Each of the elements in the halogen family contains seven electrons in its outermost electron shell. To be stable it needs to have eight. As a result the atoms react with other elements to gain an electron and fill the shell.
The smallest atoms try to fill their electron shell most vigorously, and as fluorine is the smallest of the family, it is the most reactive. Iodine has the largest atoms and is the least reactive of the family.