| WEIGHT | | | | DECOMPOSES
| | | | SULPHATE| HYDROX-|
| | | | | IDE |
__________|________|________|_________|_________|________|_________________
| | | | | |
Calcium | Ca | 40.1 | 1.54 | 2070.00 | 1670. | At dull red heat
Strontium | Sr | 87.6 | 2.50 | 170.00 | 7460. | At white heat
Barium | Ba | 137.4 | 3.75 | 2.29 | 36300. | Scarcely at all
===========================================================================

~The family.~ The alkaline-earth family consists of the very abundant
element calcium and the much rarer elements strontium and barium. They
are called the alkaline-earth metals because their properties are
between those of the alkali metals and the earth metals. The earth
metals will be discussed in a later chapter. The family is also
frequently called the calcium family.

1. _Occurrence._ These elements do not occur free in nature. Their most
abundant compounds are the carbonates and sulphates; calcium also occurs
in large quantities as the phosphate and silicate.

2. _Preparation._ The metals were first prepared by Davy in 1808 by
electrolysis. This method has again come into use in recent years.
Strontium and barium have as yet been obtained only in small quantities
and in the impure state, and many of their physical properties, such as
their densities and melting points, are therefore imperfectly known.

3. _Properties._ The three metals resemble each other very closely. They
are silvery-white in color and are about as hard as lead. Their
densities increase with their atomic weights, as is shown in the table
on opposite page. Like the alkali metals they have a strong affinity for
oxygen, tarnishing in the air through oxidation. They decompose water at
ordinary temperatures, forming hydroxides and liberating hydrogen. When
ignited in the air they burn with brilliancy, forming oxides of the
general formula MO. These oxides readily combine with water, according
to the equation

MO + H_{2}O = M(OH)_{2}.

Each of the elements has a characteristic spectrum, and the presence of
the metals can easily be detected by the spectroscope.

4. _Compounds._ The elements are divalent in almost all of their
compounds, and these compounds in solution give simple, divalent,
colorless ions. The corresponding salts of the three elem