CRYSTALS
In many solid substances, the atoms or molecules are fixed in place but they are not positioned at random. They are arranged in an orderly or regular pattern known as a crystalline framework. The result is that the substance forms crystals. These are not irregular lumps but orderly, geometric shapes with sharp edges and flat sides at certain angles to each other. Many pure metals have a crystalline structure. So do minerals in the rocks, and sugar and salt.
There are seven basic shapes or systems of crystals. Simplest is the cubic shape which is like a box. Diamonds are cubic crystals. The monoclinic system is like a matchbox which has been squashed slightly flat. The calcium-rich mineral gypsum has this shape. Some natural minerals like ruby and emerald form large, shiny crystals with beautiful colours. They are cut and polished as gemstones.
Crystals of quartz in sand grains have a triangular shape. Ice crystals form snowflakes and are six-sided.
Tiny crystals of minerals in many kinds of rocks fit together like miniature building bricks to form much bigger regular shapes. The hexagonal (six-sided) pillars are made of the rock basalt.
SOLUTIONS
Stir a teaspoon of table salt into a glass of water – and the salt disappears. However, tasting the water shows the salt is still there. It has dissolved. The large grains or crystals of salt have broken down into their individual atoms. These are too small to see and float about freely among the molecules of water. The substance which dissolves, which is usually a crystalline solid, is the solute. The substance it dissolves in, usually a liquid is the solvent. The solute in the solvent is known as a solution.
When substances dissolve, their atoms or molecules usually gain or lose electrons. For example table salt, sodium chloride (NaCl), dissolves and breaks apart into its atoms of sodium (Na) and chlorine (Cl). Sodium loses an electron and becomes positive (Na+), while chlorine gains an electron and becomes negative (Cl-). Atoms which are positive or negative are known as ions. Many solutes form ions.
Shaking salt and sand together produces a mixture, which is different from a solution. In a mixture two or more substances intermingle but they do not dissolve and their molecules do not alter by becoming ions. Add water to the mixture and the salt dissolves. It is soluble. The sand grains do not dissolve. They are insoluble.
The sea contains dissolved salt. The Dead Sea has so much that no more can dissolve: a saturated solution.
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