OCR C4 chemical patterns part 2.

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Elements in the same group of the periodic table have similar properties. Group 1 elements are called the alkali metals. Group 7 elements are known as the halogens. The elements in a particular group have different reactivities depending on their place in that group.

Group 1 appearance.

Lithium, sodium and potassium are all soft metals that are easily cut with a scalpel or knife. The freshly cut surface is a shiny, silver colour, but this tarnishes quickly to a dull grey as the metal reacts with oxygen and water in the air. Pieces of such metals are stored in oil to prevent these reactions.

The shiny surface of sodium tarnishes more quickly than that of lithium. And potassium tarnishes more quickly than sodium. This shows the increasing reactivity of the metals as we go down the group.

Handling alkali metals.

Because the alkali metals are so reactive, care has to be taken when using them. They must not be touched because they will react with the water in sweat on the skin. Gloves may be used, and goggles should be worn.

The alkali metals show trends in physical properties down the group.

Melting point.

The alkali metals have low melting and boiling points compared to most other metals. Apart from the other alkali metals, only three metals (indium, gallium and mercury) have lower melting points than lithium. You can see from the graph that lithium, at the top of group 1, has the highest melting point in the group. The melting points then decrease as you go down the group.

Boiling point.

The boiling points of these alkali metals show a similar pattern to the melting points.

Density.

The density of a substance is a measure of how much mass it has for its size. It is measured in grams/cubic centimetre. For example gold and lead are very dense metals - even a small lump of either of them can still feel heavy. The alkali metals have low densities compared with most other metals. This means a piece of an alkali metal would feel lighter than a same size piece of another metal. You can see from the graph that lithium, at the top of group 1, has the lowest density in the group. The densities then generally increase as you go down the group.

Hardness.

The alkali metals are very soft. Lithium is the hardest alkali metal and they become softer as you go down the group.

Reaction with cold water

All the alkali metals react vigorously with cold water. In each reaction, hydrogen gas is given off and the metal hydroxide is produced. The speed and violence of the reaction increases as you go down the group. This shows that the reactivity of the alkali metals increases as you go down group 1.

Lithium.

When lithium is added to water, it floats. It fizzes steadily and becomes smaller, until it eventually disappears.

Lithium + water, makes, lithium hydroxide + hydrogen.

2Li(s) + 2H2O(l), makes, 2LiOH(aq…

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