Which of the following metals can react with water? (a) \(\mathrm{Au},(\mathrm{b}) \mathrm{Li},(\mathrm{c}) \mathrm{Hg},(\mathrm{d}) \mathrm{Ca},(\mathrm{e}) \mathrm{Pt}\)

Alkali metals make up Group 1 of the periodic table and include elements like lithium (Li), sodium (Na), and potassium (K). These metals are characterized by their single valence electron, which they lose easily, making them highly reactive. When alkali metals come in contact with water, vigorous reactions occur, producing hydrogen gas and alkali hydroxides. For example, lithium reacts with water to form lithium hydroxide.
For safety reasons, these elements are generally stored under oil to prevent reactions with moisture in the air. In terms of reactivity with water, lithium is the least reactive of the alkali metals, while the others become progressively more reactive as you move down the group.

Moving to the second column of the periodic table, we find the alkaline earth metals like beryllium (Be), magnesium (Mg), and calcium (Ca). These elements have two valence electrons and while they're not as reactive as their neighbors, the alkali metals, they still readily react with water, especially as you go down the group. Calcium, for example, reacts with water at room temperature to form calcium hydroxide and release hydrogen gas.
However, not all alkaline earth metals react with water at the same rates. Magnesium reacts slowly with cold water but rapidly with hot water, while beryllium does not react at all. This variance is due to factors such as the metal's atomic size and the strength of the metallic bonding.

Noble metals, which include gold (Au), mercury (Hg), and platinum (Pt), are situated towards the bottom of the reactivity series and are known for their resistance to corrosion and oxidation. Unlike alkali and alkaline earth metals, noble metals do not react with water. They have very stable electronic configurations which make them less prone to losing electrons.
Because of their stability, noble metals are highly sought after for jewelry and industrial applications, and their non-reactivity with water is a key property that contributes to their desirability and usefulness in various fields including electronics and catalysis.

The reactivity series is a tool used by chemists to predict the outcomes of metal-water reactions, among other types of reactions. It lists metals in order of decreasing reactivity. Alkali metals top the series, followed by alkaline earth metals, making them both reactive with water. Noble metals are found at the bottom of the series, making them the least reactive.
Understanding the reactivity series is crucial for predicting which metals can displace others in compounds or which will react with substances like water. In the given exercise, lithium and calcium were correctly identified as reactive based on their positions in the series, while gold, mercury, and platinum do not react due to their low reactivity.