(a) The order of reactivity is: Y > X > Z. Metal Y reacts fastest, followed by Metal X, and then Metal Z reacts slowest.

(b) The reaction rate is faster when a metal is more reactive. A more reactive metal readily loses electrons to form ions, leading to a quicker reaction with hydrochloric acid. Therefore, the faster reaction rate of metal Y indicates it releases electrons more readily than metal X and metal Z.

Magnesium (Mg) reacts with steam (H₂O(g)) to produce hydrogen gas and magnesium oxide. The reaction is:

However, when magnesium reacts with steam, the reaction is:

The reaction occurs because the magnesium is reacting with the water vapour, which contains oxygen. The magnesium metal reacts with the oxygen in the steam to form magnesium oxide. The reaction is not particularly vigorous, but it is noticeable. The reaction is endothermic, meaning it requires heat to occur. The heat is supplied by the reaction of the magnesium with the oxygen in the steam.

Potassium (K) reacts vigorously and exothermically with cold water. The reaction is very rapid and produces hydrogen gas and potassium hydroxide. The heat produced is often sufficient to ignite the hydrogen gas, leading to a small explosion. The reaction equation is:

Sodium (Na) also reacts vigorously with cold water, although less so than potassium. It produces hydrogen gas and sodium hydroxide. The reaction is exothermic, but the heat produced is usually not enough to ignite the hydrogen. The reaction equation is:

Calcium (Ca) reacts with cold water, but the reaction is much slower than that of sodium or potassium. It produces hydrogen gas and calcium hydroxide. The reaction is not exothermic and the hydrogen gas is not usually ignited. The reaction equation is:

The reactivity of these metals with water increases down the group (K > Na > Ca). This is because the ionization energy decreases down the group, making it easier to lose electrons and form the metal ion. The smaller ionic radius of potassium also contributes to the higher reactivity.