<span>Water is a polar molecule. If a solute dissolved in water is polar molecule, it will dissolve in water. If a solute dissolved in water is non-polar like oil it will not dissolve in water. Polar dissolves in polar.</span>
Calcium with water:
When a metal reacts with water( cold water or hot water) then the products formed are metal hydroxide and hydrogen gas.
Metal + steam --------> Metal hydroxide + Hydrogen
Calcium reacts with cold water to form calcium hydroxide and hydrogen gas:
potassium with water.
Potassium reacts violently with cold water to form potassium hydroxide and hydrogen gas:
2K(s)+ 2H2O(l) --------> 2KOH(aq) + H2(g) + heat.
In this reaction so much heat is produced get hydrogen gas formed catches fire and burns explosively.
Magnesium ribbon dissolves in zinc nitrate and copper nitrate solutions.
Even though the question is incomplete and refers to your practical work, however, I will try to help you as much as I can.
Metals dissolves in solutions of other metals that are lower than them in the electrochemical series. Hence, copper strip will show no change in magnesium nitrate or zinc nitrate solution. A zinc strip will not show any change in magnesium nitrate or zinc nitrate.
However, a magnesium ribbon will dissolve very quickly in zinc nitrate and copper nitrate solutions.
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