Which metal element is not solid at room temperature?

What is the order of ionic conductivity for the given cations at a specific concentration?

When substance $A$ reacts with water,it produces a combustible gas $B$ and a solution of substance $C$ in water. When another substance $D$ reacts with solution $C$,it produces the same gas $B$,even when reacting with dilute sulfuric acid at room temperature. $A$ gives a golden yellow flame in a Bunsen burner flame. What are $A, B, C,$ and $D$ respectively?

The hydroxides and carbonates of sodium and potassium are easily soluble in water,while the corresponding salts of magnesium and calcium are sparingly soluble in water. Explain.

Give the preparation and uses of sodium hydrogen carbonate (Baking soda - $NaHCO_3$).

Which of the following solubility orders is correct?