Here's how to write the balanced equation for the single replacement reaction of calcium metal with hydrochloric acid:

1. Identify the reactants and products:

* Reactants: Calcium metal (Ca) and hydrochloric acid (HCl)

* Products: Calcium chloride (CaCl₂) and hydrogen gas (H₂)

2. Write the unbalanced equation:

Ca(s) + HCl(aq) → CaCl₂(aq) + H₂(g)

3. Balance the equation:

* Calcium (Ca): One Ca atom on each side, so it's balanced.

* Chlorine (Cl): Two Cl atoms on the product side (CaCl₂) and one on the reactant side (HCl). To balance, put a coefficient of 2 in front of HCl.

* Hydrogen (H): Two H atoms on the product side (H₂) and two on the reactant side (2HCl), so it's balanced.

4. The balanced equation is:

Ca(s) + 2HCl(aq) → CaCl₂(aq) + H₂(g)