Preparing a Salt Using a Metal and an Acid

When a reactive metal reacts with a dilute acid, a salt is formed and hydrogen gas is released. This is one of the simplest ways to prepare a salt in basic laboratory chemistry.

1. The General Reaction

Metal+Acid→Salt+Hydrogen gas\text{Metal} + \text{Acid} \rightarrow \text{Salt} + \text{Hydrogen gas}Metal+Acid→Salt+Hydrogen gas

Examples:

• Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen

• Zinc + Sulfuric acid → Zinc sulfate + Hydrogen

• Iron + Hydrochloric acid → Iron(II) chloride + Hydrogen

2. Choosing the Metal

The metal must be above hydrogen in the reactivity series but not so reactive that it’s dangerous.

Good choices:

• Magnesium

• Zinc

• Iron

Metals to avoid:

• Sodium, potassium, and similar highly reactive metals (too violent).

• Copper, silver, gold (do not react with dilute acids to form salts).P

3. Safety Essentials

Any metal-acid reaction produces hydrogen gas, which is flammable.

Important precautions:

• Use dilute acids only (e.g., 1 M HCl or H₂SO₄).

• Work in a well-ventilated area or fume hood.

• Keep sparks/flames far away.

• Never seal the reaction vessel—gas needs to escape.

4. Step-by-Step Laboratory Method

Step 1 — Add the Metal to the Acid

• Pour a measured amount of dilute acid into a beaker.

• Add small pieces of the metal.

• The metal dissolves with bubbling (hydrogen gas).

Observation: effervescence (bubbles), heat release.

Step 2 — Wait for Reaction to Finish

• Continue until all bubbling stops.

• You should still see some unreacted metal present. This ensures all acid has been consumed (prevents excess acid in your salt solution).

Step 3 — Filter the Mixture

• Filter off the excess metal.

• The filtrate (liquid that passes through) is the salt solution.

Example:Mg + 2HCl → MgCl₂ (in solution) + H₂↑

Step 4 — Concentrate the Salt Solution

• Heat the salt solution gently to evaporate some water.

• Stop heating before crystallization begins.

Step 5 — Crystallize the Salt

• Allow the warm concentrated solution to cool slowly.

• Salt crystals form as solubility decreases.

Step 6 — Filter and Dry the Crystals

• Filter off the crystals.

• Dry them using filter paper or a warm oven (50–60°C).

5. Example Preparation

Preparing zinc sulfate from zinc and sulfuric acid:

1. Add zinc pieces to dilute sulfuric acid.

2. Wait for bubbling to stop; ensure some zinc remains.

3. Filter off unreacted zinc.

4. Heat the filtrate gently until concentrated.

5. Cool to crystallize zinc sulfate.

6. Filter and dry the crystals.

6. Why This Method Works Well

• It avoids leftover acid (the metal ensures all acid is neutralized).

• It gives a pure salt solution.

• It’s simple and suitable for many metals.