The metal takes hydrogen’s seat

The idea

A metal displaces hydrogen from an acid as hydrogen gas and forms a salt: Acid + Metal → Salt + Hydrogen gas.

Fizzing zinc

Drop zinc granules into dilute acid. Bubbles stream up. A burning candle brought near the gas goes pop and the gas burns.

Which gas is it?

• Hydrogen
• Oxygen
• Carbon dioxide

Zinc in acid

First, do the experiment. A teacher does this in the school lab.

Now zoom in to the atoms. Add each metal to its acid and watch hydrogen.

A metal swaps places with hydrogen

A metal displaces hydrogen from an acid as hydrogen gas and forms a salt: Acid + Metal → Salt + Hydrogen gas.

The metal takes the place of hydrogen. The hydrogen atoms pair up and leave as H₂ gas.

• Metal | Acid | Salt | Gas
• Zn | HCl | ZnCl₂ | H₂
• Mg | H₂SO₄ | MgSO₄ | H₂
• Fe | HCl | FeCl₂ | H₂

Notes

A metal displaces hydrogen from an acid as hydrogen gas and forms a salt: Acid + Metal → Salt + Hydrogen gas.

Check yourself

Zinc is dropped in dilute acid. Which gas bubbles out?

Zn + H₂SO₄ → ? + H₂. What is the salt?

What does the metal do to the acid?

Acid + Metal → ?

• Oxygen. The gas burns with a pop. That is hydrogen, not oxygen.
• Carbon dioxide. Carbon dioxide does not burn. The pop test shows hydrogen.
• Hydrogen — correct. Yes! Metal + acid gives hydrogen gas.
• ZnSO₄ — correct. Yes! Zinc takes the place of hydrogen next to SO₄.
• ZnH₂. The hydrogen leaves as gas. It does not stay with zinc.
• ZnO. There is no oxygen gained here. The zinc joins SO₄.
• It joins the hydrogen to make H₂. The hydrogen atoms pair up with each other. The metal joins the rest of the acid.
• It takes the place of hydrogen — correct. Yes! Hydrogen is pushed out and the metal joins the rest of the acid.
• Nothing, it only sits there. The metal changes. It forms a salt.
• Salt + water. Salt + water comes from an acid and a base.
• Salt + carbon dioxide. Carbon dioxide comes from an acid and a carbonate.
• Salt + hydrogen gas — correct. Yes! That is the pattern.