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Acids and reactive metals

Acids will react with reactive metals, such as magnesium and zinc, to make a salt and hydrogen.

acid + metal → salt + hydrogen

hydrochloric acid + zinc → zinc chloride + hydrogen

2HCl + Zn → ZnCl₂ + H₂

The hydrogen causes bubbling during the reaction, and can be detected using a burning splint which produces a squeaky pop sound.

In general, the more reactive the metal, the faster the reaction. This is indicated by more bubbles being given off per second from the metals with higher reactivity, as shown by this diagram.

The diagram shows that aluminium is the most reactive of the four metals, followed by zinc, then iron and finally, copper.

Also note that the reaction of metals with acids is exothermic (ie heat energy is given out).

Acids and metal hydroxides (alkalis)

When acids react with metal hydroxides (commonly known as alkalis), a salt and water are made.

acid + metal hydroxide → salt + water

nitric acid + lithium hydroxide → lithium nitrate + water

HNO₃ + LiOH → LiNO₃ + H₂O

[Higher tier only]

Note that the reaction between a metal hydroxide and an acid can be represented by an ionic equation between the hydrogen ions and the hydroxide ions to form water molecules.

H⁺(aq) + OH^–(aq) → H₂O(l)

Also note that the reaction of metal hydroxides with acids is exothermic (ie heat energy is given out).

Acids and bases

When acids react with a base, a salt and water are made.

acid + base → salt + water

nitric acid + magnesium oxide → magnesium nitrate + water

2HNO₃ + MgO → Mg(NO₃)₂ + H₂O

Also note that the reaction of metal oxides with acids is exothermic (ie heat energy is given out).

Acids and metal carbonates

When acids react with carbonates, such as calcium carbonate (found in chalk, limestone and marble), a salt, water and carbon dioxide are made.

acid + metal carbonate → salt + water + carbon dioxide

sulfuric acid + iron(II) carbonate → iron(II) sulfate + water + carbon dioxide

H₂SO₄ + FeCO₃ → FeSO₄ + H₂O + CO₂

The carbon dioxide causes bubbling during the reaction, which is observed as fizzing. It can be detected by passing the gas through limewater, which will go cloudy.

Also, the reaction of metal carbonates with acids is exothermic (ie heat energy is given out).

This type of reaction can be used to test unknown solutions to see if they are acidic. Simply add a solution of sodium carbonate to the solution and if carbon dioxide gas is given off, the solution is acidic.

This type of reaction can also be used to test unknown solutions for the presence of carbonate (CO₃^–) ions. Simply add an acid to the solution and if bubbles of carbon dioxide are given off, the solution contains carbonate ions.