Breaking Bonds: Decomposition

Why it matters

You cannot separate salt from water by picking it out, nor can you split water into hydrogen and oxygen with a filter. Breaking a compound's bonds takes a chemical change, driven by an energy input such as heat or electricity.

The idea in plain words

Compare the reactant and products: 2HgO breaks down into 2Hg and O₂. Note that one compound yields two simpler substances.

Predict first

Why can't we separate the elements in a compound using physical methods like filtration or distillation?

In a compound, atoms are locked together by chemical bonds. Only a chemical change can break these bonds to release the constituent elements, and we bring it about with an energy input such as heat or an electric current.

• The elements are mixed too tightly to be seen.
• The elements are chemically bonded and require energy to break apart. — correct
• Physical methods are only for solids.
• Compounds are too small to be separated.

What you do

First, heat the red mercuric oxide powder. Watch it turn into silvery liquid mercury and oxygen gas. Next, pass electricity through water. Observe the gases collected at the electrodes.

Check yourself

In the thermal decomposition of mercuric oxide (2HgO → 2Hg + O₂), what visual change confirms the reaction?

The reaction data specifies a colour change from red to silvery mercury droplets, indicating the formation of elemental mercury.

During the electrolysis of water (2H₂O → 2H₂ + O₂), what is the volume ratio of Hydrogen to Oxygen gas collected?

The equation 2H₂O → 2H₂ + O₂ shows two moles of hydrogen gas for every one mole of oxygen gas, resulting in a 2:1 volume ratio.

Which condition is strictly required for the electrolysis of water to occur?

Electrolysis is defined by the use of electrical energy to drive a non-spontaneous chemical reaction, as seen in the water decomposition data.

• The powder turns from red to silvery droplets. — correct
• The powder turns black.
• The powder dissolves in water.
• The powder glows blue.
• 1 volume H₂ : 1 volume O₂
• 2 volumes H₂ : 1 volume O₂ — correct
• 1 volume H₂ : 2 volumes O₂
• 4 volumes H₂ : 1 volume O₂
• High temperature
• Sunlight
• Electric current — correct
• A catalyst like manganese dioxide