How can you prove that mercury oxide and chalk are complex substances? What phenomena (physical or chemical) will this proof be based on?

Complex substances consist of atoms of different elements, therefore, when they decompose, new substances can be obtained. This phenomenon is based on a chemical phenomenon.
Decomposition of mercury (II) oxide is orange. observes a color change to black – mercury is formed, and a gas is also released – oxygen.
2HgO = 2Hg + O2
When chalk decomposes, carbon dioxide is released:
CaCO3 = CaO + CO2

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