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A combustion reaction is a type of chemical reaction where a substance combines with oxygen to produce heat and light. This kind of reaction can usually be observed in the form of a fire or flame. For a combustion reaction to occur, there must be a fuel (the substance that burns), oxygen, and an ignition source, like a spark or heat.
In the case of acetone, a common organic solvent, combustion involves reacting with oxygen to produce carbon dioxide and water. The balanced reaction for the combustion of acetone is:

• Unbalanced: \( \text{CH}_3\text{COCH}_3 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} \)
• Balanced: \( \text{CH}_3\text{COCH}_3 + 4\text{O}_2 \rightarrow 3\text{CO}_2 + 3\text{H}_2\text{O} \)

Balancing a combustion reaction often involves matching the number of carbon, hydrogen, and oxygen atoms on both sides of the equation. This ensures the reaction follows the law of conservation of mass, meaning no atoms are created or destroyed.

A decomposition reaction is a chemical reaction where a single compound breaks down into two or more simpler substances. These types of reactions are the opposite of combination reactions and often require energy in the form of heat, light, or electricity to occur.
For the decomposition of mercury(I) carbonate, the compound Hg₂CO₃ decomposes into carbon dioxide, mercury, and mercury oxide. This reaction can be depicted as:

• Unbalanced: \( \text{Hg}_2\text{CO}_3 \rightarrow \text{CO}_2 + \text{Hg} + \text{HgO} \)
• Balanced: \( \text{Hg}_2\text{CO}_3 \rightarrow \text{CO}_2 + 2\text{Hg} + 2\text{HgO} \)

Balancing this reaction requires ensuring that the number of each type of atom is the same on both sides. In this case, it involves adjusting coefficients to account for two mercury atoms and balancing the mercury oxide produced.

A combination reaction, also known as a synthesis reaction, involves two or more simple substances combining to form a more complex compound. These reactions can be seen as the reverse of decomposition reactions and usually release energy, making them somewhat exothermic.
In the combination reaction between sulfur dioxide and water, these reactants form sulfurous acid as shown in the equation:

• Equation: \( \text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \)

Interestingly, this particular reaction does not need any additional balancing, as all the atoms are already balanced in the given equation. This is because the simple addition of the reactants directly forms the product without any left-over atoms, perfectly adhering to the conservation of mass. Combination reactions like this indicate a straightforward path from reactants to products.