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A chemical formula represents the types and numbers of atoms in a molecule of a compound. It acts like a shorthand notation that tells us the composition of a compound precisely. For example, when aluminum and bromine react, we first identify the ions they form. Aluminum, being a metal, forms a cation with a +3 charge, written as \( \text{Al}^{3+} \), whereas bromine, a non-metal, forms an anion with a -1 charge, \( \text{Br}^{-} \).

To form a neutral compound, these charges need to be balanced. This means we need three bromine atoms for each aluminum atom to balance the charges. The resulting chemical formula is \( \text{AlBr}_3 \), which indicates that each molecule of this compound consists of one aluminum atom and three bromine atoms.

The importance of the chemical formula is that it not only tells us the proportion of the elements but also helps to predict the possible reactions with other chemicals, as it reflects the internal structure of the molecule.

A balanced chemical equation is a statement that uses chemical formulas to show the identities and amounts of substances involved in a chemical reaction. This equation follows the law of conservation of mass, which states that matter cannot be created or destroyed in a closed system.

For a chemical reaction between aluminum and bromine, we have the formula \( 2 \text{Al}(s) + 3 \text{Br}_2(l) \rightarrow 2 \text{AlBr}_3(s) \). This shows that two moles of aluminum react with three moles of bromine to produce two moles of aluminum bromide.

Balancing chemical equations involves adjusting the coefficients (the numbers in front of the molecules) to ensure that there are equal numbers of each type of atom on both sides of the equation. This balance ensures that the mass and the atoms are conserved in the reaction. Balancing equations is crucial as it reflects the actual ratios in which substances react and informs us about the stoichiometry of the reaction.

A combustion reaction involves the burning of a substance in the presence of oxygen, releasing energy in the form of heat and light. Many organic compounds, particularly those containing carbon (C), hydrogen (H), and sometimes oxygen (O), undergo combustion.

During combustion, complete oxidation of a carbon-containing compound leads to the formation of carbon dioxide (\( \text{CO}_2 \)) and water (\( \text{H}_2\text{O} \)). In the case of acetone, an organic compound with the formula \( \text{C}_3\text{H}_6\text{O} \), the combustion reaction in air can be represented as:

\[ \text{C}_3 \text{H}_6 \text{O}(l) + 4 \text{O}_2(g) \rightarrow 3 \text{CO}_2(g) + 3 \text{H}_2\text{O}(l) \]

This balanced reaction equation reveals that one molecule of acetone requires four molecules of oxygen to produce three molecules each of carbon dioxide and water. Understanding combustion reactions is essential because it relates to everyday events such as burning fuels in engines, cooking, and even breathing.