# 7: Chemical Reactions and Equations


Chemistry is largely about chemical changes. Indeed, if there were no chemical changes, chemistry as such would not exist! Chemical changes are a fundamental part of chemistry. Because chemical changes are so central, it may be no surprise that chemistry has developed some special ways of presenting them.

• 7.1: Prelude to Chemical Reactions
The space shuttle-and any other rocket-based system-uses chemical reactions to propel itself into space and maneuver itself when it gets into orbit. The rockets that lift the orbiter are of two different types. Although the solid rocket boosters each have a significantly lower mass than the liquid oxygen and liquid hydrogen tanks, they provide over 80% of the lift needed to put the shuttle into orbit-all because of chemical reactions.
• 7.2: The Chemical Equation
A chemical equation is a concise description of a chemical reaction. Proper chemical equations are balanced.
• 7.3: Composition, Decomposition, and Combustion Reactions
A composition reaction produces a single substance from multiple reactants. A decomposition reaction produces multiple products from a single reactant. Combustion reactions are the combination of some compound with oxygen to make oxides of the other elements as products (although nitrogen atoms react to make $$N_2$$).
• 7.4: Types of Chemical Reactions - Single and Double Replacement Reactions
A single-replacement reaction replaces one element for another in a compound. The periodic table or an activity series can help predict whether single-replacement reactions occur. A double-replacement reaction exchanges the cations (or the anions) of two ionic compounds. A precipitation reaction is a double-replacement reaction in which one product is a solid precipitate. Solubility rules are used to predict whether some double-replacement reactions will occur.

Thumbnail: Reaction of sodium and water breaks the glass vessel. Image used with permission (CC SA-BY-3.0; Tony Mach).

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