Classify each chemical reaction as a synthesis, decomposition, single- displacement, or double-displacement reaction. (a) \(\mathrm{CaSO}_{4}(g) \underset{\text { heat }}{\longrightarrow} \mathrm{CaO}(s)+\mathrm{SO}_{3}(g)\) (b) \(2 \mathrm{Na}(s)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{Na}_{2} \mathrm{O}_{2}(s)\) (c) \(\mathrm{Pb}(s)+2 \mathrm{AgNO}_{3}(a q) \longrightarrow \mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(a q)+2 \mathrm{Ag}(s)\) (d) \(\mathrm{HI}(a q)+\mathrm{NaOH}(a q) \longrightarrow \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{NaI}(a q)\)

In a decomposition reaction, a single compound breaks down into simpler substances. This type of chemical reaction is like a puzzle being taken apart into smaller pieces. It usually requires an external source of energy, such as heat, light, or electricity, to initiate the breaking of bonds.

Using our real-life example from the exercise, when calcium sulfate (\textbf{CaSO}\(_4\)) is heated, it decomposes into calcium oxide (\textbf{CaO}) and sulfur trioxide (\textbf{SO}\(_3\)). Think of it as the compound 'unpacking' into simpler containers when heat knocks on its door.

These reactions are essential in numerous processes, from the calcination of limestone in cement production to the breakdown of organic matter in composting. Understanding decomposition helps shed light on both the creation of basic building blocks in manufacturing and the natural cycle of matter in the environment.

A synthesis reaction is a chemical dance where two or more reactants come together to form a single new product. It's like two clouds merging in the sky to form a single, larger cloud. In a classroom setting, this could be introduced as the 'coming together' of elements or compounds.

For instance, sodium (\textbf{Na}) combines with oxygen (\textbf{O}\(_2\)) to form sodium peroxide (\textbf{Na}\(_2\)\textbf{O}\(_2\)). This transformation can be visualized as two individual substances joining hands to create a new one. It's foundational to learning how complex substances are built up from more elementary ones, such as in the formation of water from hydrogen and oxygen.

Synthesis reactions are the backbone of chemical manufacturing, leading to the creation of a plethora of materials and substances that are integral to modern life, like plastics, pharmaceuticals, and fertilizers.

Imagine a dance floor where a partner in a dancing couple is replaced by someone else; this is the essence of a single-displacement reaction. Here, one element takes the place of another within a compound, creating a new element and a new compound.

An exemplar reaction is lead (\textbf{Pb}) swapping with silver (\textbf{Ag}) in silver nitrate (\textbf{AgNO}\(_3\)) to yield lead(II) nitrate (\textbf{Pb(NO}\(_3\))\(_2\)) and silver. It's a classic 'one-in, one-out' scenario at a molecular level. Such reactions are not just textbook examples but are utilized in processes like metal extractions and making batteries.

This type of reaction helps in demonstrating the reactivity series of metals, where a more reactive metal will displace a less reactive metal from its compound. It's integral for students to learn about reactivity and how it governs the ability of elements to replace one another.

Double-displacement reactions are the 'trading places' of the chemical world, where compounds swap parts and produce two new compounds. This is akin to two couples exchanging partners in a dance. In the context of an aqueous solution, it involves the exchange of ions between two reacting compounds.

Using the exercise's example, when hydroiodic acid (\textbf{HI}) reacts with sodium hydroxide (\textbf{NaOH}), they exchange ions to form water (\textbf{H}\(_2\)\textbf{O}) and sodium iodide (\textbf{NaI}). Students might envision this as the ions reorganizing in a social dance to find a more stable configuration.

These reactions are prevalent in chemistry labs and are particularly easy to visualize when they result in the formation of a precipitate or gas. Known also as metathesis, these reactions are central in the study of ionic compounds and are key in numerous industrial processes, including the synthesis of various salts and the workings of ionic batteries.