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Recognize combustion, rusting, precipitation, and gas-forming reactions through reaction observations and balanced chemical equations.

---

## Reactions Have Patterns

When substances react, we do more than notice clues such as heat, color, precipitates, or gas. We can also read the **reaction type** from the pattern of reactants and products.

This lesson uses four common examples: combustion, rusting, precipitation, and gas formation. OpenStax Chemistry 2e also groups reactions through examples such as precipitation, acid-base reactions, and oxidation-reduction in [Classifying Chemical Reactions](https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions).

## Reaction Type Lab

Choose one reaction type. Watch the visible change first, then match it with the chemical equation.

Component: ChemicalReactionTypesLab
Props:
- title: Reaction Type Reader
- description: This model places reactants on the left and products on the right. Focus on
the new substance that forms, not only on the shape of the model.
- labels: {
chooseType: "Choose a chemical reaction type",
before: "Reactants",
after: "Products",
reactionView: "Three-dimensional model of chemical reaction types",
equationLabel: "Balanced equation",
visibleCueLabel: "What appears",
readingLabel: "Type to read",
checkLabel: "Quick check",
types: {
combustion: {
tab: "Combustion",
helperCaption: (
<>
Phosphorus and oxygen form an oxide while releasing heat and light.
</>
),
equation: (
$$\mathrm{P_4} + 5\mathrm{O_2} \rightarrow 2\mathrm{P_2O_5}$$
),
visibleCue: <>Heat and light appear.</>,
reading: (
<>
The reactant combines with $$\mathrm{O_2}$$.
</>
),
check: <>The atoms on the left and right must match.</>,
},
rust: {
tab: "Rusting",
helperCaption: <>Iron reacts with oxygen and water until a rust layer forms.</>,
equation: (
$$4\mathrm{Fe} + 3\mathrm{O_2} + 2x\mathrm{H_2O} \rightarrow 2\mathrm{Fe_2O_3} \cdot x\mathrm{H_2O}$$
),
visibleCue: <>The surface color changes.</>,
reading: <>Iron is oxidized into rust.</>,
check: (
<>
$$x$$ means the amount of water attached to each{" "}
$$\mathrm{Fe_2O_3}$$.
</>
),
},
precipitate: {
tab: "Precipitate",
helperCaption: <>Ions in solution assemble into a poorly soluble solid product.</>,
equation: ... [truncated; 1883 chars]
  Visible text: {
chooseType: "Choose a chemical reaction type",
before: "Reactants",
after: "Products",
reactionView: "Three-dimensional model of chemical reaction types",
equationLabel: "Balanced equation",
visibleCueLabel: "What appears",
readingLabel: "Type to read",
checkLabel: "Quick check",
types: {
combustion: {
tab: "Combustion",
helperCaption: (
<>
Phosphorus and oxygen form an oxide while releasing heat and light.
</>
),
equation: (

),
visibleCue: <>Heat and light appear.</>,
reading: (
<>
The reactant combines with .
</>
),
check: <>The atoms on the left and right must match.</>,
},
rust: {
tab: "Rusting",
helperCaption: <>Iron reacts with oxygen and water until a rust layer forms.</>,
equation: (

),
visibleCue: <>The surface color changes.</>,
reading: <>Iron is oxidized into rust.</>,
check: (
<>
 means the amount of water attached to each{" "}
.
</>
),
},
precipitate: {
tab: "Precipitate",
helperCaption: <>Ions in solution assemble into a poorly soluble solid product.</>,
equation: ... [truncated; 1883 chars]

## Four Examples to Recognize

The four examples below show different clues: energy release, metal change, solid formation, and acid-base neutralization.

### Combustion

Combustion happens when a substance reacts with oxygen and usually releases energy as heat or light. For burning phosphorus, a common school-level equation is:

```math
\mathrm{P_4} + 5\mathrm{O_2} \rightarrow 2\mathrm{P_2O_5}
```

Some chemistry sources write the phosphorus oxide as $$\mathrm{P_4O_{10}}$$. That does not contradict the equation above because $$\mathrm{P_2O_5}$$ is its empirical formula. For coefficient practice here, $$\mathrm{2P_2O_5}$$ keeps the numbers of $$\mathrm{P}$$ and $$\mathrm{O}$$ atoms balanced.

Visible text: Some chemistry sources write the phosphorus oxide as . That does not contradict the equation above because is its empirical formula. For coefficient practice here, keeps the numbers of and atoms balanced.

### Rusting

Rusting is an example of oxidation in metals. Iron reacts with oxygen and water, then forms rust, often written as hydrated iron(III) oxide.

```math
4\mathrm{Fe} + 3\mathrm{O_2} + 2x\mathrm{H_2O} \rightarrow 2\mathrm{Fe_2O_3} \cdot x\mathrm{H_2O}
```

The letter $$x$$ means the amount of water in rust is not always one fixed number. Because the product has coefficient $$2$$, the water on the left is written as $$2x\mathrm{H_2O}$$ so the equation stays balanced. The key pattern here is that iron changes into a new reddish-brown substance.

Visible text: The letter means the amount of water in rust is not always one fixed number. Because the product has coefficient , the water on the left is written as so the equation stays balanced. The key pattern here is that iron changes into a new reddish-brown substance.

### Precipitation

Precipitation happens when a newly formed substance does not dissolve well in the mixture. For example, limewater can react with dissolved baking soda and form calcium carbonate as a white precipitate.

```math
\mathrm{Ca(OH)_2} + \mathrm{NaHCO_3} \rightarrow \mathrm{CaCO_3} + \mathrm{NaOH} + \mathrm{H_2O}
```

The product $$\mathrm{CaCO_3}$$ matters because this compound appears as the solid. A precipitate is a newly formed product with low solubility, not just material that falls to the bottom.

Visible text: The product matters because this compound appears as the solid. A precipitate is a newly formed product with low solubility, not just material that falls to the bottom.

### Gas Formation

In some reactions, one product escapes as a gas. Fermentation of glucose can produce ethanol and carbon dioxide.

```math
\mathrm{C_6H_{12}O_6} \rightarrow 2\mathrm{C_2H_5OH} + 2\mathrm{CO_2}
```

The coefficient $$2$$ before ethanol and carbon dioxide should not be dropped. Without it, the carbon, hydrogen, and oxygen atoms would not be balanced.

Visible text: The coefficient before ethanol and carbon dioxide should not be dropped. Without it, the carbon, hydrogen, and oxygen atoms would not be balanced.

## Coefficients Do Not Change Formulas

A coefficient is the number placed before a chemical formula. We may change coefficients so the number of atoms before and after the reaction matches.

```math
\mathrm{P_4} + 5\mathrm{O_2} \rightarrow 2\mathrm{P_2O_5}
```

In this equation, $$5$$ before $$\mathrm{O_2}$$ and $$2$$ before $$\mathrm{P_2O_5}$$ are coefficients. The formula $$\mathrm{O_2}$$ must not be changed into $$\mathrm{O_5}$$, because that would change the substance.

Visible text: In this equation, before and before are coefficients. The formula must not be changed into , because that would change the substance.

## The Name Helps, the Equation Decides

Names such as combustion, precipitation, or gas formation help us predict the reaction pattern. The final check still comes from the balanced chemical equation.

Use this quick check.

| Reaction type | Nearby clue | What to check |
| :------------ | :---------- | :------------ |
| Combustion | $$\mathrm{O_2}$$ appears and energy is released. | Atoms on the left and right are balanced. |
| Rusting | A metal changes color into rust. | The new product contains metal and oxygen. |
| Precipitation | A new solid appears from solution. | The solid product is written as a new compound. |
| Gas formation | Bubbles or gas leave the mixture. | A gas product such as $$\mathrm{CO_2}$$ appears. |

Visible text: | Reaction type | Nearby clue | What to check |
| :------------ | :---------- | :------------ |
| Combustion | appears and energy is released. | Atoms on the left and right are balanced. |
| Rusting | A metal changes color into rust. | The new product contains metal and oxygen. |
| Precipitation | A new solid appears from solution. | The solid product is written as a new compound. |
| Gas formation | Bubbles or gas leave the mixture. | A gas product such as appears. |

Once the reaction type is readable, the next move is to write the reaction neatly. Put reactants on the left, products on the right, then adjust coefficients until the atoms balance.