---
title: "Chemical Reactions: Reactants and Products"
book: "School Chemistry — Grades 1 to 12"
subject: chemistry
language: en
chapter: 12
exercises: 12
source: https://one-course.com/books/chemistry/1/en/chapter/12-chemical-reactions-reactants-and-products
license: CC-BY-NC-SA-4.0
credit: "One Chemistry Book, One Course (one-course.com)"
---

# Chapter 12 — Chemical Reactions: Reactants and Products

Charcoal glows in a barbecue. An hour later almost nothing is left of the black lumps — a little grey ash, and the heat that cooked the meal. Where has the black carbon gone? It has not vanished: it has met the oxygen of the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) and become a new, invisible gas. With [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) and [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) in hand, we can now say exactly what happens in such a change.

**You already know.**

In a [chemical change](https://one-course.com/books/chemistry/1/en/chapter/6-changes-that-cannot-be-undone#def-g5-irreversible-changes-chemical-change) new substances appear and there is no simple way back ([Chapter 6](https://one-course.com/books/chemistry/1/en/chapter/6-changes-that-cannot-be-undone#ch-g5-irreversible-changes)). Limewater turns milky with carbon dioxide, anhydrous copper sulfate turns blue with water ([Chapter 10](https://one-course.com/books/chemistry/1/en/chapter/10-identifying-substances#ch-g7-identifying-substances)). All matter is made of [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom), which group into [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule); a formula such as $\ce{CO2}$ lists the [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) of a [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) ([Chapter 11](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#ch-g7-atoms-and-molecules)).

![Glowing charcoal: carbon reacting with the oxygen of the air.](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-chemical-reactions/img-8e82256031ce.jpg)

*Glowing charcoal: carbon reacting with the oxygen of the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air).*

## 12.1 Physical and chemical transformations

**Definition 12.1 (Physical transformation).**

A *physical transformation* changes the state or the arrangement of matter without changing the [chemical species](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-species): melting, freezing, boiling, dissolving, mixing. The same species are present before and after; only their form or their arrangement has changed.

**Example 12.2 (Water, three ways).**

Ice melting into water, water boiling into water vapour, and salt dissolving in water are [physical transformations](#def-g7-chemical-reactions-physical-transformation): before and after, there are water [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) (and salt). Zooming in, the [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) are the same; only how they are arranged and how they move has changed.

**Proposition 12.3 (Chemical transformation).**

In a [chemical change](https://one-course.com/books/chemistry/1/en/chapter/6-changes-that-cannot-be-undone#def-g5-irreversible-changes-chemical-change), also called a chemical transformation, some [chemical species](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-species) disappear and new ones appear. It is told apart from a [physical transformation](#def-g7-chemical-reactions-physical-transformation) by its [products](#def-g7-chemical-reactions-reactant): new species, which can be detected with [characteristic tests](https://one-course.com/books/chemistry/1/en/chapter/10-identifying-substances#def-g7-identifying-substances-characteristic-test).

![Two kinds of transformation.](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-chemical-reactions/fig-7cfe6dd4469b.svg)

*Two kinds of transformation.*

## 12.2 Reactants and products

**Definition 12.4 (Chemical reaction).**

A *chemical reaction* is the model chemists use for a chemical transformation: it lists the species that are used up and the species that are formed.

**Definition 12.5 (Reactants and products).**

The species used up in a [chemical reaction](#def-g7-chemical-reactions-reaction) are the *reactants*; the species formed are the *products*.

**Example 12.6 (Carbon burning in oxygen).**

When charcoal burns, the [reactants](#def-g7-chemical-reactions-reactant) are carbon (the charcoal) and oxygen (from the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air)). The [product](#def-g7-chemical-reactions-reactant) is carbon dioxide: shaken with limewater, the gas left in the jar turns it milky.

**In the lab — Charcoal in a jar of oxygen.**

The teacher heats a small piece of charcoal until it glows, on a deflagrating spoon, then lowers it into a jar of oxygen. The charcoal glows far brighter than in [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) and shrinks until it has gone. The jar is covered, a little limewater is poured in and shaken: it turns milky. Carbon and oxygen have been used up; carbon dioxide has been formed.

![Glowing charcoal lowered on a spoon into a jar of oxygen burns brightly: carbon and oxygen react to form carbon dioxide.](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-chemical-reactions/fig-b53412d48eb0.svg)

*Glowing charcoal lowered on a spoon into a jar of oxygen burns brightly: carbon and oxygen react to form carbon dioxide.*

**Example 12.7 (Steel wool).**

A pad of fine steel wool, which is mostly iron, can burn in [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) with showers of sparks. The [reactants](#def-g7-chemical-reactions-reactant) are iron and oxygen; the [product](#def-g7-chemical-reactions-reactant) is a dark, crumbly solid, an iron oxide.

![Steel wool burning in the laboratory: iron and oxygen react.](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-chemical-reactions/img-4611a640b665.jpg)

*Steel wool [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning) in the laboratory: iron and oxygen react.*

## 12.3 The word equation

**Definition 12.8 (Word equation).**

A *word equation* writes a [chemical reaction](#def-g7-chemical-reactions-reaction) in words: the names of the [reactants](#def-g7-chemical-reactions-reactant) on the left, joined by $+$, an arrow, and the names of the [products](#def-g7-chemical-reactions-reactant) on the right. The arrow reads “react to give”.

**Example 12.9 (Three word equations).**

| carbon $+$ oxygen $\longrightarrow$ carbon dioxide |
| --- |
| methane $+$ oxygen $\longrightarrow$ carbon dioxide $+$ water |
| iron $+$ oxygen $\longrightarrow$ iron oxide |

**Method 12.10 (Writing a word equation).**

1. Find the [reactants](#def-g7-chemical-reactions-reactant) : the species that disappear.
2. Find the [products](#def-g7-chemical-reactions-reactant) : the new species, identified by tests.
3. Write the [reactants](#def-g7-chemical-reactions-reactant) on the left, the [products](#def-g7-chemical-reactions-reactant) on the right, with an arrow between them; never an equals sign.

A species that is present but takes no part — the nitrogen of the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) around a flame — is not written.

## 12.4 A reaction is a rearrangement of atoms

**Proposition 12.11 (Atoms are rearranged).**

In a [chemical reaction](#def-g7-chemical-reactions-reaction) the [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) of the [reactants](#def-g7-chemical-reactions-reactant) are broken apart and their [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) join up differently to build the [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) of the [products](#def-g7-chemical-reactions-reactant). The [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) themselves are neither created nor destroyed: the same [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom), in the same numbers, are there before and after.

**Example 12.12 (Methane burning, molecule by molecule).**

When the methane of a gas cooker burns, one methane [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) $\ce{CH4}$ meets two oxygen [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) $\ce{O2}$. Their [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) rearrange into one carbon dioxide [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) $\ce{CO2}$ and two water [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) $\ce{H2O}$. Before: 1 carbon, 4 hydrogen and 4 oxygen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom). After: 1 carbon, 4 hydrogen, and $2 + 2 = 4$ oxygen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom). Nothing is lost, nothing appears from nowhere.

![Methane burning, drawn with molecular models: the atoms of the reactants rearrange into the products. The same atoms are counted before and after.](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-chemical-reactions/fig-5200d5590e6e.svg)

*Methane [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning), drawn with molecular models: the [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) of the [reactants](#def-g7-chemical-reactions-reactant) rearrange into the [products](#def-g7-chemical-reactions-reactant). The same [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) are counted before and after.*

**Remark 12.13 (Counting the molecules).**

The [word equation](#def-g7-chemical-reactions-word-equation) does not say how many [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) react: for that, the equation is written with formulas and numbers in front of them. Writing such equations, and finding the right numbers, is the subject of a later chapter.

## 12.5 Exercises

**Exercise 12.1 ★.**

Physical or chemical transformation? Ice melting, a match [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning), sugar dissolving, bread toasting, a nail rusting.

**Solution of Exercise 12.1.**

Physical: ice melting, sugar dissolving. Chemical: a match [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning), bread toasting, a nail rusting.

**Exercise 12.2 ★.**

What are the [reactants](#def-g7-chemical-reactions-reactant) and the [products](#def-g7-chemical-reactions-reactant) of a [chemical reaction](#def-g7-chemical-reactions-reaction)?

**Solution of Exercise 12.2.**

The [reactants](#def-g7-chemical-reactions-reactant) are the species used up; the [products](#def-g7-chemical-reactions-reactant) are the new species formed.

**Exercise 12.3 ★.**

Write the [word equation](#def-g7-chemical-reactions-word-equation) of the [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning) of carbon in oxygen.

**Solution of Exercise 12.3.**

carbon $+$ oxygen $\longrightarrow$ carbon dioxide.

**Exercise 12.4 ★.**

In the [word equation](#def-g7-chemical-reactions-word-equation) “iron $+$ oxygen $\longrightarrow$ iron oxide”, name the [reactants](#def-g7-chemical-reactions-reactant) and the [product](#def-g7-chemical-reactions-reactant).

**Solution of Exercise 12.4.**

[Reactants](#def-g7-chemical-reactions-reactant): iron and oxygen. [Product](#def-g7-chemical-reactions-reactant): iron oxide.

**Exercise 12.5 ★★.**

When methane burns, a cold glass held above the flame mists up, and the gas collected turns limewater milky. Which [products](#def-g7-chemical-reactions-reactant) do these two tests show?

**Solution of Exercise 12.5.**

The mist shows water; the milky limewater shows carbon dioxide.

**Exercise 12.6 ★★.**

Zinc reacts with hydrochloric acid: the zinc disappears, bubbles of hydrogen are given off, and zinc chloride stays in the solution. Write the [word equation](#def-g7-chemical-reactions-word-equation).

**Solution of Exercise 12.6.**

zinc $+$ hydrochloric acid $\longrightarrow$ zinc chloride $+$ hydrogen.

**Exercise 12.7 ★★.**

Look at the model figure of methane [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning). Count the hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) before and after the reaction.

**Solution of Exercise 12.7.**

Before: 4 (in the methane [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule)). After: $2 + 2 = 4$ (two in each water [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule)).

**Exercise 12.8 ★★.**

Why is dissolving salt in water not a [chemical reaction](#def-g7-chemical-reactions-reaction), although the salt disappears from sight?

**Solution of Exercise 12.8.**

No new species appears: the salt is still there, spread through the water, and it is got back by [evaporating](https://one-course.com/books/chemistry/1/en/chapter/3-separating-mixtures#def-g3-separating-mixtures-evaporate) the water. It is a [physical transformation](#def-g7-chemical-reactions-physical-transformation).

**Exercise 12.9 ★★.**

A log of wood burns down to a small heap of ash. Explain, using the word “[products](#def-g7-chemical-reactions-reactant)”, why the ash is so much lighter than the log.

**Solution of Exercise 12.9.**

Most of the wood has reacted with oxygen to give [products](#def-g7-chemical-reactions-reactant) that are gases, carbon dioxide and water vapour, which leave into the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air). Only the ash stays.

**Exercise 12.10 ★★.**

Hydrogen burns in oxygen to give water only. Write the [word equation](#def-g7-chemical-reactions-word-equation), and say which test would identify the [product](#def-g7-chemical-reactions-reactant).

**Solution of Exercise 12.10.**

hydrogen $+$ oxygen $\longrightarrow$ water. The [product](#def-g7-chemical-reactions-reactant) turns anhydrous copper sulfate blue.

**Exercise 12.11 ★★★.**

Two hydrogen [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) $\ce{H2}$ react with one oxygen [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) $\ce{O2}$ to give two water [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) $\ce{H2O}$. Count the [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) of each kind before and after, and show that none has been created or destroyed.

**Solution of Exercise 12.11.**

Before: $2 \times 2 = 4$ hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) and 2 oxygen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom). After: two water [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) hold $2 \times 2 = 4$ hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) and $2 \times 1 =
2$ oxygen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom). The same [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom), in the same numbers.

**Exercise 12.12 ★★★.**

A student draws the [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning) of carbon as: one carbon [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) $+$ one oxygen [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) $\longrightarrow$ one carbon dioxide [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) and one oxygen [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) left over. What is wrong with this drawing? Draw the correct one in words.

**Solution of Exercise 12.12.**

An oxygen [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) cannot come from nowhere: the drawing has 2 oxygen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) before and 3 after. Correct: one carbon [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) $+$ one oxygen [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) $\longrightarrow$ one carbon dioxide [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule), with nothing left over.

## 12.6 Problem: Inside a Gas-Cooker Flame

**Problem 12.1.**

Weekend problem — what burns in a gas-cooker flame, what it makes, and how many molecules take part

The gas of a kitchen cooker is mostly methane, $\ce{CH4}$. In its blue flame, methane reacts with the oxygen of the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air). In a laboratory, the teacher holds a cold, dry beaker upside down above such a flame for a few seconds, then turns it over, pours in a little limewater and shakes it.

**Part I — [Reactants](#def-g7-chemical-reactions-reactant) and [products](#def-g7-chemical-reactions-reactant).**

1. Name the two [reactants](#def-g7-chemical-reactions-reactant) of this reaction.
2. Droplets appear inside the cold beaker; a drop of them turns anhydrous copper sulfate blue. Which [product](#def-g7-chemical-reactions-reactant) does this show?
3. The limewater turns milky. Which other [product](#def-g7-chemical-reactions-reactant) does this show?
4. The [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) also contains nitrogen, which passes through the flame unchanged. Is nitrogen a [reactant](#def-g7-chemical-reactions-reactant) ? Should it appear in the [word equation](#def-g7-chemical-reactions-word-equation) ?

**Part II — The [word equation](#def-g7-chemical-reactions-word-equation).**

5. Write the [word equation](#def-g7-chemical-reactions-word-equation) of the reaction.
6. Is the [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning) of methane a physical or a chemical transformation? Give two reasons.
7. Write the formulas of the four species of the reaction.

**Part III — Counting [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule).** Each methane [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) that burns reacts with two oxygen [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule), and gives one carbon dioxide [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) and two water [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule).

8. Count the carbon, hydrogen and oxygen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) in one methane [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) and two oxygen [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) , together.
9. Count them in one carbon dioxide [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) and two water [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) , together. What do you notice?
10. The flame burns 10 methane [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) . How many oxygen [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) does it use?
11. How many carbon dioxide [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) does it make?
12. How many water [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) does it make?

**Solution of Problem 12.1.**

**1.** Methane and oxygen.

**2.** Water.

**3.** Carbon dioxide.

**4.** No: it is not used up, and it is not written in the [word equation](#def-g7-chemical-reactions-word-equation).

**5.** methane $+$ oxygen $\longrightarrow$ carbon dioxide $+$ water.

**6.** Chemical: two species disappear (methane, oxygen), and two new species appear, detected by tests; and heat and light are given out.

**7.** $\ce{CH4}$, $\ce{O2}$, $\ce{CO2}$, $\ce{H2O}$.

**8.** 1 carbon, 4 hydrogen, $2 \times 2 = 4$ oxygen.

**9.** 1 carbon; $2 \times 2 = 4$ hydrogen; $2 + 2 \times 1 = 4$ oxygen. The same [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) in the same numbers: they have only been rearranged.

**10.** $10 \times 2 = 20$ oxygen [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule).

**11.** 10 carbon dioxide [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule).

**12.** $10 \times 2 = 20$ water [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule).