Chemical Reaction Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Chemical Reaction.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

A process in which one or more substances (reactants) are transformed into entirely different substances (products) through the breaking and forming of chemical bonds, accompanied.

Old bonds break, new bonds form. You end up with different stuff than you started with.

Read the full concept explanation โ†’

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Chemical Reaction starts by naming reactants and products, then checks conservation with a balanced equation.

Common stuck point: Students often know a formula related to chemical reaction but skip the recognition step: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?

Worked Examples

Example 1

easy
Identify the reactants and products in the following reaction: 2H2+O2โ†’2H2O2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}.

Answer

Reactants:ย H2,ย O2Product:ย H2O\text{Reactants: H}_2\text{, O}_2\quad\text{Product: H}_2\text{O}

First step

1
Reactants are the substances on the left side of the arrow: H2\text{H}_2 and O2\text{O}_2.

Full solution

  1. 2
    Products are the substances on the right side: H2O\text{H}_2\text{O} (water).
  2. 3
    The coefficients (2, 1, 2) tell us the molar ratios in which the substances react and are produced.
A chemical reaction involves the transformation of reactants into products. The arrow separates the two sides, and coefficients indicate the proportions.

Example 2

medium
List four signs that a chemical reaction has occurred, and give an example of each.

Example 3

medium
A bubbling tablet drops into water and the beaker grows colder while gas escapes. What two facts has the experiment shown?

Example 4

medium
Two clear liquids are mixed and the test tube becomes warm but no gas, color change, or precipitate appears. What evidence (if any) suggests a chemical reaction?

Example 5

hard
Design a quick test to decide whether a fizzing reaction is producing CO2_2 or H2_2.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy
Is dissolving sugar in water a chemical reaction? Explain.

Example 2

easy
A student heats sugar until it turns brown and gives off a new smell. Is this a chemical reaction or just a physical change? Explain.

Example 3

easy
Is melting ice a chemical reaction or a physical change?

Example 4

easy
Iron rusting forms iron oxide. Is this a chemical reaction?

Example 5

easy
In a chemical reaction, what happens to chemical bonds?

Example 6

easy
Does a chemical reaction always require heating?

Example 7

easy
Dissolving sugar in water: chemical reaction or physical change?

Example 8

easy
Burning wood produces ash, smoke, and gases. Chemical or physical?

Example 9

easy
Name two pieces of evidence that a chemical reaction has occurred.

Example 10

easy
In the reaction C+O2โ†’CO2C + O_2 \rightarrow CO_2, has a chemical reaction occurred?

Example 11

medium
A student mixes two clear solutions and a solid forms at the bottom. What does this indicate, and what is the solid called?

Example 12

medium
Classify each: (a) boiling water, (b) souring milk, (c) cutting paper, (d) baking soda + vinegar fizzing.

Example 13

medium
Why does a color change during dye dissolving NOT prove a chemical reaction, but rusting's color change does?

Example 14

medium
A sealed flask of reactants weighs 50 g. After reacting, what does the flask weigh, and why?

Example 15

medium
Identify reactants and products in: 2H2+O2โ†’2H2O2H_2 + O_2 \rightarrow 2H_2O.

Example 16

medium
Photosynthesis: 6CO2+6H2Oโ†’C6H12O6+6O26CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2. Is energy absorbed or released, and is it a chemical reaction?

Example 17

medium
A reaction shows fizzing and the beaker feels cold. What two things has this told you?

Example 18

medium
Explain why 'the word reaction appears' is not enough to confirm a chemical reaction.

Example 19

medium
Methane burns: CH4+2O2โ†’CO2+2H2OCH_4 + 2O_2 \rightarrow CO_2 + 2H_2O. State the type of reaction and one piece of evidence you would observe.

Example 20

challenge
Two students disagree: one says a glowing splint relighting in a gas proves a chemical reaction occurred in the flask; the other says it only identifies the gas. Who is right and why?

Example 21

challenge
A reaction in an open beaker appears to lose 2 g. Argue, using bonds and atoms, why no atoms were destroyed.

Example 22

challenge
Design a single experiment to prove that mixing solution A and B is a chemical reaction AND classify its energy change. Describe expected observations.

Example 23

easy
In the reaction 2Na+Cl2โ†’2NaCl2\text{Na} + \text{Cl}_2 \rightarrow 2\text{NaCl}, name the reactants.

Example 24

easy
In the reaction Zn+2HClโ†’ZnCl2+H2\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2, name the products.

Example 25

easy
A student lights a match. Heat, light, and smoke are produced. Is this a chemical reaction?

Example 26

easy
Bending an aluminum can: chemical reaction or physical change?

Example 27

easy
Baking a cake batter into a cake: chemical or physical?

Example 28

medium
Classify each: (a) ice cube forming in a freezer, (b) silver tarnishing, (c) shredding paper, (d) milk turning to yogurt.

Example 29

medium
In CaCO3โ†’CaO+CO2\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2, count atoms of each element on each side to confirm balance.

Example 30

medium
Identify the type of evidence in each: (a) blue solution turns green, (b) thermometer rises from 20ยฐC to 35ยฐC, (c) white solid sinks in clear liquid after mixing.

Example 31

medium
Why is freezing water NOT a chemical reaction, even though it forms a hard solid?

Example 32

medium
A bottle of hydrogen peroxide forms tiny bubbles over time. Chemical or physical change? Explain.

Example 33

medium
Why must reactions be written as balanced equations rather than just lists of substances?

Example 34

medium
Sodium reacts with water producing fizzing and heat. Write the word equation.

Example 35

medium
In the equation AgNO3+NaClโ†’AgCl+NaNO3\text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} + \text{NaNO}_3, which product is the precipitate (a solid)?

Example 36

hard
Explain why the equation H2+O2โ†’H2O\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O} is incorrect even though the substances are right.

Example 37

hard
A reaction is fast at first then slows down. Reactants are not used up. What might be true about the reaction's reverse direction?

Example 38

hard
In a sealed bag, 10 g of baking soda is mixed with 15 g of vinegar. Gas is released and the bag puffs. What is the mass of the contents of the bag now? Why?

Example 39

hard
Two students hold a hot pad after pouring two reactants together. Could 'feels hot' alone prove a chemical reaction? What other check would strengthen the conclusion?

Example 40

hard
A student observes that an iron nail in a copper sulfate solution becomes coated with copper and the blue color fades. Explain why this is a chemical reaction and write the products.

Example 41

challenge
Argue from atoms why a chemical reaction can change the number of moles of substance but never the total mass.
โ† Back to Chemical Reaction Practice Mode

Background Knowledge

These ideas may be useful before you work through the harder examples.

atomchemical bond