English:Chemical Reactions
Chemical Reactions
Introduction
Chemical reactions are happening all around you. Iron rusts, food cooks, fuels burn, plants build sugars, batteries produce electrical energy, and your cells release energy from food. In each case, substances change because their atoms are rearranged into new combinations.
A chemical reaction is a process in which one or more starting substances, called reactants, are transformed into one or more new substances, called products. The atoms themselves are not created or destroyed in an ordinary chemical reaction; instead, chemical bonds are broken, formed, or rearranged.
In this aiMOOC, you will learn how to recognize chemical reactions, read simple chemical equations, use the law of conservation of mass, distinguish important reaction patterns, explain energy changes, and investigate factors that affect reaction rate. The course is designed for Grades 7–8 and connects chemistry to everyday life and safe school experiments.
Learning Goals
By the end of this course, you should be able to explain the difference between a physical change and a chemical change, identify reactants and products, interpret evidence that a chemical reaction may have occurred, represent reactions with word and symbol equations, check whether a simple equation follows conservation of atoms, compare exothermic and endothermic reactions, and predict how conditions can change reaction rate.
You should also be able to plan a fair investigation, record observations carefully, use evidence to support a conclusion, and communicate chemical ideas using clear scientific language.
What Changes in a Chemical Reaction?
In a physical change, the substance may change shape, size, or state without becoming a different substance. Melting ice is a physical change because the water molecules remain water molecules. In a chemical change, the particles are rearranged so that substances with different properties form.
For example, when magnesium reacts with oxygen, magnesium oxide forms. The product has different properties from the magnesium metal and oxygen gas that reacted. This is a chemical change.
Reactants, Products, and Particle Rearrangement
A reaction can be written as a word equation:
reactants → products
For the combustion of methane:
methane + oxygen → carbon dioxide + water
At the particle level, the carbon, hydrogen, and oxygen atoms present before the reaction are rearranged. The reaction does not turn one kind of atom into another. It changes which atoms are bonded together.
This particle view helps explain why equations must be balanced: the same number of atoms of each element must be present before and after a chemical reaction.
Evidence of Chemical Reactions
You cannot always identify a chemical reaction just by looking, but several observations can provide useful evidence. Possible signs include formation of a gas, formation of a new solid called a precipitate, a lasting color change, a temperature change that is not caused by external heating or cooling, and the production of light.
A single observation does not always prove that a chemical reaction happened. For example, bubbles can also appear when water boils, which is a physical change. Good scientific reasoning combines observations with knowledge about the substances and conditions.
Gas Formation: Baking Soda and Vinegar
Baking soda contains sodium hydrogen carbonate, and vinegar contains acetic acid. When they react, carbon dioxide gas is produced along with water and dissolved sodium acetate. The fizzing is evidence of gas formation.
A simplified word equation is:
sodium hydrogen carbonate + acetic acid → sodium acetate + water + carbon dioxide
In a school investigation, you can capture the carbon dioxide in a balloon placed over the mouth of a bottle. Use small quantities, wear eye protection, and never close a gas-producing reaction inside a rigid sealed container.
Slow Reactions: Rusting
Rusting is a slower chemical process. Iron reacts with oxygen in the presence of water and forms hydrated iron oxides that we call rust. Salt can speed up rusting because dissolved ions help electrical charge move through the thin water layer on the metal surface.
You can investigate rusting safely by comparing clean iron nails kept under different conditions, such as dry air, tap water, and salt water. A fair test changes one main factor while keeping other conditions as similar as possible.
Chemical Equations and Conservation of Mass
A chemical equation is a symbolic way to represent a reaction. Chemical formulas show the substances, plus signs separate different reactants or products, and an arrow means “forms” or “produces.”
For example:
CH4 + 2 O2 → CO2 + 2 H2O
The large numbers placed before formulas are called coefficients. They show the relative numbers of particles or amounts of substances. In the methane equation, there is one carbon atom, four hydrogen atoms, and four oxygen atoms on both sides.
Why Equations Must Balance
The law of conservation of mass states that mass is conserved in a closed system during an ordinary chemical reaction. Atoms are rearranged, but they are not created or destroyed.
When balancing a simple equation, change only the coefficients in front of formulas. Do not change the small numbers inside a chemical formula, because that would represent a different substance.
For example, this equation is not balanced:
H2 + O2 → H2O
A balanced version is:
2 H2 + O2 → 2 H2O
Both sides now contain four hydrogen atoms and two oxygen atoms.
Common Reaction Patterns
Chemists group reactions into patterns to help describe and predict them. At this level, the most useful patterns are combination, decomposition, combustion, oxidation, acid-base reactions, and precipitation reactions. Some real reactions can fit more than one description.
Combination and Decomposition
In a combination reaction, two or more reactants form a smaller number of products. Burning magnesium in oxygen is an example because magnesium and oxygen combine to form magnesium oxide.
In a decomposition reaction, one compound breaks into simpler substances. Some decomposition reactions require energy from heat, light, or electricity.
Electrolysis can use electrical energy to drive a chemical change. Water electrolysis can produce hydrogen and oxygen gases, although the practical setup requires suitable electrodes and an electrolyte and should be carried out under teacher supervision.
Combustion and Oxidation
Combustion is a reaction in which a substance reacts rapidly with oxygen and releases energy. Hydrocarbon fuels can undergo complete combustion to produce carbon dioxide and water, provided enough oxygen is available.
Oxidation is a broader idea involving reactions in which a substance loses electrons; at this level, many familiar oxidation examples involve substances combining with oxygen. Rusting is a slow oxidation process, while burning magnesium is a fast one.
Combustion can involve fire, hot equipment, harmful gases, or very bright light. Experiments involving flames, burning magnesium, or unknown fuels must be teacher demonstrations or follow your school laboratory’s approved safety procedure.
Acid-Base and Precipitation Reactions
In an acid-base reaction, an acid reacts with a base. Many acid-base reactions are called neutralization reactions because they can form water and a salt.
In a precipitation reaction, dissolved substances react to form a solid that is poorly soluble in the liquid. The new solid is the precipitate. Observing a precipitate is useful evidence that new substances have formed.
Do not mix household chemicals unless a teacher or a reliable laboratory procedure specifically says that the combination is safe. Some common products can release toxic gases when mixed.
Energy in Chemical Reactions
Chemical reactions involve energy because bonds must be broken and new bonds form. Some reactions transfer energy to the surroundings, while others take in energy from the surroundings.
An exothermic reaction transfers energy from the reacting system to the surroundings. The surroundings may become warmer, and light may also be released. Combustion is a familiar exothermic process.
An endothermic reaction takes in energy from the surroundings. The surroundings may become cooler. Some instant cold packs rely on endothermic dissolving processes; not every temperature change in a mixture is necessarily a chemical reaction, so you must consider the evidence carefully.
Activation Energy
Even an exothermic reaction may need an initial energy input. The minimum energy needed for reacting particles to begin a successful reaction is called activation energy. A spark can provide activation energy for fuel combustion, for example.
Reaction Rate
The rate of reaction describes how quickly reactants are used or products are formed. Reactions can be extremely fast, like some combustion reactions, or very slow, like rusting.
Increasing temperature usually increases reaction rate because particles move faster and collide more often and with greater energy. Increasing concentration usually increases the number of reacting particles in a given volume. Increasing the surface area of a solid exposes more particles to collisions. A catalyst provides a different reaction pathway with lower activation energy and is regenerated rather than used up overall.
Planning a Fair Rate Investigation
Suppose you compare an effervescent tablet reacting in water at different temperatures. The independent variable could be water temperature, and the dependent variable could be the time until visible bubbling stops. Keep the tablet type, water volume, container, and method as similar as possible.
Repeat measurements when practical, record results in a table, and look for patterns. A fair test does not guarantee a correct conclusion, but it makes your evidence more trustworthy.
Chemistry in Everyday Life
Chemical reactions make many technologies and natural processes possible. Batteries use redox reactions to transfer chemical energy into electrical energy. Cooking uses complex reactions that create new flavors, colors, and textures. Digestion uses enzyme-catalyzed reactions to break large food molecules into smaller ones. Photosynthesis stores energy by building energy-rich molecules, while cellular respiration releases usable energy through linked chemical reactions.
Understanding chemical reactions also helps you think about environmental questions. Fuel combustion can release carbon dioxide and air pollutants. Corrosion can weaken structures. Industrial chemistry can create useful materials but must be designed to reduce hazards, waste, and unnecessary energy use.
Laboratory Safety and Responsible Investigation
Wear appropriate eye protection when instructed, tie back long hair, keep food and drink away from laboratory work, and follow the teacher’s directions. Never taste laboratory substances. Do not smell an unknown chemical directly; if an approved procedure requires odor observation, follow the teacher’s method. Never mix cleaning products or unknown substances.
Treat flames, hot equipment, concentrated acids or bases, and reactive metals as teacher-supervised hazards. Dispose of chemicals according to local school procedures rather than pouring unknown materials into a sink.
Interactive Tasks
Quiz: Test Your Knowledge
What is a reactant? (A starting substance in a chemical reaction) (!A substance that is always a gas) (!A tool used to measure temperature) (!A product formed after every reaction)
Which observation is strong evidence that a gas may have formed during a reaction? (Bubbles appear in a mixture that is not boiling) (!A solid is cut into smaller pieces) (!Ice melts at room temperature) (!Water changes shape inside a container)
Why must a chemical equation be balanced? (To show that each type of atom is conserved) (!To make every formula have the same subscript) (!To make all substances have the same state) (!To ensure every reaction happens quickly)
What does a coefficient in a chemical equation show? (The relative number of particles or amounts of a substance) (!The electric charge on every atom) (!The temperature needed for the reaction) (!The color of the substance)
Which description best fits an exothermic reaction? (It transfers energy to the surroundings) (!It always produces a gas) (!It always needs freezing conditions) (!It creates atoms from energy)
What is activation energy? (The minimum energy needed to start a successful reaction) (!The total mass of all products) (!The energy stored only in metals) (!The temperature at which water boils)
Which change usually increases the rate of a reaction involving a solid reactant? (Increasing the surface area of the solid) (!Using larger pieces of the same solid) (!Removing all contact between reactants) (!Lowering the temperature in every case)
What does a catalyst do? (It provides a lower energy pathway for the reaction) (!It becomes the main product of every reaction) (!It increases the number of atoms in the system) (!It forces every reaction to become endothermic)
Which process is a chemical change? (Iron rusting in moist air) (!Ice melting into liquid water) (!Paper being cut into strips) (!Salt crystals being crushed)
What is a precipitate? (A solid formed from substances reacting in solution) (!A gas released during boiling) (!A liquid used to heat a reaction) (!A metal that never reacts)
Memory Game
| Reactant | Starting substance in a chemical reaction |
| Product | New substance formed by a chemical reaction |
| Catalyst | Substance that speeds a reaction by providing a lower energy pathway |
| Precipitate | Solid that forms from a reaction in solution |
| Exothermic | Describes a process that transfers energy to the surroundings |
| Endothermic | Describes a process that takes in energy from the surroundings |
Drag and Drop
| Match the correct terms. | Topic |
|---|---|
| Starting substances | Reactants |
| New substances formed | Products |
| Atoms remain accounted for | Conservation of mass |
| Solid appears from a solution | Precipitation reaction |
| Faster reaction pathway | Catalyst |
...
Crossword Puzzle
| Reactant | What do you call a starting substance in a chemical reaction? |
| Product | What do you call a new substance formed by a chemical reaction? |
| Catalyst | What substance speeds a reaction without being used up overall? |
| Oxidation | What process includes the slow reaction that causes iron to rust? |
| Precipitate | What solid can form when dissolved substances react? |
| Exothermic | What word describes a reaction that transfers energy to the surroundings? |
LearningApps
Cloze Text
Open-Ended Tasks
Easy
- Reaction Hunt: Find four examples of chemical reactions in everyday life, photograph or sketch them safely, and explain what evidence suggests that new substances form.
- Particle Storyboard: Draw a before-and-after particle storyboard for a simple reaction and show how the same atoms can be rearranged into new combinations.
- Kitchen Chemistry Interview: Interview an adult about one cooking process that involves chemical change, then write a short explanation separating observation from scientific interpretation.
- Equation Card: Create a study card that explains reactants, products, the reaction arrow, coefficients, and one correctly balanced simple equation.
Standard
- Rust Investigation: Design a fair test comparing how two safe environmental conditions affect rusting of iron, record observations over several days, and explain your variables and controls.
- Gas Collection Model: With teacher approval, model the baking soda and vinegar reaction using a balloon to capture carbon dioxide, then create a labeled diagram that explains where the gas comes from.
- Reaction Rate Test: Investigate one safe factor that changes the rate of an effervescent tablet reaction, collect repeated measurements, graph the results, and explain the pattern.
- Energy Change Explanation: Produce a one-minute video or narrated slide that compares exothermic and endothermic processes using one carefully chosen example of each.
Advanced
- Conservation Investigation: Plan a closed-system experiment that can test conservation of mass during a safe gas-producing reaction, predict the result, identify measurement uncertainty, and justify your design before carrying it out with teacher approval.
- Catalyst Research Project: Research one catalyst used in biology, industry, or environmental technology and create an illustrated explanation of how it changes reaction rate without changing the overall reactants and products.
- Reaction Evidence Critique: Create a case study in which an observation could be mistaken for evidence of a chemical reaction, then explain what additional evidence would strengthen or weaken the conclusion.
- Sustainable Chemistry Challenge: Investigate a real process involving combustion, corrosion, batteries, or industrial reactions and propose one realistic change that could reduce energy use, hazards, emissions, or material waste.
Learning Assessment
- Explaining Evidence: Given observations from an unfamiliar experiment, decide which observations support the claim that a chemical reaction occurred, identify any ambiguous evidence, and justify your conclusion.
- Balancing and Meaning: Balance a simple chemical equation and explain how the coefficients connect the symbolic equation to conservation of atoms.
- Rate Reasoning: Predict how changing temperature, concentration, surface area, or a catalyst would affect a reaction rate and explain your prediction using particle collisions and activation energy.
- Energy Transfer: Compare two reaction scenarios, determine which is likely exothermic or endothermic from the evidence, and explain the direction of energy transfer.
- Fair-Test Design: Evaluate a proposed reaction-rate experiment, identify one uncontrolled variable, improve the procedure, and explain why the change makes the evidence more reliable.
- Chemistry Transfer: Choose an everyday process such as rusting, cooking, fuel combustion, or battery use and connect it to at least three course ideas in a coherent scientific explanation.
Evidence of Learning
Important evidence of learning includes accurate use of the terms reactant, product, precipitate, catalyst, exothermic, endothermic, activation energy, and reaction rate; explanations that connect visible observations to particle rearrangement; correctly interpreted and balanced simple equations; fair-test planning with controlled variables; careful tables, graphs, diagrams, or laboratory notes; safe practical behavior; and reasoned conclusions that acknowledge limits in the evidence.
Strong products may include a reaction storyboard, investigation report, graph, short explanatory video, annotated equation, research poster, or sustainability proposal. Transfer is shown when you can apply the ideas of conservation, energy, reaction evidence, and reaction rate to an unfamiliar chemical situation rather than only repeating a memorized example.
OERs on the Topic
The English Wikipedia article on chemical reactions provides an open reference for further reading.
Linked Learning Areas
Chemical reactions connect particle models of matter with energy, measurement, environmental science, biology, and technology. The links below can help you extend the course.