Chemical reaction facts for kids

A bright thermite reaction producing liquid iron and throwing off sparks.

A chemical reaction is a process where one set of substances transforms into completely new substances. When a reaction happens, atoms rearrange themselves by breaking and making chemical bonds. The starting materials in a reaction are called reactants. The new materials created at the end are called products.

Chemical reactions happen everywhere around us. They occur when a campfire burns wood, when iron rusts in damp air, and when our bodies digest food. While atoms rearrange during these changes, the nuclei of the atoms remain untouched. This means the elements themselves do not turn into different elements during regular chemical reactions.

How Chemical Reactions Work
- Reactants and Products
- Clues That a Chemical Reaction Is Happening
History of Chemistry and Discoveries
- Ancient Times and Alchemy
- The Birth of Modern Chemistry
Writing and Balancing Chemical Equations
- How to Read a Chemical Equation
- The Law of Conservation of Mass
Four Basic Types of Chemical Reactions
Other Important Types of Reactions
Energy in Chemical Reactions
- Exothermic Reactions
- Endothermic Reactions
Reaction Rates and Chemical Kinetics
- Factors That Change Reaction Speed
- Activation Energy
Catalysts and Enzymes
Reversible Reactions and Equilibrium
Chemical Reactions in Daily Life and Industry
- Industrial Manufacturing
- Specialized Applications
Images for kids
See Also

How Chemical Reactions Work

Reactants and Products

Every chemical reaction starts with reactants and ends with products. Reactants are the original ingredients mixed together. When these ingredients interact, bonds between atoms break apart. The atoms then assemble in new combinations to build products.

Because the products are brand new substances, they have different physical and chemical properties from the reactants. For instance, combining dangerous sodium metal with toxic chlorine gas produces harmless sodium chloride, commonly known as table salt.

Clues That a Chemical Reaction Is Happening

Scientists look for specific signs to tell if a chemical reaction has occurred. These visual and physical clues include:

Formation of a gas, often visible as fizzing or bubbles.
A solid settling out of a liquid, known as a precipitate.
A distinct change in temperature, such as releasing heat or absorbing heat.
A noticeable permanent change in color.
An emission of light, such as a flame or a glow.
A change in odor or smell.

History of Chemistry and Discoveries

Antoine Lavoisier, a pioneer of modern chemistry who explained how burning works.

Ancient Times and Alchemy

Human curiosity about chemical transformations goes back thousands of years. Early civilizations made use of reactions like fire, brewing, and smelting metal ores from rocks. Ancient Greek thinkers tried to explain matter through simple models. The philosopher Empedocles proposed that everything consisted of four basic elements: earth, water, air, and fire.

During the Middle Ages, alchemists experimented extensively with materials. Many alchemists searched for ways to turn cheap metals like lead into precious gold. While they never succeeded in making gold, alchemists invented useful laboratory tools and discovered substances such as sulfuric acid and nitric acid.

The Birth of Modern Chemistry

In the late 18th century, French scientist Antoine Lavoisier transformed the study of chemistry. Before his work, many people believed things burned because they contained a mysterious fluid called phlogiston. Lavoisier proved that burning, or combustion, actually involves a material combining with oxygen gas from the air.

Lavoisier also helped establish the law of conservation of mass. This scientific law states that matter cannot be created or destroyed during a chemical reaction. In the early 19th century, John Dalton developed modern atomic theory, and Joseph Proust showed that elements always combine in fixed proportions. In 1828, Friedrich Wöhler showed that substances found in living things could be created in a laboratory from simple minerals, creating the field of modern organic chemistry.

Writing and Balancing Chemical Equations

Methane gas burns with oxygen to form carbon dioxide and water molecules.

How to Read a Chemical Equation

Scientists use chemical equations to describe what happens during a reaction. An equation acts like a recipe written in chemical shorthand. The reactants are placed on the left side, and the products sit on the right side. An arrow (→) points from the reactants to the products, meaning "yields" or "produces."

For example, when natural gas (methane) burns in oxygen, the equation looks like this:

CH
4 + 2O
2 → CO
2 + 2H
2O

The Law of Conservation of Mass

Because atoms cannot be created or destroyed in ordinary reactions, every atom present at the start must exist at the end. An equation must be balanced, meaning there are equal numbers of each type of atom on both sides.

Chemists balance equations by placing numbers called coefficients in front of formulas. In the methane example above, the number 2 before oxygen (O
2) and water (H
2O) ensures there are four hydrogen atoms and four oxygen atoms on each side.

Four Basic Types of Chemical Reactions

Diagram showing the four main patterns of chemical reactions.

Synthesis Reactions

In a synthesis reaction, two or more simple substances combine to form a single, more complex product. Synthesis reactions follow this general pattern: A + B → AB

An everyday example occurs when iron metal reacts with sulfur powder under heat to produce iron(II) sulfide: 8Fe + S
8 → 8FeS

Another basic synthesis reaction occurs when pure hydrogen gas burns in oxygen gas to create pure water.

Decomposition Reactions

A decomposition reaction is the exact opposite of a synthesis reaction. A complex compound breaks down into two or more simpler substances. Its formula is: AB → A + B

A famous example is using electricity to split water into hydrogen and oxygen gases through a process called electrolysis: 2H
2O → 2H
2 + O
2

Single Displacement Reactions

In a single displacement reaction, one uncombined element takes the place of another element within a compound. Its pattern is: A + BC → AC + B

For instance, when pure magnesium metal is placed into water, magnesium pushes out the hydrogen to form magnesium hydroxide and hydrogen gas: Mg + 2H
2O → Mg(OH)
2 + H
2

Double Displacement Reactions

In a double displacement reaction, parts of two different compounds switch places to build two brand-new compounds. The pattern is: AB + CD → AD + CB

An example happens when clear solutions of lead(II) nitrate and potassium iodide mix together. They instantly trade partners to create potassium nitrate and a bright yellow solid called lead(II) iodide: Pb(NO
3)
2 + 2KI → PbI
2 + 2KNO
3

Other Important Types of Reactions

Combustion Reactions

A combustion reaction occurs when a substance reacts rapidly with oxygen gas, releasing lots of energy as heat and light. Most combustion reactions involve burning fuels like wood, coal, or gasoline. When octane, a main ingredient in gasoline, burns in an engine, it produces carbon dioxide, water vapor, and energy: 2C
8H
18 + 25O
2 → 16CO
2 + 18H
2O

Oxidation and Reduction (Redox)

Table salt is formed when sodium atoms transfer electrons to chlorine atoms.

In a redox reaction, one atom loses electrons while another atom gains them.

Redox is short for reduction-oxidation. These reactions involve the transfer of tiny subatomic particles called electrons between atoms:

Oxidation occurs when an atom or molecule loses electrons.
Reduction occurs when an atom or molecule gains electrons.

You can remember this with the phrase "OIL RIG": Oxidation Is Loss, Reduction Is Gain. When sodium metal reacts with chlorine gas, each sodium atom loses one electron (oxidation) and each chlorine atom gains one electron (reduction) to make stable table salt. Redox reactions are the basis for electric batteries that power mobile phones and electric cars.

Acid-Base Reactions

An acid–base reaction occurs when an acid mixes with a base. In the common definition, an acid donates a tiny positive particle called a proton (H+
), and a base accepts that proton. When an acid and a base combine in matching quantities, they undergo neutralization, creating water and a salt.

For example, when hydrochloric acid mixes with sodium hydroxide, they neutralize each other: HCl + NaOH → NaCl + H
2O

Precipitation Reactions

Precipitation happens when dissolved chemicals react to create a solid.

A precipitation reaction occurs in a liquid solution when two soluble salts react to form an insoluble solid called a precipitate. The solid particles clump together, making the liquid turn cloudy or settling to the bottom of the container. Geologists study natural precipitation reactions because they help create limestone caves and mineral veins underground.

Photochemical Reactions

A chemical transformation triggered by light energy.

Photochemical reactions are driven by energy from light photons rather than heat. When molecules absorb light, their electrons gain energy, allowing new bonds to form.

The most famous photochemical reaction on Earth is photosynthesis. Green plants capture sunlight to turn carbon dioxide and water into energy-rich glucose sugar and fresh oxygen gas. In human skin, sunlight drives the photochemical reaction that produces essential vitamin D.

Energy in Chemical Reactions

Exothermic Reactions

Chemical reactions always involve changes in energy. Every chemical bond contains stored energy. Breaking bonds requires an input of energy, while forming new bonds releases energy.

If a reaction releases more energy than it absorbs, it is called an exothermic reaction. These reactions release excess energy into the surrounding environment as heat, light, or sound. Burning wood in a fireplace and hand-warmer packets are everyday examples of exothermic reactions.

Endothermic Reactions

If a reaction absorbs more energy from its surroundings than it releases, it is called an endothermic reaction. These reactions take in thermal energy, causing the temperature of the nearby environment to drop.

Instant cold packs used for sports injuries rely on endothermic reactions. When a water pouch inside the pack breaks, it dissolves ammonium nitrate crystals, rapidly absorbing heat and turning freezing cold to soothe bruises.

Reaction Rates and Chemical Kinetics

Catalysts lower the activation energy barrier, speeding up the reaction.

Some chemical reactions occur in the blink of an eye, like a firework exploding. Other reactions take days or years, like iron turning into rust. The speed at which reactants convert into products is called the reaction rate.

Factors That Change Reaction Speed

Scientists can control the speed of a reaction by adjusting several key conditions:

Concentration: Crowding more reactant particles into a space causes them to collide more frequently, speeding up the reaction.
Surface Area: Breaking a solid reactant into fine powder exposes more atoms to collisions, making it react much faster than a single large block.
Temperature: Heating substances gives particles more kinetic energy. They move faster and collide harder, causing bonds to break and reform much more quickly.
Pressure: Increasing pressure on gases squeezes the molecules together, increasing collision rates.
Catalysts: Adding a catalyst provides an easier chemical pathway for the reaction, greatly increasing speed without being consumed in the process.

Activation Energy

For any reaction to begin, reactant molecules must collide with enough force to break their existing bonds. The minimum amount of energy needed to kick off a reaction is called the activation energy. Think of activation energy like a hill: reactants must climb over the energy hill before they can roll down to become stable products.

Catalysts and Enzymes

The inside of a car catalytic converter, which uses metal catalysts to clean exhaust gases.

An enzyme acts like a lock, gripping specific molecules in its active site to speed up body chemistry.

A catalyst is a substance that speeds up a chemical reaction without undergoing any permanent change itself. Catalysts work by lowering the activation energy hill, allowing reactions to occur quickly at lower temperatures. Because catalysts are not consumed, a tiny amount can process millions of reactant molecules.

Cars use metallic catalysts made of platinum and rhodium in their catalytic converters. These devices rapidly turn harmful exhaust gases into harmless nitrogen, water vapor, and carbon dioxide before they leave the tailpipe.

In living creatures, biological catalysts are known as enzymes. Enzymes are specialized proteins folded into precise shapes with an "active site." The reactant fits into this active site like a key in a lock. Without enzymes, essential life processes like breaking down sugars in our cells would take years instead of fractions of a second.

Reversible Reactions and Equilibrium

Not all chemical reactions run in only one direction until the ingredients run out. Many reactions are reversible, meaning the products can react together to reform the original starting materials.

Reversible reactions use a double arrow () in their equations:

When a reversible reaction takes place in a closed container, the forward reaction and the reverse reaction eventually happen at the exact same speed. When this balance is reached, the amounts of reactants and products stop changing. This balanced state is known as chemical equilibrium.

If conditions like temperature or pressure change, the reaction shifts its balance to relieve that stress, a rule known as Le Chatelier's principle.

Chemical Reactions in Daily Life and Industry

Workers using an intense thermite reaction to weld steel train rails together.

Chemical reactions are the engine behind modern civilization and technology. Chemical engineers use controlled reactions to manufacture clean fuels, medicines, plastic materials, and fertilizers that help grow food for billions of people.

Industrial Manufacturing

One of the most important chemical reactions in human history is the Haber process, invented in the early 20th century. This reaction combines nitrogen from the air with hydrogen gas to create ammonia: N
2 + 3H
2 → 2NH
3

Ammonia is a vital ingredient in crop fertilizers. Scientists estimate that roughly half of the food grown on Earth today depends on nitrogen fertilizers made through this single reaction.

Specialized Applications

Some chemical reactions are chosen for extreme power or high heat. The thermite reaction mixes aluminum powder and iron oxide. When ignited, it produces molten liquid iron reaching temperatures above 2500 C (4500 F). This reaction is so hot and portable that railroad crews use it to weld steel tracks together in remote outdoor locations.