Chemistry is not something that only happens in laboratories with bubbling flasks. It happens all around us, every second. When you light a candle, toast bread, digest your breakfast, wash your hands with soap or watch an iron gate slowly turn rusty, chemical reactions are at work. Your body carries out thousands of reactions every moment to keep you alive. Understanding a few basic ideas about chemical reactions helps explain everyday events and the technology we rely on. This explainer describes what chemical reactions are and explores some of the most familiar examples.
What is a chemical reaction?
A chemical reaction is a process in which substances, called reactants, are transformed into new substances, called products, with different properties. During a reaction, chemical bonds between atoms are broken and new bonds are formed. The atoms themselves are not created or destroyed; they are rearranged.
This is different from a physical change, in which a substance changes form but not identity. Melting ice, boiling water or dissolving sugar are physical changes: water remains water, and sugar remains sugar. Burning wood or rusting iron, on the other hand, are chemical changes: new substances are formed.
Signs of a chemical reaction
Clues that a chemical reaction is happening include:
- A change in color, such as an apple turning brown or iron turning reddish.
- The production of a gas, seen as bubbles or fizzing.
- A change in temperature, either releasing heat or absorbing it.
- The emission of light, as in fire or glow sticks.
- The formation of a solid, called a precipitate, when two liquids are mixed.
- A new smell.
None of these signs alone proves that a reaction occurred, but together they are good indicators.
Chemical equations
Chemists describe reactions using chemical equations, which show reactants on the left and products on the right. For example, the burning of methane, the main component of natural gas, is written as:
CH₄ + 2 O₂ → CO₂ + 2 H₂O
This means that one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water. The equation is balanced: there are the same number of each type of atom on both sides, reflecting the law of conservation of mass, first clearly demonstrated by the French chemist Antoine Lavoisier in the eighteenth century.
Energy in reactions
All chemical reactions involve energy changes, because breaking bonds requires energy and forming bonds releases energy.
- Exothermic reactions release energy, usually as heat or light. Burning fuel, rusting and the reactions in hand warmers are exothermic.
- Endothermic reactions absorb energy from their surroundings, making them feel cold. Instant cold packs, which use the dissolving of ammonium nitrate in water, and photosynthesis are examples.
Activation energy
Even reactions that release energy usually need a little energy to get started, called activation energy. A match needs friction to ignite; wood needs a spark or flame to start burning. Once started, the energy released keeps the reaction going.
Catalysts
A catalyst speeds up a reaction without being used up. It works by lowering the activation energy. In cars, catalytic converters use metals like platinum and palladium to convert harmful exhaust gases into less harmful ones. In living things, catalysts called enzymes control almost every chemical reaction, from digesting food to copying DNA.
Reaction rates
Several factors affect how fast a reaction happens:
- Temperature: Higher temperatures make molecules move faster and collide more often and with more energy, speeding up reactions. That is why food spoils faster at room temperature than in the refrigerator.
- Concentration: More reactant molecules in a given space lead to more collisions.
- Surface area: Smaller pieces react faster. Powdered sugar dissolves faster than a sugar cube, and fine flour dust can even explode in the air.
- Catalysts: Speed up reactions by providing an easier pathway.
Everyday chemical reactions
Fire: combustion
Combustion is a fast reaction between a fuel and oxygen that releases heat and light. When wood, gas or candle wax burns, carbon and hydrogen in the fuel combine with oxygen from the air to form carbon dioxide and water vapor. Fire needs three things, often called the fire triangle: fuel, oxygen and heat. Removing any one of them puts out the fire. Fire extinguishers work by cooling the fire, cutting off oxygen or interrupting the chemical reaction.
Rusting: slow oxidation
Rust forms when iron reacts with oxygen and water over time, creating hydrated iron oxide, the reddish-brown flaky substance we call rust. This is a type of oxidation, a reaction in which a substance loses electrons. Salt speeds up rusting, which is why cars rust faster in places where roads are salted in winter. Paint, oil and coatings like zinc in galvanized steel protect iron from rust by keeping out air and water.
Cooking and baking
Cooking is full of chemistry:
- The Maillard reaction between sugars and amino acids gives browned foods like toast, roasted meat and coffee their rich flavors and colors.
- Caramelization turns sugar into golden, flavorful caramel.
- Proteins in eggs denature and coagulate when heated, turning liquid egg white into a firm solid.
- In baking, baking soda (sodium bicarbonate) reacts with acids, such as buttermilk or yogurt, to produce carbon dioxide gas that makes cakes and pancakes rise.
Fruits turning brown
When you cut an apple, banana or avocado, enzymes in the fruit react with oxygen in the air, producing brown pigments. Lemon juice slows this browning because its acidity and vitamin C interfere with the enzyme.
Digestion and respiration
Your body is a chemical factory. During digestion, enzymes break down large food molecules into smaller ones. During cellular respiration, cells combine glucose with oxygen to release energy, producing carbon dioxide and water. This is chemically similar to burning, but it happens in small, controlled steps.
Photosynthesis
Plants, algae and some bacteria use sunlight to convert carbon dioxide and water into glucose and oxygen:
6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂
Photosynthesis provides the oxygen we breathe and the energy at the base of almost every food chain.
Batteries
Batteries produce electricity through redox reactions, in which electrons are transferred from one substance to another. The electrons flow through an external circuit, powering devices.
Acids and bases
Acids and bases are two important groups of chemicals. Acids, such as lemon juice and vinegar, taste sour and release hydrogen ions in water. Bases, such as baking soda and soap, taste bitter, feel slippery and can neutralize acids.
The pH scale measures how acidic or basic a solution is, from 0 to 14:
| pH | Example |
|---|---|
| about 1 to 2 | stomach acid |
| about 2 | lemon juice |
| about 3 | vinegar |
| about 7 | pure water (neutral) |
| about 8 | seawater |
| about 9 | baking soda solution |
| about 12 | household ammonia |
When an acid and a base react, they neutralize each other, often forming water and a salt. Antacid tablets neutralize excess stomach acid, and a classic science experiment mixes baking soda and vinegar to produce a fizzy eruption of carbon dioxide.
Cleaning chemistry
Soap and detergents work through clever chemistry. Their molecules have a water-loving head and an oil-loving tail. When you wash your hands, the oil-loving tails attach to grease and dirt, while the water-loving heads face outward, forming tiny clusters called micelles that carry the dirt away in water. Soap also disrupts the fatty outer layer of some viruses and bacteria.
Never mix cleaning products such as bleach and ammonia, or bleach and acids, because they can react to produce toxic gases.
More reactions you meet every day
- Glow sticks: Bending a glow stick breaks an inner glass vial, mixing two chemicals. Their reaction releases energy that excites a dye, which then emits light without heat, a process called chemiluminescence. Fireflies produce light through a similar reaction controlled by an enzyme called luciferase.
- Hand warmers: Disposable hand warmers contain iron powder that rusts rapidly when exposed to air, releasing heat. Salt and activated carbon speed up the reaction.
- Tarnished silver: Silver darkens when it reacts with sulfur compounds in the air, forming a thin layer of black silver sulfide.
- Hair dye and bleach: Many hair dyes use oxidation reactions with hydrogen peroxide to change the pigments in hair.
- Concrete: When cement is mixed with water, a set of reactions called hydration forms new crystals that bind sand and gravel together. Concrete continues to harden for weeks.
- Fireworks: Metal salts in fireworks produce different colors when heated: strontium for red, barium for green, copper for blue and sodium for yellow.
Chemistry and safety at home
Many products at home are reactive chemicals and deserve respect. Read labels, store cleaning products in their original containers out of reach of children, keep them away from food and never mix different cleaning products. Use good ventilation when working with strong-smelling substances such as paint strippers or oven cleaners, and wear gloves when the label recommends it. In case of accidental swallowing or contact with the eyes, contact a poison control center or emergency services immediately.
Questions readers ask
What is the difference between a physical and a chemical change? In a physical change, a substance changes form but remains the same substance. In a chemical change, new substances are formed.
Why do reactions happen faster when it is warm? Molecules move faster and collide more often and with more energy.
What is a catalyst? A substance that speeds up a reaction without being used up, by lowering its activation energy.
Why does iron rust? Iron reacts with oxygen and water to form iron oxide, a process sped up by salt.
Why does baking soda make cakes rise? It reacts with acids to produce carbon dioxide gas, which forms bubbles in the batter.
In short
Chemical reactions rearrange atoms to form new substances, breaking and making chemical bonds and releasing or absorbing energy. They can be recognized by changes in color, gas, temperature or light. Temperature, concentration, surface area and catalysts control how fast they happen. From fire and rust to cooking, digestion, photosynthesis, batteries and cleaning, chemical reactions shape our everyday lives. Knowing a little chemistry turns ordinary moments into fascinating science.