A chemical reaction is when substances interact and rearrange their atoms to form new substances with different properties. The starting materials are called reactants; the results are called products. Nothing is created or destroyed — the same atoms are there at the end as at the beginning, just bonded in different configurations.
Chemical reactions are everywhere. Burning a log: chemical reaction. Digesting your lunch: chemical reaction. Rusting metal: chemical reaction (just a very slow one). Your muscles moving: chemical reactions. Baking a cake: dozens of simultaneous chemical reactions. Life itself is essentially a sustained, incredibly complex sequence of chemical reactions.
Imagine you have a bag of red Lego bricks connected in various ways, and a bag of blue Lego bricks. A chemical reaction is like dismantling those specific connections and reconnecting the same bricks into completely new shapes — some red-blue, some all-red, some all-blue. You started with the same number of bricks; you end with the same number of bricks. But what you've built is completely different. That's chemistry: rearranging atomic "bricks" into new structures.
What makes a reaction happen?
Chemical bonds have energy stored in them. To break old bonds takes energy; forming new bonds releases it. If a reaction releases more energy than it takes to start, it can sustain itself — like burning wood (once you light it, it keeps going). These are called exothermic reactions. Reactions that absorb more energy than they release — and need continuous energy input — are endothermic. Photosynthesis is endothermic: it uses energy from sunlight to build sugar from carbon dioxide and water.
What speeds reactions up?
Temperature: hotter means faster-moving molecules, more collisions, more reactions. Concentration: more reactant molecules in a space means more chance of collision. Surface area: a log burns more slowly than sawdust made from the same wood, because the sawdust has far more surface exposed to oxygen. And catalysts — substances that speed up a reaction without being used up themselves. Your body is full of biological catalysts called enzymes, which make reactions happen at body temperature that would otherwise need enormous heat.
A chemical reaction happens when substances mix together and change into new things. The starting materials are called reactants. The new things made are called products. Nothing gets lost or created. The same tiny parts are there at the end. They just get joined up in different ways.
Chemical reactions happen everywhere around us. Burning a log is a chemical reaction. Your body breaking down your lunch is a chemical reaction. Metal getting rusty is a chemical reaction. It just happens very slowly. Your muscles moving uses chemical reactions. Baking a cake uses lots of chemical reactions at once. Life is made up of millions of chemical reactions happening together.
Think about having a bag of red Lego bricks stuck together. You also have a bag of blue Lego bricks. A chemical reaction is like taking apart those connections. Then you stick the same bricks together in totally new ways. You might make some red-blue shapes. You might make some all-red or all-blue shapes. You still have the same number of bricks. But what you built looks completely different. Chemistry works like this. It takes apart tiny building blocks called atoms. Then it sticks them back together in new ways.
What makes a reaction happen?
Chemical bonds hold energy inside them. Breaking old bonds needs energy. Making new bonds lets energy out. Some reactions give out more energy than they need to start. These can keep going by themselves. Burning wood is like this. Once you light it, it keeps burning. These are called exothermic reactions. Some reactions need more energy than they give out. These need energy put in all the time. These are called endothermic. Plants making sugar from sunlight is endothermic. It uses energy from the sun. It turns air and water into sugar.
What speeds reactions up?
Heat makes reactions faster. Hot things move around more. They bump into each other more often. More bumping means more reactions happen. Having more reactants in the same space helps too. More stuff means more chance of things bumping together. Surface area matters as well. A whole log burns slowly. Sawdust from the same log burns much faster. This is because sawdust has much more surface touching the air. Catalysts are special substances that make reactions go faster. They don't get used up in the reaction. Your body has biological catalysts called enzymes. These make reactions happen at body temperature. Without them, these reactions would need very high heat.