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Molecules must crash hard enough to react. What makes that happen more often?
Chemical Reactions lab · Last updated: 16 October 2026
You watch molecules zoom about and crash into each other, and see that only the hardest crashes make them react. Then they look for ways to make those crashes happen more often. It explains why food spoils faster in the heat, why fridges help and why powdered sugar dissolves and burns more easily than lumps.
What you'll discover
- Molecules only react when they collide hard enough to break their old bonds.
- Heating makes molecules move faster, so more collisions are hard enough to react.
- Crowding molecules together, or breaking solids into smaller pieces, gives more collisions every second.
The science in plain words
In a chemical reaction, atoms are rearranged: old bonds break and new ones form, making new substances. For that to happen, the reacting particles must meet. Collision theory says that particles react only when they collide with enough energy, and often in the right direction. The minimum energy needed is called the activation energy. Most collisions are too gentle and the particles simply bounce apart.
Anything that increases the number of successful collisions speeds up a reaction. Raising the temperature makes particles move faster, so they collide more often and, more importantly, a much larger share of collisions have enough energy. That is why food goes off faster on a hot day and lasts longer in a fridge. Increasing concentration, or the pressure of a gas, crowds more particles into the same space, so they meet more often. Breaking a solid into smaller pieces increases its surface area, exposing more particles to collisions, which is why powders react faster than lumps.
A catalyst speeds up a reaction without being used up. It offers a different route with a lower activation energy, so more collisions succeed even at the same temperature. Catalytic converters in car exhausts and the enzymes in our bodies both work this way.
Where it fits
- India, NCERT: Class 10 science, Chapter 1: Chemical Reactions and Equations
- India, NCERT: Collision theory and reaction rates go beyond the NCERT syllabus for classes 6 to 10; the lab builds on Class 10 Chemical Reactions and Equations.
- United States: Grades 9 to 12
- Next Generation Science Standards: HS-PS1-5
Common questions
What makes a chemical reaction go faster?
Four main things: a higher temperature, a higher concentration or pressure, a larger surface area, such as using a powder instead of a lump, and a catalyst. Each one increases how often particles collide with enough energy to react.
Why does food go bad faster when it is warm?
Spoiling is caused by chemical reactions and by microbes, whose growth depends on chemical reactions too. At higher temperatures particles move faster and collide harder, so these reactions speed up. Keeping food cold in a fridge slows them down, so food stays fresh for longer.
What is a catalyst?
A catalyst is a substance that speeds up a chemical reaction without being used up itself. It gives the reaction an easier route that needs less energy to get started. Enzymes in our body are natural catalysts, and cars use catalysts to clean their exhaust gases.
Related labs
- Enzyme Activity: Enzymes speed up reactions. Why do they stop working when it gets too hot?
- Combustion: Put a jar over a candle. Why does the flame go out, and when?
- Redox Reactions: Two metals and two solutions make a battery. Where do the electrons come from?
See all chemistry labs or every Taz science lab.
Try it yourself
The Chemical Reactions lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.