Particles in Motion, Energy & Concentration ഗതികോർജം · ഗാഢത
The second half of this chapter explains why some reactions are fast and others slow. The explanation, collision theory, is built from four simple ideas. Particles are always moving. A moving particle has kinetic energy, and a higher temperature means faster particles. Concentration tells us how many particles are packed into each unit volume. And cutting a solid into small pieces increases its surface area. This page explains each idea with a lab, so that pages 6–8 will feel easy.
- Matter is made of tiny particles: atoms, molecules or ions (Class 8).
- Solid, liquid and gas, and how to find the area of a square (side × side).
1. Particles are always moving
Every substance is made of particles, and they never stop moving. In a solid they are packed tightly and only vibrate about fixed places. In a liquid they are close together but slide past one another. In a gas they are far apart and fly about fast in all directions, bumping into each other and into the walls. This is why you can smell a curry from the next room: the smell molecules move through the air (the book's opening picture, page 95).
The particles of all three move faster when they are heated.
2. Kinetic energy and temperature
Anything that moves has kinetic energy (ഗതികോർജം): the energy of motion. A fast cricket ball hurts more than a slow one, and a heavy ball more than a light one at the same speed. Particles are the same: the faster a particle moves, the more kinetic energy it has.
Temperature (താപനില) is a measure of how fast the particles move on average. When we heat a substance, its particles move faster, gain kinetic energy, and collide more often and harder. When we cool it (in a fridge, for example), the particles slow down. Not all particles have the same speed: at any temperature a few are slow, most are medium and a few are very fast.
Watch the collision counter. More particles in the same box → more collisions. Hotter → faster particles → more collisions every second, and harder ones. The speeds are exaggerated so that you can see the change; in real life, doubling the speed of the particles needs a very large rise in temperature.
3. Concentration: particles per unit volume
Imagine the box above cut into equal small boxes, each of the same size, called a unit volume (യൂണിറ്റ് വ്യാപ്തം), for example 1 mL or 1 L. The concentration (ഗാഢത) of a substance is the number of its particles (molecules) in a unit volume. A concentrated (ഗാഢ) solution has many particles in each unit volume; a dilute (നേർപ്പിച്ച) solution has few. Adding water to an acid does not remove any acid particles. It spreads them through more volume, so each unit volume holds fewer of them: the acid becomes dilute.
4. Cutting a solid increases its surface area
Only the particles on the outside of a solid can be hit by the particles of a liquid or gas around it. The total area of the outside is the surface area (പ്രതലപരപ്പളവ്). A cube has 6 square faces, so its surface area = 6 × side × side. Watch what happens when one cube of side 2 cm is cut into small cubes of side 1 cm:
The amount of solid is the same (8 cm³), but much more of it is now on the outside. This is why a powder reacts faster than a lump (marble powder in acid, page 7), why split firewood catches fire more easily than a log, and why we chew food well.
- Particles are always moving: they vibrate in solids, slide in liquids and fly about in gases.
- A moving particle has kinetic energy (ഗതികോർജം). Higher temperature (താപനില) → faster particles → more and harder collisions.
- Concentration (ഗാഢത) = the number of particles in a unit volume. Concentrated: many; dilute: few.
- Cutting a solid into smaller pieces increases its surface area (പ്രതലപരപ്പളവ്): one 2 cm cube has 24 cm², eight 1 cm cubes have 48 cm².