Particles, Solids, Liquids and Gases
Your textbook says that sound needs a medium (മാധ്യമം) to travel, that it can go through a solid, a liquid and a gas (ഖരം / ദ്രാവകം / വാതകം), but not through a vacuum (ശൂന്യത). To understand why, you need one simple picture: all matter is made of tiny particles. This page shows how the particles are arranged in solids, liquids and gases, how a push is passed from particle to particle, and what a vacuum is – including why the Moon and outer space have no air.
- To-and-fro motion and vibration (page 1).
- Nothing else – every idea is built up here.
1. Everything is made of particles
Break a piece of chalk into smaller and smaller bits. Even the finest powder is still chalk. If you could go on dividing, you would finally reach particles far too small to see even with a microscope – the molecules (or atoms) of the substance. Water, iron, wood, air – all matter is made of particles. Three facts about them explain almost everything in this chapter:
- The particles are always moving – they jiggle, slide or fly about.
- They move faster when the substance is hotter (at a higher temperature, താപനില).
- There are forces between neighbouring particles, so when one particle is pushed it pushes its neighbour.
2. Solids, liquids and gases
The same particles can make a solid, a liquid or a gas – ice, water and steam are all water. What changes is how closely the particles are packed and how freely they move:
| State | Arrangement of particles | How they move | Shape and volume |
|---|---|---|---|
| Solid ഖരം | Very close together, in a fixed pattern | Only vibrate about fixed places | Fixed shape, fixed volume |
| Liquid ദ്രാവകം | Close together, but no fixed pattern | Slide past one another | Takes the shape of the vessel; fixed volume |
| Gas വാതകം | Far apart – in air about 10 times farther apart than in water (approx.) | Fly about freely and bump into each other and the walls | Fills the whole vessel |
Raise the temperature: in all three states the particles move faster. In the solid they still stay in their places – they only vibrate harder. (A drawing, not to scale: real particles are far too small and far too many to show.)
On cold December mornings – especially in Munnar or Wayanad – the coconut oil in the bottle turns white and solid. Warm it in the sun and it becomes a clear liquid again (coconut oil melts at about 24 °C). The particles are the same; only their packing and motion change. And when someone fries pappadam in the kitchen, you soon smell it in the next room: particles of the smell move through the air, a gas, bumping their way across the house.
3. Passing on a push – why sound needs a medium
Hit one end of a row of particles. The first particle moves forward, pushes its neighbour and comes back; the neighbour pushes the next one, and so on. The push travels along the row, but each particle only moves a little forward and back – no particle travels from one end to the other. This is exactly how sound travels, which is why it needs particles – a medium (മാധ്യമം). The closer and more tightly held the particles are, the faster the push is passed on.
The motion is slowed down enormously, but the speeds are to scale using your textbook's Table 8.3: steel 5944 m/s, pure water 1485 m/s, air 347 m/s – steel is about 17 times and water about 4 times faster than air. Watch the red particle: it goes forward and comes back to its own place. In a vacuum there is nothing to pass the push on.
4. Vacuum – a space with no particles
If you remove all the particles from a space, nothing is left – not even air. Such an empty space is a vacuum (ശൂന്യത). In the laboratory, a vacuum pump (വാക്വം പമ്പ്) sucks air out of a glass bell jar (ബെൽജാർ). Every stroke of the pump removes some of the particles, so the air inside gets thinner and thinner. Even a good pump cannot remove every particle, but it can come very close.
Each dot stands for an enormous number of real particles. At 100 % the jar holds ordinary air at about 100 kPa (the normal air pressure). At 0 % it would be a perfect vacuum – no particles at all.
Worked example · How empty is “empty”?
Q. One cubic centimetre of air at room temperature contains about 2.5 × 1019 particles (approx.). A bell jar holds 5000 cm3 of air. A pump removes 99 % of the air. (a) How many particles were in the jar at first? (b) How many are left?
(a) Number in the jar = number in each cm3 × number of cm3. (b) If 99 % is removed, 1 % = 1/100 is left.
5. Why the Moon and outer space have no air
The air around the Earth – the atmosphere – is held by the Earth's gravity. The Moon has no atmosphere, and the space between the planets is almost a perfect vacuum. Step through to see why. This explains your textbook's question about astronauts (page 163) and “Let us assess” question 6.
- All matter is made of tiny particles that are always moving, faster when hotter.
- Particles are closest and most tightly held in solids, close but free to slide in liquids, and far apart in gases.
- A push is passed from particle to particle: the push travels, the particles only move a little to and fro. That is why sound needs a medium.
- A vacuum has no particles, so it cannot pass on a push – sound cannot travel through it. The Moon and outer space are (almost) a vacuum.