The core idea
Matter has mass and occupies space. Its particles move and interact; changing their arrangement, separation and energy can change the state without changing the substance itself.
1. Start with a model you can test
A stone, a spoonful of water and the air inside a ball are all matter. Mass describes how much matter is present; volume describes the space occupied. Their units differ: grams or kilograms for mass, millilitres or cubic metres for volume. In the particle model, a visible sample contains enormously many tiny particles. Depending on the substance these may be atoms, molecules or ions. Drawn circles are symbols, not miniature photographs. The spaces and forces between particles help explain behaviour that our eyes can see, but the drawing alone cannot prove which particles a material contains.
Sources: NCERT via IIT Kanpur SATHEE: Matter in Our Surroundings ↗ · NIOS: Matter in Our Surroundings ↗
2. Arrangement explains shape and compression
In a solid, neighbouring particles remain around relatively fixed positions while vibrating. In a liquid they stay close but can rearrange, so a liquid flows and takes the shape of the occupied part of a container. Gas particles are much farther apart and move throughout their container. Pressing a gas into a smaller space mainly reduces the gaps; it does not squash each molecule into a smaller molecule. Liquids and solids resist compression much more. A heap of sand can pour, yet each grain is solid: changing the arrangement of grains is different from melting the material inside each grain.
One particle model, three states
| State | Particle arrangement and movement | Macroscopic result |
|---|---|---|
| Solid | Close together; vibrate about positions | Definite shape and volume |
| Liquid | Close together; rearrange and move past one another | Definite volume; takes container’s shape |
| Gas | Far apart relative to their size; move freely | Spreads through available space |
Sources: NCERT via IIT Kanpur SATHEE: Matter in Our Surroundings ↗
3. Random motion can produce a pattern
Particles in liquids and gases move in many directions. If more of one kind begin in one region, random movement usually produces a net spread towards regions containing fewer of that kind. This is diffusion. Individual particles do not know where to go, and they continue moving after an even distribution develops. A smell crossing a room may involve both diffusion and moving air; it is not evidence for diffusion alone. Increasing temperature usually increases particle motion, but changes in air currents can also change how quickly a smell reaches someone. A fair comparison must keep those other influences similar.
Sources: NCERT via IIT Kanpur SATHEE: Matter in Our Surroundings ↗
4. State changes redistribute energy
Melting changes solid to liquid; freezing is the reverse. Vaporisation changes liquid to gas, and condensation changes gas to liquid. Sublimation goes directly from solid to gas; deposition reverses that route. These names describe direction, not new chemical substances. Water molecules remain water molecules when ice melts. Heating generally increases motion within one state. During a pure substance’s change of state at fixed pressure, supplied energy can instead change particle interactions while temperature stays nearly constant. Temperature therefore does not measure all the energy in a sample. Melting and boiling temperatures also depend on pressure and on what substances are present.
Sources: NCERT via IIT Kanpur SATHEE: Matter in Our Surroundings ↗
5. Drying does not require boiling
Evaporation happens at a liquid surface when some particles escape into the gas phase. Boiling involves vapour bubbles forming throughout the liquid at its boiling condition. A wet cloth dries below the boiling temperature because evaporation still occurs. A larger exposed surface provides more escape opportunities; moving air removes moist air near that surface. High humidity reduces net drying under otherwise similar conditions. Escaping particles carry energy, so evaporation can cool the remaining liquid and its surroundings. Water vapour is invisible. The mist near a cold surface consists of tiny liquid droplets, and the wet outside of a sealed cold bottle comes from surrounding air.
Sources: NCERT via IIT Kanpur SATHEE: Matter in Our Surroundings ↗
6. Worked example: follow missing water
Illustrative data: an empty dish has mass 40 g, and the dish with water initially has mass 160 g. Later its mass is 145 g. Initial water mass is 160 − 40 = 120 g; remaining water is 145 − 40 = 105 g. Therefore 15 g left the dish, assuming no spilling and no other material change. The dish is an open system because matter can cross its boundary. Water did not cease to exist: it entered the surrounding air as vapour. If the entire closed enclosure, including that air, were weighed, its total mass would remain the same within measurement uncertainty.
Sources: NCERT via IIT Kanpur SATHEE: Matter in Our Surroundings ↗ · NIOS: Matter in Our Surroundings ↗
7. Worked example: interpret a flat reading
Illustrative record for pure ice at ordinary atmospheric pressure: while energy enters, readings remain about 0 °C until the ice finishes melting, then rise to 4 °C. The flat part does not mean energy transfer stopped; melting used energy to change the arrangement and interactions. Celsius is written °C. Kelvin, written K without a degree sign, is related by T(K) = t(°C) + 273.15. Thus 4 °C is 277.15 K. Here T and t label the same temperature on different scales. A rise from 0 to 4 °C is a difference of 4 degrees Celsius or 4 kelvin, not 277.15 kelvin.
Sources: NCERT via IIT Kanpur SATHEE: Matter in Our Surroundings ↗
PUT IT INTO PRACTICE
Apply your understanding
- Draw equal numbers of labelled water particles in solid, liquid and gas arrangements. Keep particle size unchanged; alter spacing and freedom of movement.
- Use this illustrative record: a dish and water fall from 210 g to 198 g. Calculate the change and state two assumptions needed to call it evaporation.
- Check: 12 g left. No spill and no other material loss are required assumptions. Explain where that matter went, and why a picture cannot establish the rate.
Check your understanding
Why is an inflated ball evidence that air occupies space?
Air pushes outward against the flexible wall and fills its interior. Being invisible does not mean being absent; the effect on the wall is observable.
Why can a solid contain moving particles without flowing?
Vibrations occur around constrained positions. Flow requires continuing rearrangement of neighbours, which ordinary solid structure restricts. Motion and unrestricted movement are different ideas.
Does a puddle disappearing prove matter was destroyed?
No. Track the surrounding air as well as the ground. The system being observed lost liquid water while another region gained water vapour.
Why can two samples at the same temperature have different total energies?
They may contain different amounts of matter or be in different states. Temperature alone does not specify sample mass or the energy associated with particle interactions.
Why does condensation on a bottle not establish a leak?
Water vapour already in surrounding air can become liquid on the cold exterior. A leak is only one possible explanation and needs separate evidence.