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Why can you pile up sand but not water? Why does a balloon full of air weigh more than an empty one? Chapter 7 of Class 8 science answers questions like these with one idea: everything is made of tiny particles. This post explains the chapter's key ideas, works through the sugar-in-water activity with sample numbers, lists the mistakes that cost marks, and ends with a practice routine for students and parents.
Where is this chapter in the new book?
In the current NCERT book Curiosity, Textbook of Science for Grade 8 (reprint 2026-27), Chapter 7 is "Particulate Nature of Matter". It comes just before Chapter 8, "Nature of Matter: Elements, Compounds, and Mixtures", and Chapter 9 on solutes, solvents and solutions. The Chapter 7 PDF has four main questions: what matter is made of, what decides the state of matter, how spacing differs between states, and how particles move.
What is matter made of?
The chapter starts with a stick of chalk. Break it, break it again, then grind it to powder. Each speck is still chalk. If you could keep going, you would reach units that cannot be broken further. The book calls these constituent particles: the basic units that make up a substance. They are so small they cannot be seen "even through an ordinary microscope".
Sugar shows the same thing. Stir it into water and you cannot see it, but the water tastes sweet. The sugar has broken up into its particles, which fill the interparticle spaces between water particles.
The book also mentions Acharya Kanad's idea of the Parmanu, a tiny indivisible particle, in the Vaisheshika Sutras.
What decides whether something is a solid, liquid or gas?
Particles are held together by interparticle attractions. The chapter says these depend on the substance and on the distance between particles, and that even a slight increase in distance weakens them "drastically". Its summary table compares the three states:
Solid: particles closely packed, attraction strongest, movement only vibration. Fixed shape and volume.
Liquid: particles a little further apart, attraction slightly weaker, particles move within a limited space. Fixed volume, no fixed shape.
Gas: particles far apart, attraction negligible, particles move freely in all directions. No fixed shape or volume.
Heating gives particles more thermal energy. At the melting point they leave their fixed positions. At the boiling point they escape the liquid quickly, even from inside it, which is why you see bubbles. The book's Table 7.1 gives melting points of 0 °C for ice, 133 °C for urea and 1538 °C for iron, showing how much the strength of attraction varies.
Worked example: where did the sugar go?
Activity 7.7 asks you to mark a water level, add sugar, mark again, then stir and mark a third time. Here is how to read the result. These are sample numbers to show the reasoning, not measurements from the book.
Level A, water only: 200 mL
Level B, just after adding sugar (not yet dissolved): 208 mL
Level C, after stirring until dissolved: 205 mL
Step 1. The sugar grains took up 208 − 200 = 8 mL while they sat at the bottom as solid.
Step 2. After dissolving, the total is 205 mL. If water and sugar simply added up, we would expect 208 mL. The volume is 208 − 205 = 3 mL less.
Step 3. Explain it with particles. The sugar particles moved into the spaces between water particles, so part of their volume "disappeared" into those gaps. This is the chapter's evidence that liquids have interparticle spaces.
Step 4. Compare with sand. The book says sand settles and the total volume increases, because sand does not dissolve: its particles stay bound in grains.
What other activities show particle movement?
Potassium permanganate in water (Activity 7.8). Pink streaks spread until the whole tumbler is evenly pink. The book's extension uses hot, room-temperature and ice-cold water: the colour spreads fastest in hot water, because particles move faster when heated.
Incense in a room (Activity 7.9). You smell it across the room after a few minutes because air particles keep moving and carry the fragrance particles with them.
Syringe (Activity 7.6). Air can be compressed because gas particles have large spaces between them. Water is "practically incompressible".
Which mistakes do students make?
Research shows these ideas are harder than they look. A review by de Vos and Verdonk (1996) discusses children's inclination "to attribute all kinds of macroscopic properties to particles", that is, to give tiny particles the properties of the material you can see and hold. Saying that particles melt is one example. A study of 87 middle and high school students by Aydeniz and Kotowski (2012) found misconceptions about conservation of matter, condensation and how molecules behave remained even after the topic had been taught.
Watch for these in your own answers:
Particles melt or expand. It is the spacing and movement of particles that change, not the particles themselves.
Air fills the gaps. The gaps are empty.
Evaporation only happens at 100 °C. The chapter says evaporation happens at all temperatures, slowly and only at the surface.
Rice flour is a liquid because it takes the shape of the jar. Each grain keeps its own shape. That is Question 10 in the chapter's exercises.
A practice routine
Draw it. For each state, draw a box of particles showing spacing. Then draw ice melting and water boiling, as the chapter's Question 6 asks.
Explain one everyday event a day. A wet floor drying, perfume spreading, a dented ball that recovers in sun.
Do one activity at home with an adult: sugar in water, or permanganate in hot and cold water.
Answer the chapter's "Keep the curiosity alive" questions without looking at the text, then check. Testing yourself like this is one of the two techniques rated highest in a large review of study methods by Dunlosky and colleagues (2013).
When you reach Class 9, these ideas return in more detail. Our post on Matter in Our Surroundings for Class 9 picks up from here.
If it helps to see particles moving rather than read about them, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
Try Activity 7.7 tonight with a glass of water and two spoons of sugar, and record your three levels.
