CBSE Class 8 Annual Assessment

Chapter 7: Particulate Nature of Matter

Summary

This chapter develops the idea that all matter is made of an enormous number of extremely small particles, far too small to see even under an ordinary microscope. Grinding chalk and dissolving sugar show that a substance can be broken down into constituent particles while still remaining the same substance. These particles are held together by attractive interparticle forces, and the strength of these forces, which depends on the distance between particles, decides the physical state of matter. In solids, particles are tightly packed with strong attractions, so solids have a fixed shape and volume and their particles can only vibrate. On heating, vibrations increase until the solid melts at its melting point. In liquids, attractions are slightly weaker, so liquids have a fixed volume but take the shape of their container; on further heating they boil at the boiling point and turn to vapour. In gases, attractions are negligible, so gases have neither fixed shape nor volume and spread to fill all available space. The chapter shows interparticle spacing through a syringe (gases are compressible, liquids practically not) and through dissolving sugar (the solution volume is less than the sum of the parts). Diffusion of potassium permanganate and incense smoke demonstrates that particles are in constant motion, faster when heated. Thermal energy thus governs the state of matter.

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