Class 8 Science · Chapter 7 NotesParticulate Nature of Matter

Matter is anything that has mass and occupies space, and the chapter shows that every piece of matter is built from extremely small units called constituent particles. When you break a chalk stick again and again and finally grind it into powder, each speck is still chalk — grinding is only a physical change in which the size of the pieces becomes smaller. If this breaking could continue, a stage would come when the chalk cannot be broken further; the tiny units obtained at that stage are the basic building blocks of chalk. The same is true of rocks that break into pebbles, stones, sand and clay. These particles are so small that they cannot be seen even through an ordinary microscope, yet a single grain of sugar contains millions and millions of them.

• Constituent particlethe basic unit that makes up a larger piece of a substance or material.
• Grinding chalk is a physical change — no new substance is formed.
• Matter is made of a very large number of extremely small particles.

The particles of matter are not glued tightly with no gaps; there are spaces between them called interparticle spaces. When sugar is dissolved in water, it seems to disappear, but the water tastes sweet throughout. This happens because the sugar breaks into its constituent particles, which separate and occupy the spaces between the water particles. The particles are also held together by forces that are attractive in nature, called interparticle attractions. The strength of these attractions depends on the nature of the substance and on the distance between the particles; even a slight increase in distance decreases the forces drastically. It is this strength that ultimately decides the physical state of a substance.

• Interparticle spacethe empty space between constituent particles.
• Interparticle attractionthe attractive force that holds particles together.
• Sugar dissolves because its particles fit into the spaces between water particles.

In solids such as an iron nail, rock salt, a stone, wood, a key or a piece of aluminium, the particles are tightly packed and the interparticle attractions are very strong. This holds the particles in fixed positions, so solids have a definite shape and a definite volume. The particles cannot move past one another; they can only vibrate or oscillate about their positions. When a solid is heated, its particles vibrate more vigorously, and at a certain stage they begin to leave their fixed positions, the attractions weaken and the solid changes into a liquid. The minimum temperature at which a solid melts to become a liquid at atmospheric pressure is called its melting point. Solids with weak attractions melt at low temperatures and those with strong attractions melt at high temperatures.

• Melting pointminimum temperature at which a solid becomes a liquid at atmospheric pressure.
• Melting pointsice 0 °C, urea 133 °C, iron 1538 °C.
• Ice is an exception — its particles are farther apart than those in water.

When water is poured from one container to another of a different shape, it takes the shape of the new container but its level stays at 200 mL, showing that liquids have no fixed shape but have a definite volume. This is because liquid particles can move freely, but only within a limited space, and the interparticle attractions are slightly weaker than in solids yet strong enough to keep the particles close together. You can move a finger through water without breaking it permanently, which is not possible with a solid. On heating a liquid, a stage comes when it boils: the temperature at which a liquid boils and turns into vapour at atmospheric pressure is its boiling point. At the boiling point, bubbles of vapour form within the liquid as well as at the surface. Vapour also forms slowly at the surface at all temperatures, even below the boiling point, and this slower process is called evaporation.

• Boiling pointtemperature at which a liquid boils and turns into vapour at atmospheric pressure.
• At boiling point, vapour forms throughout the liquid as bubbles.
• Evaporationslow vapour formation at the surface at all temperatures.

Gases behave very differently from solids and liquids. In the smoke-jar activity, smoke trapped in one gas jar spreads and fills the second jar completely once the glass plate is removed, showing that gases have no fixed volume and occupy all the space available to them. Like liquids, gases also take the shape of the vessel they are in. This happens because gas particles move freely in all directions and the interparticle attractions between them are negligible. The same spreading can be shown with iodine vapour obtained by keeping solid iodine in a closed gas jar. Since both liquids and gases flow and do not keep a fixed shape, they are together called fluids.

• Gases have neither a fixed shape nor a fixed volume.
• Interparticle attraction in gases is negligible.
• Fluidsliquids and gases, which flow and do not retain a fixed shape.

The spacing between particles explains many everyday observations. When air is trapped in a syringe and the plunger is pushed, the volume of air decreases because gas particles have a lot of space between them that can be reduced by pressure; when the push stops, the particles spread out again and the plunger moves back. Water, in contrast, is practically incompressible. When sugar is stirred into water, the level first rises and then falls somewhat after dissolving, because the sugar particles occupy the spaces between the water particles. Sand does not dissolve; its particles settle and add to the total volume. In solids the spacing is minimum, in liquids it is a little more, and in gases it is maximum. The spaces between particles contain nothing at all — not air.

• Gases are highly compressible; liquids are practically incompressible.
• Solidsminimum interparticle spacing; liquids: little more; gases: maximum.
• Sand is insoluble because water particles cannot pull its strongly held particles apart.

Particles of matter are in constant motion. When a few grains of potassium permanganate are dropped into water, pink streaks spread from the grains and soon the whole water becomes uniformly pink. The moving water particles pull out the permanganate particles and spread them through the liquid. In hot water the colour spreads fastest, in water at room temperature less quickly, and in ice-cold water most slowly, showing that the movement of particles increases when heat is provided. Similarly, the fragrance of a burning incense stick first stays near the stick and then fills the whole room because air particles hit the fragrance particles and carry them everywhere. The thermal (heat) energy of particles therefore decides the state of matter: low in solids, higher in liquids and highest in gases.

• Particles are in constant motion; heating increases their movement.
• In solids, particles only vibrate; in liquids they move within limited space; in gases they move freely.
• Thermal energy of particles determines the physical state of matter.

The tiny particles that make up all matter are atoms and molecules. Iron is made up of atoms of iron and gold of atoms of gold. Atoms of some elements, such as hydrogen, oxygen and sulfur, cannot exist independently; a certain number of atoms of the same element combine to form a molecule. For example, two hydrogen atoms combine to form a molecule of hydrogen, and a water molecule is made up of two hydrogen atoms and one oxygen atom. Long before modern chemistry, the ancient Indian philosopher Acharya Kanad spoke of the Parmanu — tiny, indivisible, eternal particles of which matter is made — in his work the Vaisheshika Sutras. The chapter also notes that the word particle is used loosely in daily life: Suspended Particulate Matter (SPM) refers to dust particles in air, which are themselves made of countless atoms and molecules.

• Atomthe basic particle of an element, such as iron or gold.
• Moleculea stable particle formed when atoms combine, e.g., hydrogen and water.
• Acharya Kanad proposed the idea of Parmanu in the Vaisheshika Sutras.