Particulate Nature of Matter Class 8: Particles, Interparticle Forces and the Three States

What Particulate Nature of Matter is about

Key concepts

Matter is made of tiny particles

NCERT section 7.1 What Is Matter Composed of?

Activity 7.1: break a stick of chalk again and again, then grind it to powder with a mortar and pestle. Each speck seen under a magnifying glass is still chalk: grinding is a physical change that only makes pieces smaller. If we could keep going, we would reach units that cannot be broken further: the constituent particles of chalk, the basic units that make up a larger piece of a substance. Grains of sand and clay are not the smallest units of rock either.

Activity 7.2: add sugar to water without stirring: the top layer is not sweet. After stirring until it dissolves, the top layer tastes sweet although no sugar can be seen. Sugar has broken up into its constituent particles, which spread through the water. Each sugar grain contains millions upon millions of particles.

These particles are so small that even an ordinary microscope cannot show them.

Our scientific heritage: Acharya Kanad, in the *Vaisheshika Sutras*, proposed that matter is made of tiny, indivisible, eternal particles called *parmanu*.

(Higher grades: the particles are atoms and molecules; a water molecule has two hydrogen atoms and one oxygen atom.)

Worked example

A few drops of rose water are added to a jug of drinking water and stirred. No drops can be seen afterwards, yet every glass poured from the jug smells of roses. Explain using the particle idea.

Try it first, then show the solutionSolution

1. The rose water breaks up into its constituent particles, which mix with the water.
2. These particles are far too small to see, and they spread throughout the jug, into the spaces between water particles.
3. They are still present in every part of the water, so every glass smells of roses.

Answer: The scent particles are spread invisibly through all the water but are still there.

Interparticle attractions and spaces

NCERT section 7.2 What Decides Different States of Matter?

Particles are held together by attractive forces called interparticle attractions.

• Their strength depends on the nature of the substance and the distance between particles.
• Even a small increase in distance weakens the attraction greatly.
• The strength of these attractions decides the physical state of a substance.

The gaps between particles are interparticle spaces. Even in solids, where particles are closely packed, there is some space between them. These spaces contain nothing at all, not air.

When sugar dissolves, its particles move into the spaces between water particles.

Worked example

What happens to the attraction between particles when a substance is heated and its particles move apart? Name the change this can lead to in a solid.

Try it first, then show the solutionSolution

1. Heating makes particles vibrate or move more and the distance between them increases.
2. Even a small increase in distance weakens interparticle attraction a lot.
3. In a solid this can lead to melting (solid to liquid).

Answer: The attraction weakens sharply; a solid may melt.

Solids

NCERT section 7.2.1 Solid state

Activity 7.3: iron nail, rock salt, stone, wood, key, aluminium: all keep their own shape and size.

In solids:

• particles are tightly packed with very small interparticle spaces;
• interparticle attractions are very strong;
• particles stay in fixed positions and can only vibrate about them, not move past each other.

So solids have a definite shape and definite volume.

Melting: on heating, the particles vibrate more and more until they leave their fixed positions; the attractions weaken and the solid becomes a liquid. The lowest temperature at which a solid melts at atmospheric pressure is its melting point.

Solids with weak attractions melt at low temperatures; those with strong attractions at high temperatures. Book examples: ice 0 °C, urea 133 °C, iron 1538 °C.

Usually particles are a little farther apart in a liquid than in its solid; ice is an exception: its particles are farther apart than in water.

Worked example

Iron melts at 1538 °C and urea at 133 °C. Which has stronger interparticle attractions? By how many degrees do their melting points differ?

Try it first, then show the solutionSolution

1. A higher melting point means more energy is needed to overcome the attractions.
2. So iron has much stronger interparticle attractions than urea.
3. Difference $=1538-133=1405$ °C.

Answer: Iron; the melting points differ by 1,405 °C.

Liquids

NCERT section 7.2.2 Liquid state

Activity 7.4: 200 mL of water poured from container A into B and then C (different shapes) takes each container's shape, but the level stays at 200 mL (if the containers are clean and none sticks to the walls). So liquids have no fixed shape but a fixed volume.

In liquids:

• particles can move past each other, but only within a limited space;
• the attraction between particles is a little less than in a solid, yet it still holds them close together;
• you can move a finger through water, and the water closes up again behind it.

Boiling: on heating, particles move more vigorously, move apart and the attractions weaken until particles escape as vapour. The temperature at which a liquid boils at atmospheric pressure is its boiling point. At boiling, vapour forms fast, throughout the liquid, as bubbles.

Evaporation: vapour forms slowly at all temperatures, only at the surface: that is why spilled water dries up even when it is not hot.

Worked example

Give two differences between boiling and evaporation.

Try it first, then show the solutionSolution

1. Boiling happens at a fixed temperature (the boiling point); evaporation happens at all temperatures.
2. Boiling is fast and happens throughout the liquid (bubbles); evaporation is slow and only at the surface.

Answer: Temperature: fixed versus any; place and speed: throughout and fast versus surface and slow.

Gases and fluids

NCERT section 7.2.3 Gaseous state

Activity 7.5: smoke from an incense stick is trapped in gas jar A, covered with a glass plate; gas jar B is placed upside down on top and the plate removed. The smoke spreads to fill both jars. (Iodine vapour from solid iodine in a closed jar spreads the same way.)

In gases:

• particles move freely in all directions;
• interparticle attractions are negligible;
• interparticle spaces are largest.

So gases have no fixed shape and no fixed volume; they fill whatever space is available.

The visible smoke particles are pushed about by the invisible gas particles of air hitting them, which lets us 'see' the motion of gas particles.

Liquids and gases both flow and do not keep a fixed shape, so together they are called fluids.

Note: grains of rice or flour take the shape of a jar, but each grain keeps its own shape: they are solids made of many small pieces, not liquids.

Worked example

Why does the smoke in Activity 7.5 spread into the upper gas jar as well, and what does this show about gases?

Try it first, then show the solutionSolution

1. Gas particles move freely in all directions and their attractions are negligible.
2. Air particles keep hitting the smoke particles, carrying them into all available space.
3. So gases have no fixed volume and fill the whole space available.

Answer: Gas particles move freely; gases fill all the space available.

Interparticle spacing in the three states

NCERT section 7.3 How Does the Interparticle Spacing Differ in the Three States of Matter?

Activity 7.6, syringe: with the nozzle closed by your thumb, the plunger can be pushed in a long way when the syringe holds air: gas particles have large spaces that pressure can reduce (they spring back when released). With water, the plunger hardly moves: liquids are practically incompressible.

Activity 7.7, sugar in water: mark the water level (A), add two spoons of sugar (level rises to B), then stir until it dissolves (level C). The level after dissolving is a little lower than B: the volume of the solution is less than the volumes of water and sugar added together, because sugar particles fit into spaces between water particles. Insoluble sand just settles and the volume increases by its own volume.

Summary of spacing: solids least, liquids a little more, gases most. Attraction: solids strongest, liquids slightly weaker, gases negligible. Movement: solids only vibrate, liquids move within a limited space, gases move freely everywhere.

Note: in air pollution, 'particulate matter' means tiny dust specks, which are themselves made of huge numbers of constituent particles.

Worked example

200 mL of water rises to 210 mL when sugar is added. After stirring until the sugar dissolves, the level is 205 mL. Explain the drop of 5 mL. What would happen with sand instead?

Try it first, then show the solutionSolution

1. Dissolved sugar particles move into the spaces between water particles.
2. So the solution takes up less volume than water and sugar separately: the level drops from 210 mL to 205 mL.
3. Sand does not dissolve; its grains settle and the level stays raised by the sand's volume.

Answer: Sugar particles fill spaces between water particles; sand would not reduce the level.

Particles are always moving (diffusion)

NCERT section 7.4 How Particles Move in Different States of Matter?

Activity 7.8: a few grains of potassium permanganate in water send out pink streaks; after some time the whole water is uniformly pink. Water particles are in constant motion: they pull particles out of the grain and knock them through the liquid. In substances like sand, particles are held so strongly that water cannot pull them out, so sand is insoluble.

Temperature: the pink colour spreads fastest in hot water, slower at room temperature and slowest in ice-cold water, because particles move faster when heated.

Activity 7.9: incense burning in one corner of a room is soon smelt everywhere: air particles keep moving and hitting the fragrance particles, spreading them through the room (the same reason we smell perfume from a distance).

Soap and oil: soap particles surround oil particles on cloth; one end attaches to oil and the other mixes with water, lifting the oil away.

Thermal energy decides the state: low energy keeps particles close with strong attractions (solid); at the melting point energy overcomes some attraction and particles move within a limited space (liquid); with enough energy particles overcome attractions and move freely (gas).

Worked example

A crystal of potassium permanganate colours hot water evenly in 2 minutes, room-temperature water in 6 minutes and ice-cold water in 15 minutes. Explain the pattern. How many times faster is the hot water than the ice-cold water?

Try it first, then show the solutionSolution

1. Particles move faster at higher temperature, so they spread the colour faster.
2. Hot water is fastest, ice-cold water slowest.
3. $15÷2=7.5$, so the hot water is 7.5 times faster here.

Answer: Faster particle motion at higher temperature; 7.5 times faster.

Science in real life: Particulate Nature of Matter

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Constituent Particles, Interparticle Spaces and Motion of Particles
• Solids, Liquids and Gases: Interparticle Attraction and Change of State

Particulate Nature of Matter: formulas and key facts

Constituent particle

$\text{the basic unit that makes up a larger piece of a substance}$

Attraction and distance

$\text{distance between particles}\uparrow \Rightarrow \text{attraction}\downarrow \text{ (sharply)}$

Shape and volume

$\text{solid: fixed shape, fixed volume};\text{liquid: fixed volume only};\text{gas: neither}$

Spacing order

$\text{solid}<\text{liquid}<\text{gas}$

Attraction order

$\text{solid}>\text{liquid}>\text{gas (negligible)}$

Change of state on heating

$\text{solid}\stackrel{\text{melting point}}{\to }\text{liquid}\stackrel{\text{boiling point}}{\to }\text{gas}$

Melting points (book)

$\text{ice }0{}^{\circ }\text{C},\text{urea }133{}^{\circ }\text{C},\text{iron }1538{}^{\circ }\text{C}$

Particulate Nature of Matter: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Particulate Nature of Matter

The question patterns to expect, and the method that works for each.

Tricks to remember Particulate Nature of Matter

Short memory aids. Say each one aloud once, then use it on a question the same day.

• SLG: Stuck, Loose, Gone

  Solid particles are stuck in place, liquid particles are loose, gas particles are gone everywhere.

  Use it when: Comparing the states.

• Heat shakes particles apart

  More heat, more shaking, more distance, less attraction.

  Use it when: Melting and boiling.

• Sugar hides in the gaps

  Dissolving sugar slips into the spaces between water particles, so the level drops a little.

  Use it when: Activity 7.7.

• Hot water hurries

  Colour spreads fastest in hot water because hot particles hurry.

  Use it when: Effect of temperature on motion.

• Boil all over, evaporate on top

  Boiling: everywhere, at one temperature. Evaporation: surface only, any temperature.

  Use it when: Boiling versus evaporation.

Particulate Nature of Matter: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Particulate Nature of Matter

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying the spaces between particles contain air

  Gaps are imagined as filled with air.

  Wrong working

  There is air between the particles of iron.

  Right working

  Interparticle spaces contain nothing at all.

  ✓Do this: Write 'empty space' for interparticle spaces.

  Usually costs: 1 mark.

• ✗Mistake: Saying particles in solids do not move at all

  Fixed positions are confused with no motion.

  Wrong working

  Particles of a solid are completely still.

  Right working

  They vibrate about fixed positions but cannot move past each other.

  ✓Do this: Use the word 'vibrate'.

  Usually costs: 1 mark.

• ✗Mistake: Confusing boiling and evaporation

  Both turn liquid into vapour.

  Wrong working

  Evaporation happens only at the boiling point.

  Right working

  Evaporation happens at all temperatures at the surface; boiling happens at the boiling point throughout the liquid.

  ✓Do this: Compare temperature, place and speed.

  Usually costs: 1-2 marks.

• ✗Mistake: Calling rice grains a liquid

  They take the container's shape.

  Wrong working

  Rice flows and takes the jar's shape, so it is a liquid.

  Right working

  Each grain has a fixed shape and volume; the heap takes the jar's shape only because it is made of many small solid pieces.

  ✓Do this: Check whether each individual piece keeps its shape.

  Usually costs: 1-2 marks.

• ✗Mistake: Drawing gas particles smaller than solid particles

  Students show 'thinness' by size.

  Wrong working

  Tiny dots for gas, big circles for solid.

  Right working

  Particles are the same size; only spacing and arrangement differ.

  ✓Do this: Keep particle size the same in all states.

  Usually costs: 1 mark in diagrams.

NCERT exercise guide: Particulate Nature of Matter

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Particulate Nature of Matter

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   In which state are interparticle attractions negligible?
   (a) Solid
   (b) Liquid
   (c) Gas
   (d) None

   Hint

   Particles move freely.

   Answer

   (c) Gas

2. Q2 · 1 mark · MCQExam level

   The melting point of ice is
   (a) 0 °C
   (b) 100 °C
   (c) 133 °C
   (d) −10 °C

   Hint

   Book table.

   Answer

   (a) 0 °C

3. Q3 · 2 marks · Very short answerExam level

   What are fluids? Give two examples.

   Hint

   They flow.

   Answer

   Substances that flow and have no fixed shape: liquids and gases, such as water and air.

4. Q4 · 1 mark · Very short answerExam level

   Name the ancient Indian thinker who proposed the idea of parmanu and the work in which it appears.

   Hint

   Vaisheshika.

   Answer

   Acharya Kanad; the Vaisheshika Sutras.

5. Q5 · 2 marks · Very short answerHarder exam level

   Why can we move a finger through water but not through a block of wood?

   Hint

   Strength of attraction.

   Answer

   Water's particles can move past each other because attractions are a little weaker, so they move aside and close back; in wood the particles are held in fixed positions by strong attractions.

6. Q6 · 2 marks · Very short answerHarder exam level

   Why does sand not dissolve in water while sugar does?

   Hint

   Pulling particles out.

   Answer

   Moving water particles can pull sugar particles out of the grains; in sand the particles are held so strongly that water cannot pull them out.

7. Q7 · 3 marks · Short answerHarder exam level

   Draw (describe) the arrangement of particles in a copper coin, in coconut oil, and in the air inside a football.

   Hint

   Solid, liquid, gas.

   Answer

   Copper coin (solid): same-size circles packed in neat rows. Coconut oil (liquid): circles close together but irregular, with small gaps. Air in the football (gas): a few circles far apart, with arrows showing free motion in all directions.

8. Q8 · 2 marks · Very short answerHarder exam level

   In Activity 7.4, why might the water level in container C be slightly below 200 mL?

   Hint

   Clean containers.

   Answer

   If a container is not clean and dry, a little water sticks to its walls when pouring, so slightly less reaches the next container.

9. Q9 · 2 marks · Very short answerHarder exam level

   Explain why the fragrance of an incense stick spreads through a room.

   Hint

   Air particles move.

   Answer

   Air particles are always moving; they hit the fragrance particles and spread them throughout the room.

10. Q10 · 5 marks · Long answerHarder exam level

    Compare solids, liquids and gases under five headings: shape, volume, interparticle spacing, interparticle attraction, and movement of particles.

    Hint

    Use the wrap-up table.

    Answer

    Shape: fixed, not fixed, not fixed. Volume: fixed, fixed, not fixed. Spacing: minimum, a little more, maximum. Attraction: maximum, slightly weaker, negligible. Movement: only vibration, within a limited space, free in all available space.

11. Q11 · 3 marks · Short answerHOTS / competency

    Water poured on a slope runs downhill, but a stone placed on the slope stays where it is. Explain using particles.

    Hint

    Liquid versus solid.

    Answer

    Water is a liquid: its particles can move past each other, so it flows downhill. The stone is a solid: its particles are held in fixed positions by strong attractions, so it keeps its shape and does not flow (it may roll as a whole, but its parts stay together).

12. Q12 · 3 marks · Short answerHOTS / competency

    A balloon is fixed over the neck of an empty bottle and the bottle is placed in hot water. Predict what happens and explain using particles.

    Hint

    Air particles speed up.

    Answer

    The balloon inflates a little. Heating makes the air particles inside move faster and spread out more, so the air takes up more space and pushes into the balloon.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What is a constituent particle?Show answer

   The basic unit that makes up a larger piece of a substance.

2. 2.Who proposed the idea of parmanu?Show answer

   Acharya Kanad, in the Vaisheshika Sutras.

3. 3.What are interparticle attractions?Show answer

   The attractive forces that hold the particles of matter together.

4. 4.What is in the spaces between particles?Show answer

   Nothing at all; they are empty.

5. 5.Define melting point.Show answer

   The lowest temperature at which a solid melts to a liquid at atmospheric pressure.

6. 6.Define boiling point.Show answer

   The fixed temperature, at normal atmospheric pressure, at which a liquid changes into vapour throughout its bulk.

7. 7.Which state has negligible interparticle attraction?Show answer

   Gas.

8. 8.What are fluids?Show answer

   Liquids and gases, because they flow and have no fixed shape.

9. 9.Why does potassium permanganate colour water faster when hot?Show answer

   Water particles move faster at higher temperature.

10. 10.Which common solid has its particles farther apart than in its liquid?Show answer

    Ice (compared with water).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you explain how chalk and sugar activities show particles?
• ☐Can you state what decides the strength of interparticle attractions?
• ☐Can you describe particle arrangement, spacing and motion in each state?
• ☐Can you define melting point and boiling point with examples?
• ☐Can you distinguish boiling and evaporation?
• ☐Can you explain why gases fill any container?
• ☐Can you explain the syringe activity?
• ☐Can you explain why the water level drops after sugar dissolves?
• ☐Can you explain why colour spreads faster in hot water?
• ☐Can you draw particles in a solid, a liquid and a gas?

Particulate Nature of Matter: frequently asked questions