Overview
Why can you pile up sand but not water, and why does the smell of an incense stick fill a whole room? This chapter answers such questions with one powerful idea: all matter is made of a very large number of extremely small particles. You begin by breaking chalk and dissolving sugar, which show that a substance can be divided into tiny constituent particles too small to see even with a microscope, an idea first imagined in India by Acharya Kanad as the Parmanu. You then learn that these particles attract one another, that the attraction weakens quickly with distance, and that there are empty spaces between particles. These two ideas explain the three states of matter. In solids, closely packed particles are held in fixed positions and only vibrate; heating them to the melting point lets them move. In liquids, particles slide past each other, so a liquid keeps its volume but not its shape; heating leads to boiling, while evaporation goes on at every temperature. In gases, attractions are negligible, so a gas fills any space and is easy to compress. Diffusion of colour in water and of smells in air shows that particles are always moving, faster when hotter. The chapter ends by showing how thermal energy decides the state of matter.
हिंदी में पढ़ें
आपके आसपास की हर वस्तु, पत्थर से लेकर साँस में ली जाने वाली हवा तक, ऐसे कणों से बनी है जो इतने सूक्ष्म हैं कि दिखाई नहीं देते। इस अध्याय में आप जानेंगे कि ये कण कैसे जमे होते हैं, एक-दूसरे को कितनी प्रबलता से आकर्षित करते हैं और कैसे गति करते हैं, और इससे ठोस, द्रव और गैस की व्याख्या कैसे होती है।
Learning Objectives
- Give evidence from activities that matter is made of extremely small constituent particles
- Describe interparticle attractions and interparticle spaces and show that the spaces exist
- Explain the properties of solids, liquids and gases in terms of the spacing, attraction and movement of particles
- Define melting point and boiling point and use melting points to compare interparticle attractions
- Distinguish between boiling and evaporation
- Explain why gases are highly compressible and liquids are practically incompressible
- Explain diffusion and the effect of temperature on the movement of particles
- Use thermal energy to explain changes of state and compare the three states in a table
Topics in this chapter
7 topics · tap a topic title to jump straight to it.
Matter is made of tiny particles
1Think first
2The idea, in plain words
Pick up a pebble from a riverbank. Long ago it was part of a big rock high in the mountains. Rain, wind and flowing rivers slowly broke that rock into stones, then pebbles, then sand and fine clay. This raises a simple but deep question: if we keep breaking something into smaller and smaller pieces, is there a smallest piece?
Try it with a stick of chalk. Snap it in two, then keep breaking the pieces until your fingers cannot break them any more. Grind these bits in a mortar and pestle and look at the powder through a magnifying glass. Every tiny grain is still chalk. Grinding is a physical change: only the size of the pieces changes, not the substance.
Now imagine you could keep grinding for ever. At some stage you would reach units of chalk that cannot be broken any further and still be chalk. These basic building units are called the constituent particles of chalk. A constituent particle is the basic unit that, in very large numbers, makes up a piece of a substance. Grains of sand and clay are not the smallest units either; each grain is itself made of a huge number of constituent particles.
Sugar in water tells the same story. Put two spoons of sugar in a glass of water without stirring, and the top layer hardly tastes sweet. Stir until the sugar disappears, and now the top layer is sweet. You cannot see any sugar, yet your tongue tells you it is there. On dissolving, each sugar grain has broken up into its constituent particles, which have spread through the water. A single small grain of sugar contains millions and millions of such particles.
These particles are far too small to be seen, even with an ordinary microscope. So we use evidence such as taste, smell and changes in volume to learn about them.
The idea is very old. In ancient India, Acharya Kanad taught that matter is made of tiny, indivisible, eternal particles that he called Parmanu. His ideas are recorded in the Vaisheshika Sutras. Today we call the tiniest particles of matter atoms and molecules, which you will study in higher classes.
3Worked example
- A chalk stick ground to powder is still chalk; only the size of the pieces changed.
- Stirred sugar water tastes sweet everywhere, though no sugar can be seen.
- A grain of sand is made of a very large number of constituent particles.
4Your turn
A chalk stick is ground into a very fine powder. Which statement is correct?
5Remember this
Interparticle attractions and interparticle spaces
1Think first
2The idea, in plain words
If chalk and sugar are made of separate particles, why does a piece of chalk not simply fall apart into dust? The answer is that particles attract one another. The attractive forces between the constituent particles of a substance are called interparticle attractions (or interparticle forces of attraction).
How strong these attractions are depends on two things: the nature of the substance and the distance between the particles. The second point is important. Even a small increase in the distance between particles makes the attraction much weaker. In the end, the strength of these attractions decides whether a substance is a solid, a liquid or a gas at a given temperature.
Particles are also not packed so tightly that they touch everywhere. There are gaps between them, called interparticle spaces. Here is a simple way to see this. Half fill a glass with water and mark the level A. Add two teaspoons of sugar and mark the new level B: the level rises because the sugar takes up room. Now stir until the sugar dissolves and mark the level C. You will find that C is a little lower than B.
| Level | What is in the glass | What we notice |
| A | Water only | Starting level |
| B | Water with undissolved sugar | Level rises |
| C | Sugar fully dissolved | Level falls slightly below B |
Why does the level fall? The volume of the solution is less than the volume of the water plus the volume of the sugar. The sugar particles have slipped into the spaces between the water particles. Repeat the activity with sand and the level does not fall, because sand does not dissolve; its grains just settle and take up extra space.
What fills these interparticle spaces? Many students guess air, but that is wrong. The spaces between particles contain nothing at all.
One more word of care: in news about air pollution you may hear of Suspended Particulate Matter (SPM). Those dust particles are huge compared with constituent particles; each speck of dust is itself made of an enormous number of atoms and molecules.
- Volume of sugar solution is less than volume of water + volume of sugar
3Worked example
- Sugar stirred into water lowers the level slightly from B to C, showing spaces between water particles.
- Sand added to water raises the level and does not lower it, because sand does not dissolve.
- A piece of iron holds together because its particles attract one another strongly.
4Your turn
What is present in the spaces between the particles of a solid?
5Remember this
The solid state and melting
1Think first
2The idea, in plain words
Collect a few solids: an iron nail, a piece of rock salt, a stone, a wooden block, a key and a piece of aluminium. Each has its own definite shape and definite volume. Tap them with a hammer and most of them hardly change. Why are solids so firm?
In a solid, the particles are tightly packed and the interparticle attractions are very strong. These strong attractions hold each particle in a fixed position. The particles are not completely still: each one keeps moving to and fro about its own position. This motion is called vibration. But a particle in a solid cannot move past its neighbours. That is why a solid keeps its shape and volume, and why you can pile up stones or sand but never a heap of water.
What happens when we heat a solid? The particles gain energy and vibrate more and more strongly. At a certain temperature the vibrations become so strong that particles start leaving their fixed positions. The attractions between them become weaker, and the solid turns into a liquid. This change is called melting.
The lowest temperature at which a solid melts to form a liquid, at atmospheric pressure, is called its melting point. Solids with weak interparticle attractions melt at low temperatures; solids with strong attractions need a high temperature.
| Solid | Melting point |
| Ice | 0 °C |
| Urea | 133 °C |
| Iron | 1538 °C |
Reading the table: iron melts at a temperature more than ten times higher than urea (1538 is about 11.6 times 133), which tells us that the particles of iron are held together far more strongly.
Usually, the particles in a liquid are a little farther apart than in the solid of the same substance. Ice is a famous exception: its particles are farther apart than the particles in liquid water. This is why ice floats on water.
- Melting point: the lowest temperature at which a solid melts at atmospheric pressure
3Worked example
- An iron nail keeps its shape when hammered lightly because its particles are strongly attracted.
- Ice melts at 0 °C, so it melts on a warm day; iron needs 1538 °C.
- Ice floats because its particles are farther apart than in liquid water.
4Your turn
Which statement best describes particles in a solid?
5Remember this
The liquid state, boiling and evaporation
1Think first
2The idea, in plain words
Take three containers of different shapes, A, B and C, and mark the 200 mL level on each. Fill A with water up to the mark, pour it into B, then from B into C. Each time, the water takes the shape of the container, yet it always reaches the 200 mL mark. Its volume stays the same. (If a container is not clean, a few drops may stick to its walls and the level may look slightly lower.)
So a liquid has a fixed volume but no fixed shape. In a liquid the particles are still close together, but the interparticle attractions are slightly weaker than in a solid. The particles can move about and slide past each other, but only within a limited space; they do not fly apart.
You can feel this. Move a finger through a dish of water. The water parts easily and closes again when you take your finger out; you cannot cut a solid like that. You only push the water aside for a moment. The attractions are weak enough to let the particles move, but strong enough to keep them together. The same idea explains why water takes the shape of your cupped hands and then runs away when you open them.
When a liquid is heated, its particles move faster and faster and move apart, so the attractions between them become weaker. At a certain temperature, called the boiling point, particles escape very quickly, not only from the surface but from inside the liquid too. We see this as bubbles. The temperature at which a liquid boils and turns into vapour at atmospheric pressure is its boiling point. Water, at normal atmospheric pressure, boils at about 100 °C.
But spilled water dries up on a floor that is nowhere near 100 °C. This is because some particles at the surface always have enough energy to escape. This slow change of a liquid into vapour, at any temperature and only from the surface, is called evaporation.
| Boiling | Evaporation | |
| Temperature | Only at the boiling point | At any temperature |
| Where | Surface and inside (bubbles) | Only at the surface |
| Speed | Fast | Slow |
- Boiling point: the temperature at which a liquid boils and turns into vapour at atmospheric pressure
3Worked example
- 200 mL of water fills a tall bottle and a wide bowl to different heights but is still 200 mL.
- A finger moves through water but not through a block of wood.
- Wet clothes dry in the sun by evaporation, well below 100 °C.
4Your turn
Which property is true for a liquid?
5Remember this
The gaseous state and compressing gases
1Think first
2The idea, in plain words
Hold a glass jar upside down over a burning incense stick so that smoke collects inside, then cover it with a glass plate. Place a second empty jar upside down on top, mouth to mouth, and slide the plate out. Soon the smoke spreads to fill both jars completely. (Take care with the burning stick.) A gas does not keep a fixed volume; it spreads to fill all the space it can reach, and it takes the shape of its container.
In a gas, the particles move freely in every direction and the interparticle attractions are negligible (so small that we can ignore them). So a gas has neither a fixed shape nor a fixed volume. The smoke only helps us see this: tiny smoke specks are knocked about by the invisible moving particles of air. The same can be shown with violet iodine vapour in a closed jar, which must be handled carefully as it irritates the eyes and nose.
Liquids and gases both flow and do not keep a fixed shape, so together they are called fluids.
Gases have another special property. Take a syringe without a needle, pull the plunger out, close the nozzle with your thumb, and push the plunger in. It moves in a long way: the air inside is squeezed into a smaller volume. Let go and the plunger springs back. This shows that gas particles have a lot of empty space between them, and pressing the gas pushes the particles closer together. We say gases are highly compressible.
Fill the same syringe with water and try again. The plunger hardly moves at all. Water is practically incompressible, because its particles are already close together. Solids are even harder to squeeze.
3Worked example
- Smoke from one jar spreads into an empty jar placed on top of it.
- Air in a closed syringe can be pushed into a much smaller volume.
- Cooking gas and oxygen are stored by squeezing a lot of gas into steel cylinders.
4Your turn
Why can air in a closed syringe be compressed easily, but water cannot?
5Remember this
Particles are always moving: diffusion
1Think first
2The idea, in plain words
Drop a few grains of potassium permanganate into a glass of still water, using a spoon, never your fingers. Soon pink streaks curl out from the grains. Wait longer, and the whole glass turns an even pink, although nobody stirred it.
Why? The particles of water are always moving. They pull particles of potassium permanganate away from the grain and keep bumping into them, so these particles spread all through the water. This spreading of one substance through another on its own, because of the motion of particles, is called diffusion. Sand does not spread like this: its particles are held together so strongly that water particles cannot pull them out, so sand is insoluble in water.
Does temperature matter? Take three glasses, one with hot water, one at room temperature and one with ice-cold water, and drop one small grain into each. The colour spreads fastest in hot water, more slowly at room temperature and slowest in ice-cold water. Water particles move faster when they are hotter, so heating increases the movement of particles.
Gases show diffusion even more quickly. Light an incense stick in one corner of a room. At first only people near it smell it, but in a few minutes the fragrance reaches every corner. The moving particles of air knock the fragrance particles about until they are spread through the room. This is how the smell of perfume, of food cooking in the kitchen, or of camphor being heated reaches you.
Moving particles also help in washing. When oily clothes are washed with soap, many soap particles gather round each bit of oil. One end of a soap particle holds on to the oil and the other end mixes with water, so the oil is lifted off the cloth and rinsed away.
Diffusion is fastest in gases, slower in liquids and extremely slow in solids, because particles move most freely in gases and least in solids.
- Hotter particles move faster, so diffusion is faster
3Worked example
- A grain of potassium permanganate colours a whole glass of water pink without stirring.
- The smell of an incense stick spreads across a room.
- Colour spreads faster in hot water than in ice-cold water.
4Your turn
The smell of an incense stick reaches every corner of a room. This shows that:
5Remember this
Thermal energy and the three states compared
1Think first
2The idea, in plain words
We can now put the whole picture together. Matter is made of tiny particles held by attractive forces. How strong those forces are depends on how far apart the particles are, and that depends on how much thermal (heat) energy the particles have. So, in the end, it is the thermal energy of the particles that decides the state of a substance.
- Solid: low thermal energy. Particles stay close, attract each other strongly and only vibrate.
- At the melting point: the heat supplied is used to overcome the attractions. Particles leave their fixed positions, the spacing grows a little, and they can move about within a limited space. The solid becomes a liquid.
- Gas: particles have enough energy to overcome the attractions almost completely and move freely in all directions.
Think of a burning candle that has just been put out. The solid wax at the bottom has particles tightly packed in a regular way. The pool of melted wax around the wick has particles close together but jumbled and moving. The wax vapour that rose from the wick had particles far apart and moving fast in every direction. One substance, three states, all decided by thermal energy.
| Property | Solid | Liquid | Gas |
| Interparticle spacing | Minimum | A little more than in solids | Maximum |
| Packing | Closely packed | A little loosely packed | Particles free |
| Interparticle attraction | Maximum | Slightly weaker than in solids | Minimum (negligible) |
| Movement | Only vibrations | Within a limited space | In all the available space |
| Shape and volume | Fixed shape, fixed volume | No fixed shape, fixed volume | No fixed shape, no fixed volume |
A puzzle to test yourself: rice grains and rice flour take the shape of whatever jar they are poured into. Are they liquids? No. Each grain or speck keeps its own shape and volume; only the heap changes shape, because the many small pieces can slide past each other. They are solids.
A step further. The tiny particles that make up matter are atoms and molecules. Iron is made of iron atoms. Atoms of some elements, like hydrogen and oxygen, do not usually exist alone, so a few of them join to form a molecule: two hydrogen atoms form a hydrogen molecule. A water molecule has two hydrogen atoms and one oxygen atom.
- More thermal energy → particles farther apart → weaker attraction → solid to liquid to gas
3Worked example
- Solid wax, melted wax and wax vapour show the three arrangements of the same particles.
- Rice flour takes the shape of the jar, but each speck is still a solid.
- A water molecule is made of two hydrogen atoms and one oxygen atom.
4Your turn
In which state is the interparticle attraction the strongest?
5Remember this
Key Concepts
- Matter
- Anything that has mass and takes up space; it is made of extremely small particles.
- Constituent particle
- The basic unit that, in very large numbers, makes up a piece of a substance.
- Parmanu
- Acharya Kanad's name for the tiny, indivisible, eternal particles of matter, described in the Vaisheshika Sutras.
- Interparticle attraction
- The attractive force between the constituent particles of a substance; it weakens sharply as the distance between particles grows.
- Interparticle space
- The empty gap between particles; it contains nothing at all.
- Solid
- A state with fixed shape and fixed volume; particles closely packed, strongly attracted, only vibrating.
- Liquid
- A state with fixed volume but no fixed shape; particles close but able to move within a limited space.
- Gas
- A state with no fixed shape or volume; particles far apart with negligible attraction, moving freely.
- Vibration
- The to-and-fro movement of a particle about a fixed position, as in a solid.
- Melting point
- The lowest temperature at which a solid melts into a liquid at atmospheric pressure (ice 0 °C, urea 133 °C, iron 1538 °C).
- Boiling point
- The temperature at which a liquid boils and turns into vapour at atmospheric pressure.
- Evaporation
- The slow change of a liquid into vapour from its surface at any temperature.
- Fluid
- A substance that flows and has no fixed shape; liquids and gases are fluids.
- Compressibility
- How much a substance can be squeezed into a smaller volume; gases are highly compressible, liquids practically are not.
- Diffusion
- The spreading of one substance through another on its own, due to the motion of particles; faster when hotter.
- Thermal energy
- The heat energy of particles; it decides their spacing, attraction and hence the state of matter.
- Atom and molecule
- The tiny particles that make up matter; a molecule is formed when atoms join, as two hydrogen atoms form a hydrogen molecule.
- Suspended Particulate Matter (SPM)
- Tiny dust particles suspended in air; each is itself made of a huge number of atoms and molecules.
End-of-Chapter Trial Paper & Test Questions
Topic-wise questions to test your understanding of every concept in this chapter.
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What is a constituent particle? How does dissolving sugar in water show that matter is made of tiny particles? / घटक कण क्या है? पानी में चीनी घोलने से यह कैसे पता चलता है कि पदार्थ सूक्ष्म कणों से बना है?
Show answer
A constituent particle is the basic unit that, in very large numbers, makes up a substance. When sugar is stirred into water it can no longer be seen, yet the whole solution tastes sweet, so the sugar has broken into particles too small to see that have spread through the water. / घटक कण वह मूल इकाई है जो बहुत बड़ी संख्या में मिलकर किसी पदार्थ को बनाती है। पानी में घोलने के बाद चीनी दिखाई नहीं देती, फिर भी पूरा घोल मीठा लगता है, इसलिए चीनी ऐसे सूक्ष्म कणों में टूट गई है जो दिखाई नहीं देते और पूरे पानी में फैल गए हैं।
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Who first spoke of the Parmanu, and in which work? / परमाणु का विचार सबसे पहले किसने दिया था और किस ग्रंथ में?
Show answer
Acharya Kanad, an ancient Indian philosopher, said that matter is made of tiny, indivisible, eternal particles called Parmanu. His ideas are written in the Vaisheshika Sutras. / प्राचीन भारतीय दार्शनिक आचार्य कणाद ने कहा कि पदार्थ परमाणु नामक सूक्ष्म, अविभाज्य और शाश्वत कणों से बना है। उनके विचार वैशेषिक सूत्र में लिखे हैं।
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When sugar is dissolved in water, the water level falls slightly from where it was with undissolved sugar. Why? / चीनी के घुलने पर पानी का स्तर, बिना घुली चीनी वाले स्तर से थोड़ा नीचे आ जाता है। क्यों?
Show answer
There are empty spaces between water particles. When sugar dissolves, its particles move into these interparticle spaces, so the volume of the solution is less than the volume of water plus sugar. / पानी के कणों के बीच खाली स्थान होते हैं। चीनी घुलने पर उसके कण इन अंतराकणीय स्थानों में चले जाते हैं, इसलिए घोल का आयतन पानी और चीनी के आयतनों के योग से कम होता है।
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Why does a solid have a fixed shape and volume? What happens to its particles when it is heated to its melting point? / ठोस का आकार और आयतन निश्चित क्यों होता है? गलनांक तक गर्म करने पर उसके कणों का क्या होता है?
Show answer
In a solid the particles are closely packed and held in fixed positions by strong attractions, so they can only vibrate. On heating they vibrate more strongly, and at the melting point they leave their positions, the attractions weaken and the solid becomes a liquid. / ठोस में कण पास-पास सटे होते हैं और प्रबल आकर्षण बलों के कारण अपनी निश्चित स्थितियों में बँधे रहते हैं, इसलिए वे केवल कंपन कर पाते हैं। गर्म करने पर उनका कंपन बढ़ता है और गलनांक पर वे अपनी स्थितियाँ छोड़ देते हैं, आकर्षण कमज़ोर पड़ जाता है और ठोस द्रव बन जाता है।
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The melting points of ice, urea and iron are 0 °C, 133 °C and 1538 °C. Which has the strongest interparticle attraction and why? / बर्फ़, यूरिया और लोहे के गलनांक 0 °C, 133 °C और 1538 °C हैं। किसमें अंतराकणीय आकर्षण सबसे प्रबल है और क्यों?
Show answer
Iron has the strongest interparticle attraction, because it needs the highest temperature, 1538 °C, before its particles can leave their fixed positions. Ice has the weakest. / लोहे में अंतराकणीय आकर्षण सबसे प्रबल है, क्योंकि उसके कणों को अपनी निश्चित स्थितियाँ छोड़ने के लिए सबसे अधिक ताप, 1538 °C, चाहिए। बर्फ़ में यह आकर्षण सबसे कमज़ोर है।
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Milk spilled on a table flows and spreads, but the glass tumbler keeps its shape. Explain. / मेज़ पर गिरा दूध बहकर फैल जाता है, पर काँच का गिलास अपना आकार बनाए रखता है। समझाइए।
Show answer
Milk is a liquid: its particles are close but the attractions are weak enough for them to slide past each other, so it flows and has no fixed shape. Glass is a solid: its particles are held in fixed positions by strong attractions, so it keeps its shape. / दूध द्रव है: उसके कण पास-पास होते हैं पर आकर्षण इतना कमज़ोर होता है कि वे एक-दूसरे के पास से फिसल सकते हैं, इसलिए वह बहता है और उसका कोई निश्चित आकार नहीं होता। काँच ठोस है: उसके कण प्रबल आकर्षण से निश्चित स्थितियों में बँधे रहते हैं, इसलिए वह अपना आकार बनाए रखता है।
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Distinguish between boiling and evaporation. / क्वथन और वाष्पन में अंतर बताइए।
Show answer
Boiling happens only at the boiling point, quickly, and from the whole liquid, so bubbles form. Evaporation happens at any temperature, slowly, and only from the surface of the liquid. / क्वथन केवल क्वथनांक पर, तेज़ी से और पूरे द्रव में होता है, इसलिए बुलबुले बनते हैं। वाष्पन किसी भी ताप पर, धीरे-धीरे और केवल द्रव की सतह से होता है।
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Air in a closed syringe can be pushed into a smaller volume, but water cannot. What does this tell us? / बंद सिरिंज की हवा को कम आयतन में दबाया जा सकता है, पर पानी को नहीं। इससे क्या पता चलता है?
Show answer
Gas particles have large empty spaces between them, which can be reduced by pressing, so gases are highly compressible. Water particles are already close together, so water is practically incompressible. / गैस के कणों के बीच बड़े खाली स्थान होते हैं जिन्हें दबाकर कम किया जा सकता है, इसलिए गैसें अत्यधिक संपीड्य होती हैं। पानी के कण पहले से ही पास-पास होते हैं, इसलिए पानी लगभग असंपीड्य है।
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Why does potassium permanganate colour hot water faster than ice-cold water? / पोटैशियम परमैंगनेट गर्म पानी को बर्फ़ीले ठंडे पानी की तुलना में जल्दी रंगीन क्यों कर देता है?
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Water particles move faster in hot water. They pull out and knock the potassium permanganate particles about more quickly, so the colour spreads (diffuses) faster. In ice-cold water the particles move slowly, so diffusion is slowest. / गर्म पानी में पानी के कण तेज़ी से गति करते हैं। वे पोटैशियम परमैंगनेट के कणों को जल्दी खींचकर इधर-उधर धकेलते हैं, इसलिए रंग तेज़ी से फैलता (विसरित होता) है। बर्फ़ीले ठंडे पानी में कण धीरे चलते हैं, इसलिए विसरण सबसे धीमा होता है।
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Sea water tastes salty though no salt can be seen in it. Explain using the particle idea. / समुद्र का पानी खारा लगता है, जबकि उसमें नमक दिखाई नहीं देता। कण-सिद्धांत से समझाइए।
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The salt has dissolved: it has broken into its constituent particles, which are far too small to see and have spread into the spaces between the water particles. They are still present, so we can taste them. / नमक घुल चुका है: वह अपने घटक कणों में टूट गया है, जो इतने सूक्ष्म हैं कि दिखाई नहीं देते और पानी के कणों के बीच के स्थानों में फैल गए हैं। वे पानी में मौजूद हैं, इसलिए हमें उनका स्वाद आता है।
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