Which of the following are matter? Chair, air, love, smell, hate, almonds, thought, cold, cold drink, smell of perfume.
Matter: Chair, air, almonds, cold drink.
Not matter: Love, smell, hate, thought, cold, smell of perfume.
Matter is anything that has mass and occupies space. Chair, air, almonds, and cold drink have mass and occupy space, so they are matter. Love, hate, thought, cold, and smell are feelings or sensations — they do not have mass and do not occupy space, so they are not matter.
Give reasons for the following observation: The smell of hot sizzling food reaches you several metres away, but to get the smell from cold food you have to go close.
The smell reaches us due to the process of diffusion — the intermixing of particles of two different substances on their own.
Hot food has higher temperature, which means the particles of the food have higher kinetic energy. These particles move faster and diffuse quickly into the surrounding air, spreading over a large distance. This is why the smell of hot food reaches us even from several metres away.
In cold food, the temperature is low, so the kinetic energy of particles is also low. The particles move slowly and diffusion is very slow. Hence, we need to go close to cold food to get its smell.
A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
This observation shows that matter has intermolecular spaces between its particles.
The diver is able to cut through water because the particles of water have intermolecular spaces between them. Although there is an intermolecular force of attraction between water particles, it is not strong enough to prevent the diver from pushing them apart. The water particles simply move aside to let the diver through, demonstrating the existence of spaces between them.
What are the characteristics of the particles of matter?
The characteristics of particles of matter are:
(i) Particles of matter have spaces between them. The intermolecular spaces allow one substance to diffuse into another. For example, sugar dissolves completely in water because sugar particles fit into the spaces between water molecules.
(ii) Particles of matter are constantly moving. All particles are in a state of continuous random motion. The kinetic energy of particles increases with an increase in temperature.
(iii) Particles of matter attract each other. There is a force of attraction between particles of matter, called the intermolecular force. This force varies in solids, liquids, and gases.
The mass per unit volume of a substance is called density (density = mass/volume). Arrange the following in order of increasing density: air, exhaust from chimneys, honey, water, chalk, cotton, and iron.
In order of increasing density (from least dense to most dense):
Air < Exhaust from chimneys < Cotton < Water < Honey < Chalk < Iron
Gases (air, exhaust) have the lowest density as their particles are far apart. Cotton is a light solid with many air pockets. Water is a common reference liquid (density = 1 g/cm³). Honey is denser than water. Chalk and iron are dense solids, with iron being the densest among these substances.
(a) Tabulate the differences in the characteristics of states of matter.
(b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy, and density.
(a) Differences in characteristics of states of matter:
Solid: Definite shape, definite volume, incompressible, rigid, low kinetic energy, high density, strong intermolecular forces.
Liquid: No definite shape (takes shape of container), definite volume, slightly compressible, fluid (can flow), moderate kinetic energy, moderate density, moderate intermolecular forces.
Gas: No definite shape, no definite volume (fills any container), highly compressible, fluid, high kinetic energy, very low density, very weak intermolecular forces.
(b) Comments:
Rigidity: Solids are rigid because their particles are held tightly by strong intermolecular forces and can only vibrate about fixed positions.
Compressibility: Gases are most compressible because there are very large intermolecular spaces. Solids are least compressible because particles are already very close.
Fluidity: Liquids and gases can flow because their particles can move past each other. Solids cannot flow due to rigid structure.
Filling a gas container: Gases expand to completely fill any container because gas particles have very high kinetic energy and negligible intermolecular attraction, allowing them to spread out in all directions.
Shape: Solids have a definite, fixed shape. Liquids take the shape of the container they are kept in. Gases fill the entire container.
Kinetic energy: Kinetic energy of particles increases from solids to liquids to gases. Gas particles have the highest kinetic energy.
Density: Generally, solids > liquids > gases in terms of density, because particles are most closely packed in solids and most widely spread in gases.
Give reasons: (a) A gas fills completely the vessel in which it is kept. (b) A gas exerts pressure on the walls of the container. (c) A wooden table should be called a solid. (d) We can easily move our hand in air but to do the same through a solid block of wood we need a karate expert.
(a) Gas particles have very high kinetic energy and extremely large intermolecular spaces with negligible intermolecular forces. The particles move rapidly in all directions with no fixed positions, so they spread out and fill the entire vessel completely.
(b) Gas particles are in constant random motion. They frequently collide with the walls of the container with a certain force. This force per unit area exerted on the walls constitutes the pressure that a gas exerts on the walls of its container.
(c) A wooden table has all the characteristics of a solid: it has a definite shape and definite volume, it is rigid and incompressible, it cannot flow, and its particles are closely packed with strong intermolecular forces. Therefore, a wooden table is correctly called a solid.
(d) Air particles have very large intermolecular spaces and very weak intermolecular forces. Our hand can easily displace air particles as they offer very little resistance. Wood is a solid with tightly packed particles and very strong intermolecular forces. To break through wood, one needs to overcome these strong forces, requiring the kind of force a karate expert can deliver.
Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.
Ice floats on water because ice is less dense than liquid water, which is an exceptional property of water.
When water freezes to form ice, the water molecules (H2O) arrange themselves into a special open, cage-like crystalline lattice structure held together by hydrogen bonds. This hexagonal arrangement has more empty space between the molecules compared to liquid water, where the molecules are more randomly and closely packed.
Because ice has more empty space, its volume is greater for the same mass, making its density lower (~0.917 g/cm³) than liquid water (1 g/cm³). Since ice is less dense, it floats on liquid water. This property is vital for aquatic life — floating ice insulates the liquid water below, preventing lakes from freezing solid in winter.
Convert the following temperatures to the Celsius scale: (a) 300 K (b) 573 K
Conversion formula: °C = K − 273
(a) 300 K − 273 = 27°C
(b) 573 K − 273 = 300°C
Convert the following temperatures to the Kelvin scale: (a) 25°C (b) 373°C
Conversion formula: K = °C + 273
(a) 25 + 273 = 298 K
(b) 373 + 273 = 646 K
Give reasons for the following observations: (a) Naphthalene balls disappear with time without leaving any solid. (b) We can get the smell of perfume sitting several metres away.
(a) Naphthalene is a substance that undergoes sublimation — it changes directly from the solid state to the gaseous state without passing through the liquid state. At room temperature, naphthalene slowly sublimes, converting into vapour. Since no liquid is formed, no solid residue is left behind. This is why naphthalene balls disappear over time without leaving any solid.
(b) Perfume contains highly volatile aromatic substances. When perfume is sprayed or applied, its particles mix with air and spread in all directions by the process of diffusion. The particles of perfume have sufficient kinetic energy at room temperature to move through the air and reach our nose even from several metres away.
Name the following: (a) Change in state from solid to liquid. (b) Change in state from liquid to gas. (c) Change in state from solid directly to gas.
(a) Solid → Liquid: Melting (also called Fusion). The temperature at which this occurs is called the melting point.
(b) Liquid → Gas: Vaporization (includes both Boiling and Evaporation). The temperature at which liquid boils is called the boiling point.
(c) Solid → Gas (directly): Sublimation. Examples: naphthalene, dry ice (solid CO2), camphor.
Why does a desert cooler cool better on a hot dry day?
A desert cooler works on the principle of evaporative cooling. As water evaporates from the wet pads of the cooler, it absorbs latent heat from the surroundings, lowering the temperature of the air blown into the room.
On a hot dry day, the humidity (amount of moisture in the air) is very low. Dry air can absorb much more water vapour compared to humid air. This means the rate of evaporation from the wet pads is much higher on a dry day. Higher rate of evaporation means more heat is absorbed from the surroundings, resulting in greater cooling. Hence, a desert cooler cools better on a hot, dry day than on a humid day.
How does water vapour present in air change into rain?
The water cycle explains how water vapour changes into rain:
1. Evaporation: Water from oceans, rivers, lakes, and other water bodies evaporates due to heat from the sun. This water vapour (H2O in gaseous state) rises into the atmosphere.
2. Rising and Cooling: As the water vapour rises to higher altitudes, the temperature decreases significantly.
3. Condensation: When the water vapour cools below its dew point, it condenses around tiny dust particles, forming tiny water droplets. Large collections of these droplets form clouds.
4. Precipitation (Rain): When enough water droplets accumulate and the droplets grow heavy enough, they fall down as rain (precipitation) due to gravity.
Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Acetone, petrol, and perfume are all highly volatile substances — they evaporate very quickly at room temperature.
When these substances are applied to the palm, they evaporate rapidly. The process of evaporation requires energy in the form of latent heat of vaporization. This energy is absorbed from the skin of the palm (the surroundings). The loss of heat from the skin causes the palm to feel cold.
This is an example of evaporation causing cooling. The faster the evaporation, the greater the cooling effect.
Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
A saucer has a much larger surface area compared to a cup, even though they hold a similar quantity of liquid.
The rate of evaporation is directly proportional to the surface area exposed to the air. Since the saucer has a larger surface area, more liquid surface is exposed to the atmosphere. This allows faster evaporation from the saucer. Evaporation absorbs heat from the liquid (latent heat), causing the tea or milk to cool down much faster in a saucer than in a cup. Therefore, we can sip hot tea or milk faster from a saucer.
What type of clothes should we wear in summer?
We should wear light-coloured cotton clothes in summer, for the following reasons:
Cotton: Cotton fibres have a porous, absorbing structure. They absorb sweat from the body and expose it to the air, allowing faster evaporation of sweat. This evaporation absorbs heat from the body and keeps us cool and comfortable.
Light colours: Light-coloured (white, pale) clothes reflect most of the sunlight and heat that falls on them, absorbing less heat compared to dark-coloured clothes. This keeps the body cooler.
Synthetic or dark-coloured clothes trap heat and inhibit evaporation of sweat, making us feel hot and uncomfortable in summer.
Sponge has holes in it and can be compressed. Is it a solid?
Yes, sponge is a solid.
A sponge has a definite shape and a definite volume under normal conditions, which are key properties of a solid. The sponge appears to be compressible because it has tiny pores (holes) filled with air. When we compress a sponge, the air is squeezed out of these pores, which reduces its volume temporarily. However, the basic solid framework (structure) of the sponge remains intact and it springs back to its original shape once the pressure is released.
This is similar to how rubber or foam behaves. The solid nature of the sponge material itself is not changed by the presence of air pockets. Therefore, sponge is correctly classified as a solid.
What is sublimation? Give two examples.
Sublimation is the process by which a solid substance directly converts into the gaseous state upon heating, without passing through the intermediate liquid state. The reverse process — where a gas directly converts to a solid — is called deposition (or reverse sublimation).
Examples of sublimation:
(i) Naphthalene balls (camphor/mothballs): When kept in open air, naphthalene slowly sublimes and the balls gradually disappear without leaving any liquid residue.
(ii) Dry ice (solid CO2): Solid carbon dioxide sublimes directly into gaseous CO2 at atmospheric pressure and room temperature, without forming liquid carbon dioxide.
Distinguish between evaporation and boiling.
Evaporation:
1. Occurs at all temperatures (below boiling point).
2. Occurs only at the surface of the liquid.
3. No bubble formation.
4. It is a slow, gradual process.
5. Causes cooling of the surroundings (absorbs latent heat from the surroundings).
6. Rate depends on temperature, surface area, humidity, and wind speed.
Boiling:
1. Occurs at a fixed temperature called the boiling point.
2. Occurs throughout the bulk of the liquid.
3. Bubbles are formed throughout the liquid.
4. It is a rapid, vigorous process.
5. Does not cause cooling of surroundings (heat is supplied to the liquid).
6. Boiling point depends on atmospheric pressure.
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