Basic Concepts of Matter
1. Which of the following are matter?
Chair, air, love, smell, hate, almonds, thought, cold, lemon water, smell of perfume.
Matter: Chair, air, almonds, lemon water, smell of perfume
Not matter: Love, hate, thought, cold (these are emotions, sensations or ideas)
Note: Smell itself is not matter, but the particles that cause smell are matter.
2. 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.
This happens because:
- At higher temperatures, particles of matter have more kinetic energy and move faster.
- In hot food, the particles evaporate more quickly and diffuse through air at a faster rate.
- In cold food, particles have less kinetic energy, so they diffuse slowly and don't travel far.
- This demonstrates that the rate of diffusion increases with temperature.
3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
This shows that particles of matter have space between them. The diver is able to move through water because the water molecules can be pushed aside, making space for the diver to pass through. This demonstrates the property of intermolecular spaces in liquids.
4. What are the characteristics of the particles of matter?
The main characteristics of particles of matter are:
- Particles are very small: They are tiny and cannot be seen with naked eyes.
- Particles have spaces between them: There are intermolecular spaces between particles.
- Particles are continuously moving: They possess kinetic energy and are in constant motion.
- Particles attract each other: There are intermolecular forces of attraction between particles.
1. The mass per unit volume of a substance is called density. Arrange the following in order of increasing density – air, exhaust from chimneys, honey, water, chalk, cotton and iron.
Increasing order of density:
Air < Exhaust from chimneys < Cotton < Water < Honey < Chalk < Iron
Explanation:
- Air and exhaust are gases with very low density
- Cotton is a solid but very light and porous
- Water has density of 1 g/cm³
- Honey is denser than water
- Chalk is a solid with moderate density
- Iron is a heavy metal with high density
States of Matter
2. (a) Tabulate the differences in the characteristics of states of matter.
| Property |
Solid |
Liquid |
Gas |
| Shape |
Fixed |
Not fixed, takes shape of container |
Not fixed, fills entire container |
| Volume |
Fixed |
Fixed |
Not fixed |
| Compressibility |
Negligible |
Very low |
High |
| Intermolecular spaces |
Very small |
Larger than solids |
Very large |
| Intermolecular forces |
Very strong |
Weaker than solids |
Very weak |
| Kinetic energy of particles |
Very low |
Higher than solids |
Very high |
| Density |
High |
Moderate |
Very low |
| Fluidity |
No |
Yes |
Yes |
(b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.
- Rigidity: The property of a substance to resist change in its shape. Solids are rigid while liquids and gases are not.
- Compressibility: The ability of a substance to be compressed or reduced in volume. Gases are highly compressible, liquids are slightly compressible, solids are incompressible.
- Fluidity: The ability of a substance to flow. Liquids and gases have fluidity, solids do not.
- Filling a gas container: Gases completely fill the container they are placed in due to high kinetic energy and weak intermolecular forces.
- Shape: Solids have definite shape, liquids take the shape of container, gases fill the entire container.
- Kinetic energy: Energy possessed by particles due to their motion. It is highest in gases, moderate in liquids, and lowest in solids.
- Density: Mass per unit volume. It is highest in solids, moderate in liquids, and lowest in gases.
3. Give reasons
(a) A gas fills completely the vessel in which it is kept.
Because gas particles have very high kinetic energy and very weak intermolecular forces, so they move randomly in all directions and fill the entire space available.
(b) A gas exerts pressure on the walls of the container.
Gas particles are in continuous random motion and keep colliding with the walls of the container. These collisions exert force on the walls, which is observed as pressure.
(c) A wooden table should be called a solid.
Because it has definite shape and volume, it is rigid, has high density, and strong intermolecular forces - all characteristics of solids.
(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.
Because air particles have large intermolecular spaces and weak forces, so hand can easily pass through. In solids, particles are closely packed with strong intermolecular forces, requiring great force to separate them.
4. 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 of anomalous expansion of water:
- When water freezes to form ice, it expands instead of contracting
- This expansion increases the volume of ice
- Since density = mass/volume, increased volume means decreased density
- The density of ice (0.917 g/cm³) is less than density of water (1 g/cm³)
- Therefore, ice floats on water
- This is due to the formation of a cage-like structure in ice with hydrogen bonding, creating more empty space
4. Arrange the following substances in increasing order of forces of attraction between the particles— water, sugar, oxygen.
Increasing order of forces of attraction:
Oxygen (gas) < Water (liquid) < Sugar (solid)
Explanation:
- Oxygen is a gas with very weak intermolecular forces
- Water is a liquid with moderate intermolecular forces (hydrogen bonding)
- Sugar is a solid with strong intermolecular forces
6. Give two reasons to justify—
(a) water at room temperature is a liquid.
(b) an iron almirah is a solid at room temperature.
(a) Water at room temperature is a liquid because:
- It has definite volume but no definite shape - it takes the shape of the container
- It can flow and has fluidity
(b) An iron almirah is a solid at room temperature because:
- It has definite shape and definite volume
- It is rigid and cannot be compressed
Temperature & State Changes
1. Convert the following temperatures to the celsius scale.
(a) 293 K (b) 470 K
Formula: °C = K - 273
(a) 293 K = 293 - 273 = 20°C
(b) 470 K = 470 - 273 = 197°C
1. Convert the following temperature to celsius scale:
a. 300 K b. 573 K
Formula: °C = K - 273
a. 300 K = 300 - 273 = 27°C
b. 573 K = 573 - 273 = 300°C
2. Convert the following temperatures to the kelvin scale.
(a) 25° C (b) 373° C
Formula: K = °C + 273
(a) 25°C = 25 + 273 = 298 K
(b) 373°C = 373 + 273 = 646 K
2. What is the physical state of water at:
a. 250°C b. 100°C
a. At 250°C: Gaseous state (steam) - Water exists as vapor above 100°C
b. At 100°C: Both liquid and gaseous state - At boiling point, water exists in equilibrium between liquid and vapor states
5. What is the physical state of water at—
(a) 25°C (b) 0°C (c) 100°C?
(a) At 25°C: Liquid state
(b) At 0°C: Both solid and liquid state (at melting/freezing point)
(c) At 100°C: Both liquid and gaseous state (at boiling point)
3. Give reason 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 balls disappear with time without leaving any solid because:
Naphthalene undergoes sublimation - it directly changes from solid to gaseous state without passing through liquid state. The naphthalene particles escape into the air as gas.
(b) We can get the smell of perfume sitting several metres away because:
Perfume particles diffuse through air. The particles of perfume have high kinetic energy and move randomly in all directions, spreading throughout the available space and reaching our nose from a distance.
3. For any substance, why does the temperature remain constant during the change of state?
The temperature remains constant during change of state because:
- The heat energy supplied is used to overcome the intermolecular forces of attraction
- This heat energy doesn't increase the kinetic energy of particles, so temperature doesn't rise
- Instead, it increases the potential energy of the particles by separating them
- This heat is called latent heat
- Once the change of state is complete, the temperature starts rising again
7. Why is ice at 273 K more effective in cooling than water at the same temperature?
Ice at 273 K is more effective in cooling than water at the same temperature because:
- Ice at 273 K (0°C) absorbs additional latent heat of fusion to melt into water at 273 K
- This latent heat (334 J/g) is taken from the surroundings, causing more cooling
- Water at 273 K only absorbs specific heat to warm up, which is much less than latent heat
- Therefore, ice provides extra cooling due to the absorption of latent heat during melting
8. What produces more severe burns, boiling water or steam?
Steam produces more severe burns than boiling water because:
- Steam at 100°C contains additional latent heat of vaporization (2260 J/g)
- When steam condenses on skin, it releases this latent heat in addition to the heat from cooling to body temperature
- Boiling water at 100°C only releases the specific heat as it cools to body temperature
- Therefore, steam transfers more heat energy to the skin, causing more severe burns
4. Suggest a method to liquefy atmospheric gases.
Atmospheric gases can be liquefied by:
- Applying high pressure: Compressing the gas increases intermolecular forces
- Cooling below critical temperature: Reducing temperature decreases kinetic energy of particles
- The most common method is Linde's process which uses the Joule-Thomson effect - when a gas expands suddenly, it cools and can liquefy
- Another method is Claude's process which uses adiabatic expansion
9. Name A,B,C,D,E and F in the following diagram showing change in its state
Based on the state change diagram:
- A: Melting/Fusion (Solid → Liquid)
- B: Vaporization/Evaporation (Liquid → Gas)
- C: Sublimation (Solid → Gas)
- D: Freezing (Liquid → Solid)
- E: Condensation (Gas → Liquid)
- F: Deposition (Gas → Solid)
Evaporation & Effects
1. Why does a desert cooler cool better on a hot dry day?
A desert cooler cools better on a hot dry day because:
- On hot dry days, the humidity is low and air can hold more moisture
- Evaporation occurs more rapidly in dry air
- The desert cooler works on the principle of evaporation
- When water evaporates, it absorbs latent heat from the surroundings
- This causes cooling of the air that is blown out by the cooler
- More evaporation means more cooling effect
2. How does the water kept in an earthen pot (matka) become cool during summer?
Water in an earthen pot becomes cool because:
- Earthen pots have small pores through which water seeps out slowly
- This water evaporates from the outer surface of the pot
- Evaporation requires latent heat which is taken from the water inside the pot
- This causes the temperature of the remaining water to decrease
- The porous nature of the earthen pot facilitates continuous evaporation
3. Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Our palm feels cold because:
- Acetone, petrol and perfume are volatile liquids with low boiling points
- They evaporate quickly when put on palm
- During evaporation, they absorb latent heat of vaporization from the palm
- This heat is taken from our skin, making it feel cold
- The faster the evaporation, the more cooling sensation we feel
4. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
We can sip hot tea faster from a saucer because:
- A saucer has a larger surface area compared to a cup
- Larger surface area increases the rate of evaporation
- During evaporation, the liquid cools down faster
- More heat is lost to the surroundings through evaporation
- Therefore, tea in a saucer cools faster and can be sipped sooner
5. What type of clothes should we wear in summer?
In summer, we should wear:
- Light-colored clothes: They reflect sunlight and absorb less heat
- Cotton clothes: Cotton is a good absorber of water and helps in evaporation
- Loose-fitting clothes: Allow air circulation and promote evaporation of sweat
- Breathable fabrics: That allow sweat to evaporate quickly, providing cooling effect
These clothes help keep us cool by facilitating the evaporation of sweat, which is our body's natural cooling mechanism.