Solution :: Chapter 1: Matter in our surrounding

States of Matter - Questions and Answers

1. Which of the following are matter?

Matter is anything that has mass and occupies space. Let's examine each item:

• Chair: Yes, it is matter because it has mass and occupies space.
• Air: Yes, it is matter because it has mass and occupies space.
• Love: No, it is not matter. It is an abstract concept, not something with mass or volume.
• Smell: No, it is not matter in itself. Smell is a sensation created when particles from a substance interact with your nose, but the smell itself is not matter.
• Hate: No, it is not matter. It is an emotion, not something with mass or volume.
• Almonds: Yes, it is matter because it has mass and occupies space.
• Thought: No, it is not matter. It is a mental process, not something physical.
• Cold: No, it is not matter. Cold is a sensation; it's the absence of heat, not a physical substance.
• Lemon water: Yes, it is matter because it has mass and occupies space.
• Smell of perfume: No, it is not matter in itself, but the particles of perfume that spread and cause the smell are matter.

So, the following are matter: Chair, air, almonds, lemon water.

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 observation is due to the rate of diffusion of particles. In hot food, the molecules of the food are moving faster because of the increased temperature, and as a result, the molecules of the smell (which are volatile) spread more quickly into the surrounding air. In cold food, the molecules are moving slower, so the rate of diffusion of the smell particles is slower, making it necessary to be closer to detect the smell.

3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?

The observation that a diver is able to cut through water in a swimming pool demonstrates the property of matter related to the movement of particles. The particles of matter are in constant motion, and in liquids like water, these particles are not as tightly packed as in solids. This allows the particles to move past each other, enabling the diver to move through the water.

4. What are the characteristics of the particles of matter?

The characteristics of the particles of matter are:

• Tiny Size: The particles of matter are extremely small. They cannot be seen with the naked eye.
• Inter-particle Spaces: There is space between the particles of matter. The space depends on the state of the matter (solid, liquid, or gas).
• Movement of Particles: The particles of matter are in constant motion. In solids, the particles vibrate about their fixed positions, while in liquids and gases, the particles move more freely.
• Attractive Forces: There are forces of attraction between the particles of matter. These forces are strongest in solids, weaker in liquids, and weakest in gases.
• Kinetic Energy: The particles of matter have kinetic energy, which increases with an increase in temperature (particles move faster when heated).

5. Arranging substances in order of increasing density

To arrange the following substances in order of increasing density, we need to know the densities of each substance:

• Air: Density ≈ 1.225 kg/m³
• Exhaust from chimneys: Density ≈ 1.2 kg/m³ (similar to air)
• Honey: Density ≈ 1.42 g/cm³
• Water: Density ≈ 1.0 g/cm³
• Chalk: Density ≈ 2.4 g/cm³
• Cotton: Density ≈ 1.5 g/cm³
• Iron: Density ≈ 7.87 g/cm³

Order of increasing density: Air ≈ Exhaust from chimneys < Water < Cotton < Honey < Chalk < Iron

6. (a) Differences in the characteristics of states of matter

Property	Solid	Liquid	Gas
Shape	Definite shape	No definite shape	No definite shape
Volume	Definite volume	Definite volume	No definite volume
Inter-particle Distance	Very short (close)	Moderate	Very far apart
Inter-particle Forces	Strong attraction	Moderate attraction	Weak attraction
Particle Movement	Vibration (fixed positions)	Flowing past each other	Free movement
Compressibility	Very low	Low	High
Fluidity	No fluidity	Fluid	Fluid

6. (b) Comment upon the following properties

• Rigidity: Solids are rigid because their particles are tightly packed, leaving little freedom of movement.
• Compressibility: Solids and liquids are nearly incompressible, but gases are highly compressible because their particles are far apart.
• Fluidity: Liquids and gases can flow, meaning they have fluidity, while solids are rigid and do not flow.
• Filling a gas container: Gases have the ability to fill any container due to their weak intermolecular forces and the wide spaces between particles, allowing them to spread and take the shape of the container.
• Shape: Solids have a fixed shape, liquids do not have a fixed shape but take the shape of the container, and gases have neither fixed shape nor fixed volume.
• Kinetic energy: Particles in solids have the least kinetic energy (they only vibrate), in liquids, the kinetic energy is moderate, and in gases, the kinetic energy is the highest as particles move freely.
• Density: Solids generally have the highest density, liquids have moderate density, and gases have the lowest density because their particles are the farthest apart.

7. Give reasons:

(a) A gas fills completely the vessel in which it is kept:

This happens because gas particles are in constant motion and have weak intermolecular forces, allowing them to spread and fill the entire space of the container.

(b) A gas exerts pressure on the walls of the container:

Gas particles collide with the walls of the container, and these collisions exert force on the walls, which results in pressure.

(c) A wooden table should be called a solid:

A wooden table has a definite shape and volume, and its particles are tightly packed and only vibrate in place, which are 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:

Air is a gas, and its particles are far apart and can move freely, offering little resistance. In contrast, the particles in solid wood are tightly packed and resist motion, making it hard to move through.

8. Why does ice float on water?

Ice floats on water because ice is less dense than water. When water freezes, the particles form a crystalline structure with more open spaces between them, which increases its volume and decreases its density. As a result, ice has a lower density than liquid water, causing it to float.

9. Convert the following temperatures to Celsius scale:

To convert temperature from Kelvin to Celsius, the formula is:

T_Celsius = T_Kelvin - 273.15

• (a) 300 K:
  300 K - 273.15 = 26.85 °C
• (b) 573 K:
  573 K - 273.15 = 299.85 °C

So, the temperatures are:

• (a) 300 K = 26.85 °C
• (b) 573 K = 299.85 °C

10. What is the physical state of water at:

• (a) 250°C:
  At 250°C, water is in the gaseous state (steam), as the boiling point of water is 100°C, and at 250°C, the water will be completely vaporized.
• (b) 100°C:
  At 100°C, water is at its boiling point and exists in both liquid and gaseous states (it will be at the phase transition point from liquid to gas).

11. Why does the temperature remain constant during the change of state?

During a change of state (e.g., from solid to liquid or liquid to gas), the energy supplied is used to overcome the forces of attraction between the particles of the substance, rather than increasing the kinetic energy of the particles (which would increase the temperature). This process is known as latent heat. Because all the energy is being used to change the state and not to increase the temperature, the temperature remains constant until the change of state is complete.

12. Suggest a method to liquefy atmospheric gases

To liquefy atmospheric gases, one can use the process of cooling and compression.

• Cooling: When gases are cooled to very low temperatures, their particles move slower, and eventually, the attractive forces between the particles become strong enough to bring them together into a liquid state.
• Compression: Compressing the gas increases the pressure and forces the particles closer together, which also aids in converting the gas to liquid.

In practice, atmospheric gases like oxygen and nitrogen are liquefied by first compressing them and then cooling them to their respective liquefaction points.

13. Why does a desert cooler cool better on a hot dry day?

A desert cooler works on the principle of evaporative cooling. It cools better on a hot, dry day because, in dry conditions, the rate of evaporation is higher. As the water evaporates from the cooler pads, it absorbs heat from the surrounding air, thus lowering the temperature of the air. The drier the air, the more efficiently the water can evaporate, leading to better cooling.

14. How does the water kept in an earthen pot (matka) become cool during summer?

Water kept in an earthen pot (matka) becomes cool during summer due to evaporation. The walls of the earthen pot are porous, allowing some water to seep through the walls. As this water evaporates from the surface of the pot, it absorbs heat from the water inside, lowering the temperature. This process works more efficiently in the heat of summer, where the air is dry and evaporation occurs more rapidly.

15. Why does our palm feel cold when we put some acetone or petrol or perfume on it?

When acetone, petrol, or perfume is applied to the palm, they evaporate quickly. During evaporation, these substances absorb heat from the skin to overcome the intermolecular forces holding their molecules together. This absorption of heat from the skin causes the temperature of the skin to drop, making our palm feel cold.

16. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?

We are able to sip hot tea or milk faster from a saucer because of increased surface area. When the tea or milk is poured into a saucer, the liquid spreads out in a thin layer, allowing more of the liquid to come into contact with the air. This increases the rate of evaporation and cooling, which makes the liquid cooler faster compared to being in a cup with a smaller surface area.

17. What type of clothes should we wear in summer?

During summer, we should wear light-colored and loose-fitting clothes made from natural fabrics like cotton. These materials allow sweat to evaporate easily, which helps in cooling the body. Light-colored clothes reflect sunlight and reduce the amount of heat absorbed by the body, while loose-fitting clothes allow air circulation, promoting evaporation of sweat and helping to keep the body cool.

18. Convert the following temperatures to the Celsius scale:

To convert from Kelvin to Celsius, use the formula:

T_Celsius = T_Kelvin - 273.15

• (a) 293 K:
  293 K - 273.15 = 19.85 °C
• (b) 470 K:
  470 K - 273.15 = 196.85 °C

So, the temperatures are:

• (a) 293 K = 19.85 °C
• (b) 470 K = 196.85 °C

19. Convert the following temperatures to the Kelvin scale:

To convert from Celsius to Kelvin, use the formula:

T_Kelvin = T_Celsius + 273.15

• (a) 25°C:
  25 °C + 273.15 = 298.15 K
• (b) 373°C:
  373 °C + 273.15 = 646.15 K

So, the temperatures are:

• (a) 25°C = 298.15 K
• (b) 373°C = 646.15 K

20. Give reason for the following observations:

(a) Naphthalene balls disappear with time without leaving any solid:

This happens because naphthalene undergoes sublimation. Sublimation is the process where a solid changes directly into a gas without becoming a liquid. As naphthalene absorbs heat from the surroundings, it turns into gas, leaving no solid behind.

(b) We can get the smell of perfume sitting several meters away:

This happens due to the diffusion of perfume molecules in the air. The perfume particles move from areas of high concentration (near the bottle) to areas of low concentration, allowing us to smell it even from a distance.

21. Arrange the following substances in increasing order of forces of attraction between the particles:

Oxygen (gas) has the weakest forces of attraction between its particles (because gas particles are far apart). Water (liquid) has moderate forces of attraction between particles. Sugar (solid) has the strongest forces of attraction between its particles.

Order of increasing forces of attraction: Oxygen < Water < Sugar

22. What is the physical state of water at:

• (a) 25°C:
  Water is in a liquid state at 25°C.
• (b) 0°C:
  Water is in a solid state (ice) at 0°C, because it is freezing.
• (c) 100°C:
  Water is in a gaseous state (steam) at 100°C, as it is boiling.

23. Give two reasons to justify:

(a) Water at room temperature is a liquid:

Water has a melting point of 0°C and boiling point of 100°C. At room temperature (around 25°C), it exists as a liquid. The particles of water are close enough to allow the substance to flow, which is characteristic of liquids.

(b) An iron almirah is a solid at room temperature:

Iron has a high melting point (around 1538°C), so at room temperature, it remains in a solid state. The particles of iron are closely packed and do not have enough energy to move past each other, which is a characteristic of solids.

24. 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 of its latent heat of fusion. When ice melts, it absorbs a large amount of heat from the surroundings (without a temperature change) to overcome the intermolecular forces holding its particles in a solid state. This heat absorption makes ice more effective in cooling compared to water at the same temperature.

25. What produces more severe burns, boiling water or steam?

Steam produces more severe burns than boiling water. This is because steam contains more heat energy due to its latent heat of vaporization. When steam comes into contact with the skin, it condenses into water, releasing a significant amount of heat, which can cause more severe burns compared to boiling water.

26. Name A, B, C, D, E, and F in the following diagram showing change in its state:

Answer:

• A - Fusion
• B - Vaporization
• C - Condensation
• D - Solidification (Freezing)
• E - Sublimation
• F - Deposition