Q1: What is heat? (a) A form of matter (b) Internal energy of a substance measured in joules (c) The size of molecules (d) The colour of an object
Solution:
Ans: (b) Explanation: Heat is the internal energy of a substance, which is measured in joules (symbol: J). Heat flows from hotter to colder objects when they are in contact.
Q2: At which temperature does water boil? (a) 0°C (b) 4°C (c) 100°C (d) 125°C
Solution:
Ans: (c) Explanation: Water boils at 100°C and turns into steam. During boiling, the temperature remains constant at the boiling point without further increase.
Q3: Which process occurs when a liquid changes into vapour at all temperatures from its surface? (a) Boiling (b) Evaporation (c) Melting (d) Condensation
Solution:
Ans: (b) Explanation: Evaporation is the process where a liquid changes into its vapour at all temperatures from its surface, unlike boiling which occurs at a fixed temperature.
Q4: What happens to the density of water when it is heated from 0°C to 4°C? (a) It decreases (b) It increases (c) It remains the same (d) It becomes zero
Solution:
Ans: (b) Explanation: Water contracts when heated from 0°C to 4°C, so its volume decreases while mass remains same. Therefore, density increases and reaches maximum at 4°C.
Q5: What is the relationship between the coefficients of linear, superficial, and cubical expansion? (a) α : β : γ = 1 : 1 : 1 (b) α : β : γ = 1 : 2 : 3 (c) α : β : γ = 3 : 2 : 1 (d) α : β : γ = 2 : 3 : 4
Solution:
Ans: (b) Explanation: The coefficients of linear (α), superficial (β), and cubical (γ) expansion are related as β = 2α and γ = 3α, giving the ratio α : β : γ = 1 : 2 : 3.
Fill in the Blanks
Q1: Molecules in matter are very small, about _____ meters in size.
Solution:
Ans: 10^-10
Q2: The average kinetic energy of a substance decides its _____.
Solution:
Ans: temperature
Q3: The process where a solid changes directly into a gas is called _____.
Solution:
Ans: sublimation
Q4: The maximum density of water occurs at _____.
Solution:
Ans: 4°C
Q5: _____ is an alloy of iron and nickel used to make the pendulum of a clock.
Solution:
Ans: Invar
True or False
Q1: Heat flows from a colder object to a hotter object when two objects are in contact.
Solution:
Ans: False Explanation: Heat flows from a hotter object to a colder object when two objects are in contact, not from cold to hot.
Q2: Evaporation causes cooling of the liquid.
Solution:
Ans: True Explanation: When a liquid evaporates, molecules at the surface take heat from the liquid and surroundings to change into vapour, causing cooling.
Q3: The boiling point of a liquid decreases with an increase in pressure.
Solution:
Ans: False Explanation: The boiling point of a liquid increases with an increase in pressure and decreases with a decrease in pressure, as explained by vapour pressure.
Q4: All solids, liquids, and gases expand when heated and contract when cooled.
Solution:
Ans: True Explanation: Almost all solids, liquids, and gases expand when heated and contract when cooled due to increased molecular motion and distance between molecules.
Q5: Liquids expand more than solids when heated.
Solution:
Ans: True Explanation: Liquids, like solids, get bigger when heated, but liquids expand a lot more than solids when heated to the same temperature.
Match the Following
Solution:
Ans:
1 - E: Melting is the process where a solid is heated and changes into a liquid at a fixed temperature.
2 - A: Evaporation is the process where a liquid changes into its vapour at all temperatures from its surface.
3 - B: Condensation is the process where a gas is cooled and changes into a liquid at the same fixed temperature.
4 - C: Boiling is the process where a liquid is heated and changes into its vapour at a fixed temperature called boiling point.
5 - D: Sublimation is the process where a solid is heated and changes directly into a gas at a fixed temperature.
Short Answer Questions
Q1: Explain what happens to molecules when a substance is heated.
Solution:
Ans: When a substance is heated, its molecules gain energy and move faster, increasing their kinetic energy. The increased motion makes molecules move farther apart from each other. This increased distance between molecules causes the substance to expand in size. When a substance loses heat, molecules slow down, decreasing their kinetic energy, and the substance contracts in size.
Q2: What are the three main effects of heat on a substance?
Solution:
Ans: Heat causes three main changes in a substance. First, it causes a change in temperature of the body, making it hotter or colder. Second, it causes a change in state, such as solid to liquid or liquid to gas. Third, it causes a change in size of the body through thermal expansion or contraction. These effects depend on the material and amount of heat.
Q3: How does increase in temperature affect the rate of evaporation?
Solution:
Ans: When the temperature of a liquid increases, the rate of evaporation increases. At higher temperatures, the molecules of the liquid gain more energy and move faster. More molecules then have enough energy to reach the surface and escape as vapour. This increased molecular motion and energy allows more molecules to overcome the attractive forces and escape, increasing evaporation rate significantly.
Q4: Why are small gaps left between railway tracks?
Solution:
Ans: Railway tracks are made of steel rails. Small gaps are left between the ends of two rails to allow for thermal expansion in summer. If there were no gaps, the rails would expand in summer, touch each other, and bend sideways, damaging the track. These gaps provide space for the rails to expand safely when heated without affecting the structure.
Q5: What is latent heat and why is it called latent?
Solution:
Ans: The heat absorbed or released during a change of state is called latent heat. It is called latent because it does not cause a change in temperature even though heat is being absorbed or released. This heat changes the average potential energy of the molecules, not their average kinetic energy. Latent heat is specific for each substance and remains hidden during state change.
Long Answer Questions
Q1: Analyze the differences between evaporation and boiling. Justify why evaporation causes cooling while boiling does not produce the same cooling effect.
Solution:
Ans: Evaporation occurs at all temperatures from the liquid surface, while boiling happens throughout the liquid at a fixed temperature. Evaporation is slow; boiling is fast. In evaporation, some surface molecules gain energy from collisions and escape as vapour, taking heat from surroundings without external supply. This causes cooling. In boiling, heat is continuously supplied externally at the boiling point, so temperature stays constant. Molecules throughout form bubbles and escape rapidly. Since external heat is supplied during boiling, the cooling effect is not observed as in evaporation.
Q2: Compare linear, superficial, and cubical expansion in solids. Evaluate the factors that affect each type of expansion with suitable examples.
Solution:
Ans: Linear expansion is the increase in length of a solid when heated, superficial expansion is the increase in area, and cubical expansion is the increase in volume. All three depend on the original dimension, temperature rise, and material. The formula for linear expansion is Lₜ - L₀ = L₀ α t, for superficial is Aₜ - A₀ = A₀ β t, and for cubical is Vₜ - V₀ = V₀ γ t. The coefficients are related as α : β : γ = 1 : 2 : 3. For example, a copper rod expands more than an iron rod of same length when heated equally.
Q3: Explain the anomalous behavior of water and justify its importance in nature. Analyze what would happen if water behaved like other liquids.
Solution:
Ans: Water contracts when heated from 0°C to 4°C instead of expanding, which is anomalous behavior. Water has maximum density at 4°C, which is 1000 kg/m³. Above 4°C, water expands normally. This behavior is crucial for aquatic life in cold regions. When temperature drops, water at 4°C being densest stays at the bottom while ice forms at the surface, protecting aquatic organisms. If water behaved normally, entire water bodies would freeze from bottom up, killing all aquatic life. This anomalous property ensures survival of aquatic ecosystems during winter.
Ans. The three methods of heat transfer are conduction, convection, and radiation. Conduction occurs through direct contact between materials, convection involves the movement of fluids, and radiation transfers heat through electromagnetic waves.
2. How does conduction work in solids?
Ans. In solids, conduction works by the transfer of kinetic energy from one molecule to another through direct contact. When molecules in a solid are heated, they vibrate faster and transfer their energy to adjacent molecules, resulting in heat transfer through the material.
3. Explain the role of convection in liquids and gases.
Ans. Convection plays a significant role in liquids and gases as it involves the movement of fluid particles. When a fluid is heated, it becomes less dense and rises, while cooler, denser fluid sinks. This creates a convection current that facilitates the transfer of heat throughout the fluid.
4. What is thermal radiation?
Ans. Thermal radiation is the transfer of heat in the form of electromagnetic waves, primarily infrared radiation. All objects emit thermal radiation depending on their temperature; hotter objects emit more radiation than cooler ones, allowing heat to be transferred without any medium.
5. How does insulation help in heat transfer?
Ans. Insulation helps in reducing heat transfer by providing a barrier that slows down the conduction, convection, and radiation of heat. Materials with low thermal conductivity, such as wool or fibreglass, are often used for insulation to keep buildings warm in winter and cool in summer.
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