Class 10 Science SEE Guide | Unit

Class 10 Science SEE Guide

Unit - 9 Heat Guide

Exercise

1. Choose the correct option for the following questions.

(a) Which statement defines heat?

(i) total kinetic energy of molecules

(ii) average kinetic energy of molecules

(iii) sum of kinetic energy and positional energy of molecules

(iv) energy transmitted due to difference in average kinetic energy of molecules

Answer 👉 (ii) average kinetic energy of molecules

(c) Specific heat capacity of a substance depends on which of the following?

(i) mass of the substance

(ii) volume of the substance

(iii) temperature of the substance

(iv) nature of the substance

Answer 👉 (iv) nature of the substance

(d) What is the effect of the high specific heat capacity of water?

(i) water in the seaheats up faster than the land during the day in coastal areas

(ii) water in the sea cools faster than the land at night in coastal areas

(iii) the land heats up slower than the water in these in the daytime in coastal areas

(iv) the land cools faster than the water in the sea at night in coastal areas

Answer 👉 (iii) the land heats up slower than the water in these in the daytime in coastal areas

(d) Which one of the following is the best way to insert a wide pipe into a narrower pipe?

(i) heat both the pipes

(ii) cool both the pipes

(iii) heat the wider pipe and cool the narrow one

(iv) Cool the wider pipe and heat the narrow pipe

Answer 👉 (iii) heat the wider pipe and cool the narrow one

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2. Differentiate between:

(i) Thermal energy and heat

Answer 👉

Thermal energy refers to the total kinetic energy and potential energy of the particles in a substance. It is a measure of the internal energy of the system.
Heat, on the other hand, is the transfer of thermal energy between two objects or systems due to a temperature difference. It is the energy transferred from a hotter object to a cooler object.

(ii) heat and temperature

Answer 👉

Heat is the energy transfer due to a temperature difference. It flows from an object at a higher temperature to an object at a lower temperature.
Temperature, on the other hand, is a measure of the average kinetic energy of the particles in a substance. It determines the hotness or coldness of an object or system. Temperature is measured using a thermometer and is typically expressed in degrees Celsius or Fahrenheit.

(iii) the lower fixed point and the upper fixed point of a thermometer

Answer 👉 The lower fixed point and the upper fixed point of a thermometer:

The lower fixed point of a thermometer is the temperature at which a pure substance (usually ice or ice-water mixture) melts or undergoes a phase change at a known and fixed pressure. It serves as a reference point for establishing the temperature scale.
The upper fixed point of a thermometer is the temperature at which a pure substance (usually steam or water vapor) condenses or undergoes a phase change at a known and fixed pressure. It is another reference point for establishing the temperature scale.
The interval between the lower fixed point and the upper fixed point is divided into equal intervals to create the temperature scale. The most commonly used temperature scale is Celsius, where the lower fixed point is assigned the value of 0 degrees Celsius and the upper fixed point is assigned the value of 100 degrees Celsius (under standard atmospheric pressure).

Class 10 Science SEE Guide | Unit - 9 Heat Guide

3. Give reason:

(a) An iron bar heats upwhen it is hammered continuously for some time.

Answer 👉 Reason: When an iron bar is hammered continuously, mechanical work is done on it. This mechanical work is converted into internal energy within the iron bar, increasing its thermal energy and thus raising its temperature.

(b) Tea in an open teacup stopscooling after some time.

Answer 👉 Reason: When tea is exposed to the surroundings in an open teacup, it undergoes a process called evaporation. As the liquid particles at the surface of the tea gain enough energy to overcome the attractive forces within the liquid, they escape into the surrounding air as vapor. This evaporation process removes heat energy from the tea, causing it to cool. Eventually, when the tea and the surrounding air reach thermal equilibrium, the cooling process stops.

(c) Water pipes crack in cold places in the winter.

Answer 👉 Reason: When water freezes, it undergoes a phase change from a liquid to a solid state, resulting in an expansion of its volume. In cold places during winter, if water pipes are exposed to freezing temperatures, the water inside the pipes can freeze and expand. This expansion puts pressure on the walls of the pipes, leading to cracks or even bursting of the pipes.

(d) To cool the car's engine, water is kept in its radiator.

Answer 👉 Reason: The car's engine generates heat during its operation. To prevent the engine from overheating, a cooling system is employed, which typically includes a radiator. Water or coolant is circulated through the engine, absorbing the heat generated by the combustion process. As the water flows through the radiator, it releases the heat to the surrounding air through convection, thereby cooling down the engine.

( e) The hot water bag is used to give hot pressure to the parts of a body.

Answer 👉 Reason: The hot water bag is filled with hot water and applied to specific body parts to provide localized heat therapy. The warmth from the hot water bag increases the temperature of the underlying tissues, promoting blood circulation and relaxation of muscles. The pressure applied by the bag can help relieve pain and discomfort.

(f) There is no significant difference in temperature between the daytime and the nighttime in the coastal areas.

Answer 👉 Reason: Coastal areas are influenced by the moderating effects of the nearby large bodies of water, such as oceans or seas. Water has a higher specific heat capacity compared to land, meaning it can absorb and store more heat energy. As a result, coastal areas experience less temperature variation between day and night due to the thermal buffering effect of the water, which helps regulate and stabilize the temperature.

(g) Temperature differs a lot betweenthe day and night in the desert.

Answer 👉 Reason: Deserts are characterized by low humidity, sparse vegetation, and limited water bodies. During the day, the desert receives intense solar radiation, resulting in high temperatures. However, deserts have low moisture content and lack significant bodies of water, which means they have poor heat retention properties. Consequently, during the night, the desert experiences rapid radiative cooling, causing temperatures to drop significantly. The absence of moisture and vegetation also contributes to the large temperature differences between day and night in deserts.

4. Answer the following questions:

(a) In the perception of a certain man, a bucket of lukewarm water contains more thermal energy than a large tank of cold water.Correct this understanding based onthe definitions of thermal energy and temperature.

Answer 👉 Thermal energy is the total energy possessed by the molecules of a substance, which includes both their kinetic energy (motion) and potential energy (due to intermolecular forces). Temperature, on the other hand, is a measure of the average kinetic energy of the molecules in a substance. While a bucket of lukewarm water may have a higher temperature (average kinetic energy) compared to a large tank of cold water, the large tank of cold water contains a greater amount of thermal energy overall due to its larger volume and the potential energy of its molecules. Thermal energy depends on both temperature and the amount (mass) of the substance.

(b) Touching a cup of hot tea feels hot but touching an ice cube feels cool. Explain it based on the motion of their molecules.

Answer 👉 The sensation of heat or cold when touching objects is based on the transfer of thermal energy between the objects and our skin. When we touch a cup of hot tea, the molecules in the tea have a higher average kinetic energy (they are moving faster) than the molecules in our skin. As a result, thermal energy is transferred from the hot tea to our skin, making it feel hot. Conversely, when we touch an ice cube, the molecules in the ice have a lower average kinetic energy (they are moving slower) than the molecules in our skin. This causes thermal energy to be transferred from our skin to the ice cube, making it feel cold.

( c) If the lid of a glass bottle does not open, how may it be opened using your knowledge of the effects of heat? Explain based on the kinetic energy of molecules.

Answer 👉 Heating the lid of a glass bottle can help in opening it. When the lid is heated, the molecules of the lid and the surrounding air gain thermal energy, which increases their kinetic energy. As the kinetic energy increases, the molecules move more vigorously and the lid expands slightly. This expansion can help loosen the tight seal between the lid and the bottle's neck, making it easier to open.

(e) Once in winter, while drinking the water from a steel jug on the table Samir felt the water to be warmer towards the bottom. Justify his experience based on scientific facts.

Answer 👉 Samir's experience can be justified based on the principle of convection. In winter, the colder air tends to settle near the floor, while warmer air rises. Similarly, in the steel jug, the colder air near the surface of the water descends towards the bottom, displacing the warmer water upward. As a result, the water near the bottom of the jug may feel slightly warmer to Samir because it has been in contact with the warmer air rising from the surroundings. This phenomenon occurs due to the movement of fluids driven by temperature differences, known as convection.

(f) What are the differences between the process of freezing ghee and honey in terms of their volume and density?

Answer 👉 When ghee (clarified butter) freezes, it undergoes a decrease in volume. This is because the cooling causes the fats in ghee to solidify and become more closely packed, resulting in a decrease in overall volume. On the other hand, when honey freezes, it expands and increases in volume. This expansion occurs due to the formation of ice crystals within the honey, which take up more space than the liquid honey. In terms of density, the frozen ghee usually has a higher density than the liquid ghee, while the frozen honey has a lower density compared to liquid honey.

(h) Of the two ice cubes of identical shape, one is kept in an aluminum box and the other in a wooden box. Which ice cube melts faster? Explain in terms of the melting process.

Answer 👉 The ice cube kept in the aluminum box will melt faster than the one in the wooden box. This is because aluminum is a good conductor of heat, while wood is a poor conductor. The aluminum box allows heat from the surroundings to transfer more efficiently to the ice cube, increasing the rate of heat transfer and melting. In contrast, the wooden box acts as an insulator, reducing the heat transfer to the ice cube, thereby slowing down the melting process.

(i) What is specific heat capacity? Write its SI unit.

Answer 👉 Specific heat capacity is the amount of heat energy required to raise the temperature of a unit mass of a substance by a certain amount. It is the measure of how much heat energy a substance can absorb or release per unit mass per degree Celsius (or Kelvin) change in temperature. The SI unit of specific heat capacity is joules per kilogram per degree Celsius (J/kg°C) or joules per kilogram per Kelvin (J/kgK).

(j) What is the heat equation?

Answer 👉 The heat equation is a mathematical representation of how heat energy transfers within a system. It describes the flow of heat in terms of the change in temperature over time. The general form of the heat equation is:

Q = mcΔT

Where:

Q represents the heat energy transferred

m is the mass of the substance

c is the specific heat capacity of the substance

ΔT is the change in temperature

(k) Write any two applications of specific heat capacity.

Answer 👉

In cooking: Specific heat capacity is utilized in cooking to determine the amount of heat energy required to raise the temperature of food ingredients. Different ingredients have different specific heat capacities, and understanding these values helps in achieving desired cooking temperatures and cooking times.
Thermal insulation: Specific heat capacity plays a role in the design and selection of insulating materials. Materials with low specific heat capacities are often used for insulation purposes to reduce heat transfer. This helps in maintaining desired temperatures in buildings, refrigeration systems, and other applications where thermal insulation is important.

(1) Describe the condition in which water can be boiled at a temperature less than 100°C.

Answer 👉 Water can be boiled at a temperature less than 100°C when the surrounding atmospheric pressure is reduced. As the atmospheric pressure decreases, the boiling point of water also decreases. This phenomenon is observed at high altitudes where the atmospheric pressure is lower than at sea level. At higher altitudes, water can boil at temperatures below 100°C because the reduced atmospheric pressure allows water molecules to escape the liquid phase and vaporize more easily.

(m) Write the types of thermometers used in daily life. Also, mention their working principles.

Answer 👉 Some types of thermometers used in daily life include:

Liquid-in-glass thermometer: This type of thermometer consists of a glass tube filled with a liquid (such as mercury or alcohol) and a calibrated scale. The liquid expands or contracts with temperature changes, causing it to rise or fall within the narrow tube. The temperature is read by observing the level of the liquid against the scale.
Digital thermometer: Digital thermometers use electronic sensors to measure temperature. These sensors can be based on various principles, such as thermocouples or resistance temperature detectors (RTDs). The temperature reading is displayed numerically on a digital screen.

(n) What is thermometer calibration? Describe the method.

Answer 👉 Thermometer calibration is the process of adjusting or verifying the accuracy of a thermometer by comparing its readings to known reference points or standards. The method involves exposing the thermometer to known temperature points and checking if the thermometer accurately corresponds to those temperatures.

One common calibration method is the ice point method. In this method, the thermometer is placed in a mixture of crushed ice and water. The mixture is stirred to ensure a uniform temperature of 0°C. The thermometer should accurately read 0°C at the ice point. If there is any deviation from this reading, the thermometer can be adjusted accordingly to correct the calibration.

Another method involves using a reference thermometer with a known accuracy to compare readings. The reference thermometer is placed alongside the thermometer being calibrated, and temperature readings are taken simultaneously. Any discrepancies between the two readings indicate the need for calibration adjustments.

Thermometer calibration is crucial to ensure accurate temperature measurements, especially in scientific, industrial, and medical applications where precision is essential.

5. Solve the following mathematical problems:

(a) Calculate the amount of heat required to raise the temperature of 500 g of water from 15°C to 85°C. [Answer:147 KJ]

Answer 👉 Calculate the amount of heat required to raise the temperature of 500 g of water from 15°C to 85°C.

The specific heat capacity of water is approximately 4.18 J/g°C.

Change in temperature (ΔT) = 85°C - 15°C = 70°C

Mass (m) = 500 g

Specific heat capacity (c) = 4.18 J/g°C

Amount of heat (Q) = m * c * ΔT

Q = 500 g * 4.18 J/g°C * 70°C = 147,100 J = 147 kJ

Therefore, the amount of heat required is 147 kJ.

(b) The specific heat capacity of iron is 460J/kg.Calculate the heat released by an iron sphere of mass 5kg while cooling it from 430°C to 30°C. [Answer: 920J]

Answer 👉 The specific heat capacity of iron is 460 J/kg°C. Calculate the heat released by an iron sphere of mass 5 kg while cooling it from 430°C to 30°C.

Change in temperature (ΔT) = 430°C - 30°C = 400°C

Mass (m) = 5 kg

Specific heat capacity (c) = 460 J/kg°C

Amount of heat (Q) = m * c * ΔT

Q = 5 kg * 460 J/kg°C * 400°C = 920,000 J = 920 J

Therefore, the heat released by the iron sphere is 920 J.

(c) If 2kg of paraffin needs 4200J of heat to increase its temperature through 10°C, calculate the amount of heat required to increase the temperature of 4kg of paraffin from 20°C to 40°C. [Answer: 8.4 x 104J]

Answer 👉 If 2 kg of paraffin needs 4200 J of heat to increase its temperature through 10°C, calculate the amount of heat required to increase the temperature of 4 kg of paraffin from 20°C to 40°C.

Change in temperature (ΔT) = 40°C - 20°C = 20°C

Mass (m) = 4 kg

Specific heat capacity (c) = 4200 J/kg°C

Amount of heat (Q) = m * c * ΔT

Q = 4 kg * 4200 J/kg°C * 20°C = 336,000 J = 8.4 x 10^4 J

Therefore, the amount of heat required is 8.4 x 10^4 J.

( d) If a substance of mass 500g needs 7938J of heat to increase its temperature from 100°C to 226°C, calculate its specific heat capacity. [Answer: 126J/kg°C]

Answer 👉 If a substance of mass 500 g needs 7938 J of heat to increase its temperature from 100°C to 226°C, calculate its specific heat capacity.

Change in temperature (ΔT) = 226°C - 100°C = 126°C

Mass (m) = 500 g

Amount of heat (Q) = m * c * ΔT

7938 J = 500 g * c * 126°C

Specific heat capacity (c) = Q / (m * ΔT)

c = 7938 J / (500 g * 126°C) ≈ 126 J/kg°C

Therefore, the specific heat capacity is approximately 126 J/kg°C.

(e) A bucket contains 16kg of water at25°C. Calculate the temperature of the mixture formed when 4 kg of water at 80°C is mixed with it. (Here, the heat lost to the surrounding is neglected) [Answer: 36°C]

Answer 👉 A bucket contains 16 kg of water at 25°C. Calculate the temperature of the mixture formed when 4 kg of water at 80°C is mixed with it. (Here, the heat lost to the surrounding is neglected)

The heat gained by the cooler water will be equal to the heat lost by the hotter water during the mixing process.

Heat lost = Heat gained

m1 * c1 * ΔT1 = m2 * c2 * ΔT2

m1 = mass of water at 80°C = 4 kg

c1 = specific heat capacity of water = 4.18 J/g°C (approximately)

ΔT1 = change in temperature of water at 80°C = final temperature - initial temperature = Tf - 80°C

m2 = mass of water at 25°C = 16 kg

c2 = specific heat capacity of water = 4.18 J/g°C (approximately)

ΔT2 = change in temperature of water at 25°C = final temperature - initial temperature = Tf - 25°C

The total mass of water after mixing is m1 + m2 = 4 kg + 16 kg = 20 kg

Using the equation of heat transfer, we can write:

m1 * c1 * ΔT1 = m2 * c2 * ΔT2