Activity - 1
1. 'The degree of hotness or coldness' is different for different objects. Explain this with an activity. (4 Marks)
* Keep a piece of wood and a piece of metal in a fridge for about 15 minutes and take them out.
* If you touch them with your finger you feel that metal piece is colder than the wooden piece.
* This means when you touch them heat energy is being transferred from your finger to the pieces.
* When heat energy flows out of your body you get the feeling of cold ness.
* When heat energy enters your body you get a feeling of hotness.
* The degree of coldness of the metal piece is greater than that of the wooden piece.
* This concludes that the degree of hotness or coldness is different for different objects.
Activity - 2
2. 'Temperature is a measure of thermal equilibrium' - Describe an activity to establish this. (4 Marks)
* Take two cups and fill one of them with hot water and another with cold water.
* Take a laboratory thermometer and note its mercury level in your note book.
* Keep the thermometer in hot water and note the reading of mercury level.
* Repeat the same activity by keeping the thermometer in cold water.
* When the thermometer is in hot water, the mercury level in the thermometer raises. This is because that heat got transferred from the hotter body (hot water) to the colder body (Mercury in the thermometer).
* When the thermometer is put in the cold water the mercury level comes down because of the transfer of heat from mercury (hotter body) to water (colder body).
* The steadiness of the mercury column of the thermometer indicates that flow of heat between the thermometer liquid (mercury) and water, has stopped.
* Thermal equilibrium has been attained between the water and thermometric liquid (mercury).
* The Thermometer reading at thermal equilibrium gives the temperature. Thus temperature is a measure of thermal equilibrium.
Activity - 3
3. Establish the fact that 'the average kinetic energy of molecules is directly proportional to the absolute temperature' through an activitity. (4 Marks)
* Take two bowls one with hot water and the other with cold water.
* Gently sprinkle food colour on the surface of water in both bowls.
* You will observe that the grains of food colour jiggle (move randomly).
* You further observe that the jiggling of grains of food colour in hot water is more when compared to the jiggling in cold water.
* We know that bodies possess kinetic energy when they are in motion.
* As speed of motion of particles (grain of food colour) in the bowls of water is different, we can say that they have different kinetic energies.
* We can thus conclude that the average kinetic energy of the molecules/ particles of hotter body is greater than that of a colder body.
* This means that "the average kinetic energy of the molecules is directly proportional to the absolute temperature".
Activity - 4
4. How can you show through an activity that heat energy flows from a hotter body to a colder body and temperature determines the heat energy flow. (4 Marks)
* Take water in a container and heat it to 60°C.
* Take a cylindrical transparent glass jar and fill half of it with this hot water.
* Very gently pour coconut oil over the surface of water (Care to be taken so that water and oil do not mix).
* Put a lid with two holes on the top of the glass jar.
* Take two thermometers and insert then through the holes of the lid in such a way that the bulb of one thermometer lies only inside the water and the other lies only inside the coconut oil. (See Figure)
* You observe that the reading of the thermometer kept in water decreases, while at the same time the reading of the thermometer kept in oil increases.
* This is because the average kinetic energy of the molecules of oil increases, while the average kinetic energy of the molecules of water decreases.
* It means water loses energy while oil gains energy.
* Heat energy flows from water to oil because of the temperature difference between them.
* This concludes the fact that heat is the energy that flows from a hotter body to colder body.
* Temperature determines the direction of heat (energy) flow, where as heat is the energy that flows.
Activity - 5
5. Verify the statement that the rate of rise in temperature depends on the nature of the substance' by an activity (4 Marks)
A: Take a large jar of water and heat it upto 80 °C.
* Take two identical boiling test tubes with single - holed corks.
* One of them is filled with 50 gm. of Water and the other with 50 gm. of oil both at room temperature.
* Insert two thermometers through holes of the corks, one each into two test tubes.
* Clamp them to a retort stand and place them in the jar of hot water (see the figure)
* Observe the readings in the thermometer every three minutes and note the readings in your note book.
* We have supplied the same amount of heat to water and oil as they are kept in the same hot water for the same interval of time.
* We observe that the rate of rise in temperature of oil is higher than that of the rise in temperature of the water.
* Thus we conclude that the rate of rise in temperature depends on the nature of substance.
Activity - 6
6. Deduce an expression for the specific heat of a substance by performing an activity. (4 Marks)
A: Take 250 ml. of water in one beaker and 1 litre of water in another beaker and note their initial temperatures with a thermometer.
* Heat the water in both the beakers to a temperature of 60°C and note the time required for both to rise to that temperature.
* You observe that more time is required to heat 1 litre of water to 60°C when compared to heat 250 ml. of water.
* It means you need to supply more heat energy to 1 litre water than 250 ml. water for the same change in temperature.
* For the same change in temperature the amount of heat (Q) absorbed by a substance is directly proportional to its mass (m).
∴ Q ∝ m ............. (1)
* Take 1 litre of water in a beaker and heat it over a constant flame note the temperature change (∆T) for every two minutes.
* You notice that the change in temperature rise with time is constant.
* It means for the same mass (m) of water the change in temperature is proportional to the amount of heat (Q) absorbed by it.
∴ Q ∝ m.∆T ............. (2)
* From Equations (1) & (2), We get Q ∝ ∆T (or) Q = m.S.∆T
* 'S' is a constant for a given substance. This constant is called "Specific Heat" of the substance.
Specific heat: The specific heat of a substance is the amount of heat required to rise the temperature of unit mass of the substance by one unit.
* C.G.S. Unit of Specific heat is Cal / g - °C and S.I. unit of it is J/kg - K.
Activity - 7
7. What is the Principle of method of mixtures. Describe an activity to arrive an expression for the final temperature of the mixture. (4 Marks)
A: Principle of method of mixtures: When two or more bodies at different temperatures are brought into thermal contact, then net heat lost by the hot bodies is equal to net heat gained by the cold bodies until they attain thermal equilibrium (If heat is not lost by any other process).
* Take 100 ml of water at 90 °C and 200 ml. of water at 60°C and mix the two.
* Let the initial temperatures of the samples of masses m1 and m2 be T1 and T2 (T1 > T2).
* Let T be the final temperature of the mixture.
* The temperature of the mixture is lower than the temperature of the hotter sample but higher than the temperature of the colder sample.
* This means that the hot sample has lost heat, and the cold sample has gained heat.
* Heat lost by the hot sample = Q1 = m1S (T1 − T)
* Heat gained by the Cold Sample = Q2 = m2S (T − T2)
* According to the Principle of method of mixtures
* Heat lost by the hot sample = Heat gained by the cold Sample i.e. Q1 = Q2
* m1S (T1 − T) = m2S (T − T2)
* This can be simplified to
T (m1 + m2) = (m1T1 + m2T2)
Activity - 8
8. Describe an activity to show that the rate of evaporation increases with blowing wind. (4 Marks)
A: Take few drops of spirit (say 1 ml) in two Petri dishes separately.
* Keep one of the dishes containing spirit under a ceiling fan and switch on the fan.
* Keep another dish with its lid closed. Observe the quantity of spirit in both dishes after 5 minutes.
* You notice that spirit in the dish that is kept under the ceiling fan disappears, where as some spirit is left in the dish that is kept in the lidded dish.
* If air is blown over the liquid surface in an open Pan or Petri dish, the number of molecules at the surface that fly off from the surface are more than the molecules returned to the surface.
* This is because any molecule escaping from the surface is blown away from the vicinity of the liquid.
* This increases the rate of evaporation.
* This is the reason why the spirit in Petri dish, that is kept under the ceiling fan evaporates quickly when compared to that closed.
Activity - 9
9. Explain the Process of Condensation with an activity. (4 Marks)
A: Put a glass tumbler filled half of it with cold water on a table.
* You observe water droplets formed on the outer side of the glass tumbler.
* Air contains water molecules in the form of vapour.
* When the molecules of water in air, during their motion, strike the surface of the glass tumbler which is cool, they lose their kinetic energy which lowers their temperature and they get converted into droplets.
* The energy lost by the water molecules in air is gained by the molecules of the glass tumbler.
* Hence the average kinetic energy of the glass molecules increases.
* In turn the energy is transferred from glass molecules to the water molecules in the glass.
* In this way, the average kinetic energy of water molecules in the tumbler rises and so the temperature of the water in the glass increases.
* This process is called condensation. It is defined as 'The Phase change from gas to liquid'. It is a warming process.
Activity - 10
10. Explain the Process of boiling with the help of an activity. (4 Marks)
A: Put a beaker of water on the burner and note the readings of the thermometer placed in it for every 2 minutes.
* You find that the temperature of the water rises continuously till it reaches 100 °C.
* Beyond 100 °C no further rise of temperature of water is seen. At 100 °C, though supply of heat continues, the temperature does not increase further.
* You observe a lot of bubbling at the surface of water at 100 °C. This is what we call boiling of water.
Process of boiling:
* Water is a solution, there are many impurities dissolved in it including some gases.
* When water or any liquid is heated, the solubility of gases it contains reduces. As a result, bubbles of gas are formed in the liquid.
* Evaporation of water molecules from the surrounding causes these bubbles, to become filled with saturated vapour, whose pressure increases as we increase the temperature of the liquid by heating.
* At a certain temperature, the pressure of the saturated vapour inside the bubbles becomes equal to the pressure exerted on the bubbles from outside (= Atmospheric Pressure + Pressure of the layer of water above the bubble)
* As a result, these bubbles rise rapidly to the surface and collapse at the surface releasing the vapour present in the bubbles into air at the surface.
* This process of converting the liquid into vapour (gas) continues as long as you supply heat. This appears as boiling of water.
Activity - 11
11. By an activity explain the Process of melting.
A: Take small ice cubes in a beaker. Insert the thermometer into ice cubes.
* Start heating the beaker putting it on a burner.
* Note the reading of the thermometer every 1 minute till the ice gets completely melts and gets converted into water.
* You observe, if the temperature of ice is below 0 °C, it goes on changing till it reaches 0 °C.
* When ice starts melting, you will notice no change in tempareture though you are supplying heat continuously.
* The heat energy supplied to the ice increases the internal energy of the molecules of the ice.
* This increase in internal energy of the molecules weakens the bonds as well as breaks the bonds between molecules (H2O) in ice.
* That is why ice (in solid phase) becomes water (in liquid phase).
* This process takes places at a constant temperature of 0 °C or 273 K.
* This temperature is called melting point. This process of converting solid into liquid is called 'Melting'.
Activity - 12
12. Show by an activity that water expands on freezing. (2 Marks)
A: Take a small glass bottle with a tight lid.
* Fill it completely with water and fix the lid tightly in such a way that water does not come out of it.
* Put the bottle into the deep freezer of a refrigerator for a few hours.
* Take the bottle out from the fridge and you will observe that the glass bottle breaks.
* The volume of water poured in the bottle is equal to the volume of the bottle.
* When the water freezes to ice, the bottle is broken. This means that the volume of ice should be greater than the volume of water filled in the bottle.
* In short, We say that water expands (increases in volume) on freezing.
Writer : C. V. Sarveswara Sarma