WBBSE Solutions For Class 8 School Science Chapter 1 Physical Environment Heat Long Answer Type Questions

Chapter 1 Physical Environment Long Answer Type Questions

 

Question 1. When water freezes its volume increases. Cite one advantage and one disadvantage of this property. Mention the factors that affect the melting point of substances.
Answer:

Change of State of substance

Substances generally exist in either of the three forms:

  1. Solid – for example, wood, iron, gold, aluminum, ice, etc.
  2. Liquid – for example, water, alcohol, milk, etc.
  3. Gas or vapor – for example, steam, oxygen, nitrogen, etc.

1. Melting and Freezing

These three states are inter-convertible with Melting (or fusion) meaning the transformation of exchange of heat.

For example, the supply of heat to a solid to its liquid state at some fixed solid ice produces liquid water and supplies temperature by absorption of heat energy. The more heat to water produces steam.

the fixed temperature at which melting occurs is Again, on cooling (i.e. by extracting heat) liquid called the melting point of the substance, and if water is formed from steam and extraction of different for different substances, further heat from the water will produce solid ice.

Freezing (or solidification) is the process of Thus the exchange of heat and plays the most transformation of liquid to its solid state at some important role in the interconversion of the three fixed temperatures by extraction of heat energy, states of matter.

The fixed temperature at which the freezing (or solidification) occurs is called the freezing point of the substance.

The melting point and freezing point of metals and crystalline solids are the same under the same pressure. For example, the freezing point of water and melting point of ice at normal pressure (1 atmospheric pressure) is 0°C.

But non-crystalline substances like wax, glass, butter, pitch, etc. melt and freeze at two different temperatures. For example, the butter melts at a temperature between 28°C to 37°C and freezes between 20°C to 25°C.

2. Change of volume during melting and solidification

Normally, a solid expands in volume on melting,, and liquid contracts on freezing. So, in general, the density of the liquid is less than that of its solid state.

An example is wax. But water is an exception. In the case of water, the density of ice (solid state) is less than that of water (liquid state). So, during freezing, water expands in volume as it becomes solid.

Experiment -1

Let us take two test tubes – one is filled with wax and the other is filled with ice. Both test tubes are heated separately to melt the wax and ice completely.

Now some solid wax is carefully dropped into the test tube containing liquid wax and some solid ice is dropped into the test tube containing water.

It is found that the wax sinks in the molten wax and the ice floats on the water

Inference: This confirms that solid wax has a higher density than molten wax (liquid state). Liquid wax has less density because the volume of wax increases as solid wax melts.

But the density of ice is less than that of water. This is because the volume of water increases as it freezes to ice When water freezes its volume increases. This phenomenon has certain advantages as well as disadvantages.

Disadvantages: As water freezes, its volume expands. Hence in cold countries in the winter season, pipelines used for water supply or water pipes in the radiator of cars are often found to crack and burst.

Sometimes liquid water penetrates within the solid rocks through cracks. In winter, when this water freezes, it expands and puts enormous pressure on the rocks and stones from inside.

As a result, rocks burst into pieces and may cause landslides in the hilly regions.

Advantages: The density of ice is less than that of water, and floats on water. The water underneath these ice blocks supports aquatic life.

Metallic mementos are prepared using certain substances like brass, cast iron, etc. Molten substances are poured into the molds for preparing these mementos.

When freezes, the volume of these substances increases and fits tightly against the molds. As a result, the intricate and delicate designs are properly, imprinted on the solidified material.

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3. Factors Affecting The Melting Point Of A Substance

The melting point of a solid substance depends on two factors:

  1. The pressure applied to the substance
  2. Presence of impurities in the substance
  3. Effect of pressure on the melting point of a substance

For those substances whose volume increases due to melting, their melting point increases with increasing pressure.

For example, copper, gold, etc. Increased pressure resists the volume increase, thus increasing the melting point.

Melting point of wax increases by approximately 0.04°C due to an increase of pressure by one atmosphere.

For those substances, whose volume decreases due to melting, their melting point decreases with increasing pressure. For example, ice, brass, cast iron, etc.

Increasing pressure helps in melting by decreasing their volume. The melting point of ice decreases by approximately 0.0007°C due to an increase of pressure by one atmosphere.

When two pieces of ice are pressed together for some time and then released, they form a single lump. This is because when pressure is applied, then at the contact area between two pieces of ice, the melting point decreases.

So some ice melt at the contact point. When pressure is released, the melting point again returns to the original value.

So the water formed at the contact area again freezes, forming a single lump of ice. This is called regelation.

Effect Of Presence Of Impurity In The Substance On Melting Point

The presence of impurities or presence of any other substance decreases the melting point of the substance. For example,

1. Melting point of ice is 0°C at normal pressure. But if some salt is added, its melting point becomes much lower than 0°C.

Again, a mixture of metals (called alloys) has a melting point lower than the melting point of any of its constituents.

[Example: The melting point of a Fuse wire used in safety fuse, is made up of lead and tin. The melting point of their mixture is lower than the melting point of either of them.

When excess current flows through this wire, heat is produced and the wire melts easily thereby preventing further passage of electricity through it and thus preventing fire hazards.

A freezing mixture is prepared by mixing two substances. At the existing temperature, one of them should melt requiring heat for this.

This heat is taken from the mixture and as a result, the temperature of the mixture falls. When salt is mixed with ice, its temperature decreases. This mixture is called a freezing mixture.

When ice and common salt are mixed in a 3:1 weight ratio, part of the ice undergoes melting and the heat required for this is taken from the salt mixture as a result of which the temperature becomes approximately – 23°C.

This freezing mixture is frequently used for the preservation of fish, meat, etc. and to carry medicine at low temperatures from one place to another place.

Question 2. Explain the principle of action of the pressure cooker.
Answer: The severity of burn caused by steam at 100°C is greater than that by water at 100*C – explain why?
4 pressure cooker consists of a container and the lid is provided with a rubber gasket which does not allow the steam to go out of the container.

In a pressure cooker, the steam formed from boiling water is not allowed to escape. This increases the pressure inside the cooker and so, water boils at a higher temperature.

The time required to cook food is greatly reduced if water is made to boil at a higher temperature than its boiling point. The pressure cooker raises the boiling point of water to nearly 120°C.

1 g of steam at 100°C contains 537 cal of more heat than lg of water at 100°C since the latent heat of vapourization of water at normal pressure is 537 cal/g.

Due to this greater heat content, the severity of a burn is more in the case of steam than that of water at the same temperature.

Question 3. Discuss briefly the characteristic properties of evaporation.
Answer:

Evaporation

It is our common experience that wet clothes get dry when left in the sunlight for some time. This is because of the evaporation of water from wet clothes.

During this process, the water on the surface of the cloth takes up heat from the surrounding and is slowly converted into vapor.

All liquids undergo evaporation but the rate of evaporation of volatile liquids such as spirits and alcohols is very fast.

Definition: Evaporation is a process in which liquid is slowly and gradually converted into vapor. It takes place from the surface of the liquid and it can occur at any temperature and pressure.

Characteristic properties of evaporation

1. More the exposed area of the liquid, more will be the rate of evaporation. For example, a folded sari left in the sun will take more time to dry compared to a sari left for drying in the sun without any fold.

2. Evaporation can occur at any temperature. No particular temperature is needed. But the higher the temperature the faster will be the rate of evaporation.

For example, a wet cloth becomes dry even in the winter season, but the time taken for that will be more than the time required during the summer.

3. If air flows over the liquid, it facilitates the process of evaporation and the rate of evaporation becomes faster.

Wet clothes become dry quickly if air flows over them. They dry up faster in the winter season than in the rainy season (when the moisture content in the air is very high).

4. The rate of evaporation depends on the nature of the liquids. Volatile liquids like spirits, alcohol, etc. evaporate more quickly than water.

Rate of evaporation increases as air pressure decreases.

Question 4. What is dew? Why dew is formed at the dawn?
Answer:

Condensation

This is the reverse process of vaporization. This is the phenomenon by which vapor is transformed
In summer, when we sweat heavily, it feels comfortable if we stand beneath a fan.

This is because the airflow produced by the fan facilitates into liquid. The latent heat of condensation is equal to the latent heat of vapourization. Condensation of water vapor in the air produces clouds, fog, etc. in nature.

1. Cloud: The water that evaporates from sea, ocean, river, lakes, etc. mixes with hot air. Air containing water vapor is lighter than “dry” air which contains much less water vapor.

So, lighter air goes upwards towards the higher altitudes. But with increasing altitude, the air pressure reduces. The air cools down and the water vapour of the air condenses as water droplets on the tiny dust particles floating in the air, forming clouds.

2. Fog: It is often seen during dawn in the winter season. During daytime, water evaporates and water vapor thus formed mixes with the air.

At night, when the temperature falls, the air becomes saturated with water vapor. It then condenses as water droplets on suspended dust and smoke particles, coal dust, etc.

And floats in the air, thus forming fog. In big cities and industrial belts, where the pollution level is significantly high, the air contains more dust and smoke particles, thus increasing the chance of fog formation.

At noon, the fog disappears, because, at higher temperatures, the small water droplets are again converted to water vapor.

3. Dew: In the winter season, dew is found on leaves and grasses in the morning. During the daytime, the surface of the earth and objects near it becomes hot.

But after sunset, the earth gradually cools down by radiating heat. The air in contact with the earth’s surface also cools down and when it becomes sufficiently cold, it becomes saturated with water vapor.

The water vapor then condenses into water droplets on the cold surface of leaves and grasses, which is called dew.

Dew is not formed immediately after sunset but is formed in the later part of the night and at the dawn. This is because it takes time to cool down the earth’s surface temperature so that water vapor can condense.

Question 5. How fog is formed? Why does it disappear at noon? While sweating heavily in summer, why do we feel comfortable under a fan?
Answer:

Condensation

This is the reverse process of vaporization. This is the phenomenon by which vapor is transformed
In summer, when we sweat heavily, it feels comfortable if we stand beneath a fan.

This is because the airflow produced by the fan facilitates into liquid. The latent heat of condensation is equal to the latent heat of vapourization. Condensation of water vapor in the air produces clouds, fog, etc. in nature.

1. Cloud: The water that evaporates from sea, ocean, river, lakes, etc. mixes with hot air. Air containing water vapor is lighter than “dry” air which contains much less water vapor.

So, lighter air goes upwards towards the higher altitudes. But with increasing altitude, the air pressure reduces. The air cools down and the water vapour of the air condenses as water droplets on the tiny dust particles floating in the air, forming clouds.

2. Fog: It is often seen during dawn in the winter season. During daytime, water evaporates and water vapor thus formed mixes with the air.

At night, when the temperature falls, the air becomes saturated with water vapor. It then condenses as water droplets on suspended dust and smoke particles, coal dust, etc.

And floats in the air, thus forming fog. In big cities and industrial belts, where the pollution level is significantly high, the air contains more dust and smoke particles, thus increasing the chance of fog formation.

At noon, the fog disappears, because, at higher temperatures, the small water droplets are again converted to water vapor.

3. Dew: In the winter season, dew is found on leaves and grasses in the morning. During the daytime, the surface of the earth and objects near it becomes hot.

But after sunset, the earth gradually cools down by radiating heat. The air in contact with the earth’s surface also cools down and when it becomes sufficiently cold, it becomes saturated with water vapor.

The water vapor then condenses into water droplets on the cold surface of leaves and grasses, which is called dew.

Dew is not formed immediately after sunset but is formed in the later part of the night and at the dawn. This is because it takes time to cool down the earth’s surface temperature so that water vapor can condense.

Question 6. Describe a demonstrative experiment to show that water is a bad conductor of heat.
Answer:

The flow of Heat: Conduction, Convection, and Radiation

To carry heat from one part of an object to its other part or from one object to another object is called transfer of heat.

Transfer of heat energy from one place to another with or without the help of a medium can take place in three ways

  1. Conduction
  2. Convection
  3. Radiation.

1. Conduction

It is our common experience that when one end of a long metal rod is heated, the other end of it gradually warms up.

WBBSE Soulutions for class 8 Chapter 1 Physical environment heat burner

 

Obviously, heat propagates from the hotter region of the rod to its cooler end, without any actual movement of its constituent particles. This process of transfer of heat is called conduction.

Definition: The process by which heat energy is transmitted through a substance from a region of higher temperature to a region of lower temperature without any movement of its constituent particles from one place to another is called conduction.

By the process of conduction, heat is not only transferred within the same object but heat is also transferred from one object to another provided:

The two objects are in contact with each other, and They are at different temperatures

So long as there is a temperature difference between the two objects (or between two regions at the same object), the transfer of heat continues.

Once the temperature of the two objects (or two regions of the same object) becomes the same, heat transfer ceases.

The substance through which heat can flow easily is called a good conductor of heat. The substances through which heat cannot flow easily are called the bad conductors of heat.

Generally, metals are good conductors of heat. For example, copper, steel, brass, etc. are good conductors. So cooking utensils (sometimes the base of the cooking utensils) are made up of these materials.

So, heat quickly reaches the foodstuffs that are being” cooked. Substances like wool, stone, glass, wood, water, air; diamond, paper, etc. are bad conductors of heat. Generally, most liquids and gases are also, bad conductors.

1. Demonstrative Experiments On Conduction Of Heat Describe a demonstrative experiment to show that water is a bad conductor of heat.

Apparatus needed: A glass test tube, some water, a small piece of ice, some iron wire, a test tube holder, bunsen burner.

Procedure: Let us take a small piece of ice and wrap it with iron wire such that when dropped inside a test tube filled with water, it sinks to the bottom of the test tube.

Now, by holding the test tube in a tilted position with the help of a test tube holder, the upper layer of water in the test tube is strongly heated till it starts boiling.

Observation: Even when the upper layer of water starts boiling, it is found that the piece of ice wrapped with iron wire does not melt.

WBBSE Soulutions for class 8 Chapter 1 Physical environment ice wraped

Inference: This shows that water is a bad conductor of heat. Because when the heat energy was supplied to the upper layer of water, the water in the upper part of the test tube started boiling.

But the heat energy was not transferred to the lower part of the test tube. So the ice did not melt.

Question 7. Describe a demonstrative experiment to show that the volume of ice decreases as the ice melts, but the volume of wax increases as it melts. What is the value of latent heat of vapourization of steam in an SI system?
Answer:

Apparatus needed: Two glass test tubes, some water, some wax, a small piece of ice, a small piece of wax, Bunsen burner.

Procedure: Let us take two test tubes – one is % filled with wax and the other is filled with ice.

Both the test tubes are then heated separately ‘to completely melt the wax and ice. Now some solid wax is carefully dropped into the test tube containing liquid wax and some solid ice is dropped into the test tube containing water.

Observation: It is found that the wax sinks In the molten wax and the ice floats on water.

Inference: The solid wax sinks in the molten wax. This confirms that solid wax has a higher density than molten wax (liquid state).

Liquid wax has less density because the volume of wax increases as solid wax melts. But ice floats on water.

This proves that the density of ice is less than that of water. This is because the volume of water decreases as it freezes to ice.

The latent heat of vapourization of steam as per the SI system is 2260000 J/kg.

Question 8. Write short notes on Sea breeze and Land breeze.
Answer:

Sea Breeze And Land Breeze

Water has high specific heat than that land masses. So, during day time, absorption of heat by water will produce less temperature increase compared to the land masses.

As a result, the air above the warmer land mass becomes more warm compared to air over the sea. So, the air over the land moves up and creates a partial vacuum.

Relatively colder air from the sea moves toward the land to fill up the vacuum. This flow of air from the sea towards land is called sea breeze.

After sunset, both land and sea will radiate heat and become cooler. But due to the low specific heat of land mass, the rate of decrease of temperature of the land mass will be much more than that of seawater (having higher specific heat).

So at night, the land mass becomes colder than seawater. Now, the air over the sea will become warmer and moves upwards, creating a partial vacuum.

This is filled up by the movement of colder air from the land. This flow of air from land toward the sea is called the land breeze.

WBBSE Soulutions for class 8 Chapter 1 Physical environment Day time breeze and night time breeze

3. In a refrigerator the cooling unit is placed at the top, Why?
The reason is that air in contact with the cooling unit is cooled and becomes heavier. So it moves to a lower region.

To fill the vacuum formed near the said unit, warmer air from other parts of the refrigerator moves up. This air also cools and moves downwards. Thus the whole refrigerator is cooled.

Had the cooling unit been kept at the bottom, a small quantity of air would have been cool. This air being heavier would have remained at the bottom.

The warmer and lighter air would never come down near the cooling unit. Thus there would have been little and partial cooling inside the refrigerator,

Question 9. In a refrigerator, the cooling unit is placed at the top, not at the bottom. Why? Calculate the amount of heat required to raise the temperature of 150g of water from 30°C to its normal boiling point.
Answer:

In a refrigerator, the cooling unit is placed at the top, not at the bottom.

H = m.s. (t2 – t2)

Here, m = 150g, s = 1 cal/g.°C, (t2 – t1)

= (100 – 30) °C = 70°C

The heat required, H = 150 x 1×70 = 10, 500 cal.

Question 10. What are the properties of the heat transmitted by the radiation process?

Uses Of Bad Conductors Of Heat

1. Handles of cooking utensils are generally made up of or covered with bad conductors such as wood, cane, and bakelite so that the utensil can be held by the handle with our hands even when the rest of the utensil is very hot.

2. In winter, it feels warmer and more comfortable, if we wear two garments made up of thin threads rather than one made up of thick threads, the total thickness of the garments being the same in both cases.

This is because when we wear two garments, the air is trapped between the two. Air being a bad conductor of heat prevents the outflow of heat from our body to the colder surroundings.

So two shirts, one above the other, give more warmth than a single shirt of thickness equal to that of the two together.

2. Woollen clothes are more comfortable in winter. This is because wool itself is a bad conductor of heat. It has innumerable tiny pockets of air enclosed within its fibers.

Air is also a bad conductor of heat. So wool and air pockets together prevent the outflow of heat from our body to the colder surroundings.

4. Ice blocks are covered with sawdust. Saw dust is a bad conductor of heat. It prevents the easy flow of heat from the hotter surroundings to the ice blocks. Thus melting of ice is partly avoided.

5. Ice is a bad conductor of heat. So igloos are built with ice blocks. When the temperature of the surroundings is much below zero degrees centigrade, it is warmer inside the igloos.

6. Building materials should be bad conductors of heat. Hay and mud are bad conductors of heat. So mud houses or houses made up of hay are warmer in winter while colder in the summer.

7. ln winter, birds fluff their feathers. This is because when birds bulge out feathers, layers of air are trapped within the feathers. These layers of air prevent the outflow of heat from their body to the colder surrounding.

8. Table mats are made of poor conducting materials like rubber, spun jute, etc. This is because when hot utensils are placed on it, heat from the hot utensils cannot pass on easily through the mat, and the surface of the wooden table is not damaged.

9. In winter, a new cotton quilt is more comfortable than an old one. In a new quilt, cotton fibers remain in a loose state with plenty of air pockets in between.

Air being a poor conductor of heat prevents the flow of heat through it. But in an old quilt, fibers are in a compressed state, and the quantity of trapped air within the fibers is small. So, it is less effective in preventing heat flow across it.

10. Water is a bad conductor of heat. So the upper surface of the water of a pond is warmer in summer than its lower surface.

In winter, the situation is reversed. The upper layer of water in the pond is colder compared to its bottom layers.

11. A thick glass tumbler is cracked when hot water is poured on it. This is because glass is a poor conductor of heat.

So, when hot water is poured into it, the part of the glass in direct contact with the hot water is heated and it expands.

But glass is a poor conductor, the heat is not evenly distributed throughout the tumbler. So the heated portion (in direct contact with hot water) expands more than the portion which is not in direct contact with hot water. Due to uneven expansion of a different portion of the same tumbler, it cracks.

Question 11. Which one is preferable as building material in or hay and mud? Why? Explain how all processes of heat conduction are prevented in a thermos flask.
Answer:

Building materials should be bad conductors of heat.

Hay and mud, being bad conductors of heat, make the house warm in winter and cool in summer. Tin, on the other hand, is a good conductor of heat and it allows outside heat to come in during summer and also allows inside heat to go out during winter.

Thus a house made of tin is not comfortable. The different forms of heat loss are minimized in thermos flasks in the following ways:

Conduction loss: Since there is a vacuum between the two walls, heat cannot be conducted by means of conduction. Further, the cork cap and glass wool (insulator) prevent the conduction of heat.

  1. Convection loss: Vacuum also prevents convection loss of heat.
  2. Radiation loss: The shining glass minimizes radiation loss.

Question 12. Mention one difference between sensible heat and latent heat.
Answer:

Difference between sensible heat and latent heat

lOg of ice at 0°C is melted to water at 0°C. The temperature of the water is further increased to 10°C. Calculate the total amount of heat required in the process.

Sensible heat is expended to raise the temperature of a substance. Latent heat is expended to bring about a change in the molecular structure of a substance to facilitate a change of state without altering the temperature of the substance.

The Total Heat Required Is Calculated In Two Steps:

1. Melting of ice at 0°C:

H1 = mL Here, m = lOg of ice L = 80 cal/g.

∴H1 = 10 x 80 = 800 Cal.

(ii) Raising the temperature of water:

H2 = ms (t2-t1) Here, m = 1Og of water (at 0°C) s = 1 cal/g °C

(t2-t1) = 10°C H2 = 10x1x10 = 100 cal.

∴H1 + H2 = 800 + 100 = 900 cal of heat.

Question 13. State the differences between evaporation and boiling.
Answer:

Differences between evaporation and boiling

Evaporation Boiling
1 Evaporation can occur at all temperatures. 1 At a particular pressure, boiling occurs at a particular temperature.
2 Evaporation can occur at any pressure. 2 During boiling, the pressure of vapor over the liquid surface equals the atmospheric pressure.
3 Evaporation takes place only from the upper surface of a liquid. 3 Boiling occurs at any part of a liquid.
4 The heat energy required for evaporation is gathered from the liquid itself. 4 For boiling latent heat is required and this must be supplied from outside.
5 If the temperature is increased, the rate of evaporation is increased. 5 During boiling, the temperature remains constant till all the liquid is vaporized.
6 Evaporation is a slower process. 6 Bolling is a faster process.

 

Question 14. Compare Conduction, Convection, and Radiation.
Answer:

Comparison Of Conduction, Convection, and Radiation

Conduction Convection Radiation
1. In this process of transfer of heat, a medium is required. 1. In this process of transfer of heat, a medium is required. 1. In this process for the transfer of heat, no medium is required. Radiation can occur even in absence of any intervening medium.
2. During the conduction of heat, the temperature of the medium is increased. 2. During the convection of heat, the temperature of the medium is increased. 2. During radiation of heat through any intervening medium, the temperature of the medium remains unchanged.
3. This is a slower process. 3. This is a slower process. 3. This is comparatively a faster process for the transfer of heat energy from one place to other.
4. During this process of heat transfer through a substance, there occurs no movement (or displacement) of the particles. constituting the substance. 4. During this process of heat transfer through a substance, the heat is transferred from one place to another by the actual movement of the particles constituting the substance. 4. Since no intervening medium is required for this process of heat transfer, the movement of the particles constituting the medium does not take place at all.
5. In this process, heat energy is transferred along a linear or curved path. 5. In this, process, heat energy is transferred along a linear or curved path. 5. In this process, heat energy is transferred only along a linear path.
6. Conduction occurs through all the physical states of matter (viz. solid, liquid, and gas). Generally, conduction occurs most effectively through solids. 6. Convection occurs through liquid and gas only. It does not occur through solid substances. 6. Radiation can take place through any medium, provided it allows the passage of heat energy through it.

 

Question 15. Calculate the amount of heat required to raise the temperature of a copper vessel of mass 1OOg from 50°C to 90°C.
Answer:

(Given, the specific heat of copper = 0.09 cal/g. °C). During the heating of water, the burner is placed at the top of the vessel containing water. It is found that heating has not occurred properly. Why?

Numerical problem: Do it yourself.

Water is heated by convection currents in which hot water rises and cold water sinks again and again till the entire mass is heated.

the water to be heated. Convection does not occur if water is heated at the top. If water taken in the vessel is heated at the top, then the hot water (being lighter) stays at the top of the vessel.

As a result, the water at the bottom of the vessel remains cold in such a case since no hot water can sink from the top to down below towards the bottom of the vessel. Hence, heating does not occur uniformly.

 

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