Chapter 1 Physical Environment Short Answer Type Questions
Question 1. What is meant by specific heat? What is its unit in CGS and SI unit? The specific heat of copper is 0.09 cal/g.°C. What do you mean by that?
Answer:
Specific Heat:-
Specific heat of a substance is the heat required to raise the temperature of the unit mass of the substance by 1°C.
So, a unit of “s” is calorie per gram per degree centigrade (cal/g.°C) (in CGS unit) and Joule per kilogram per degree centigrade (j/kg.°C) (in SI unit).
The specific heat of copper is 0.09 cal/g.°C. This means that to raise the temperature of 1 gram of copper by 1°C, 0.09 calorie heat energy is required.
Question 2. What do you mean by latent heat?
Answer:
Latent Heat:-
When a substance undergoes a change of state without changing temperature it either absorbs or gives out a certain quantity of heat. Latent heat is the quantity of heat absorbed or liberated by a substance of unit mass when it undergoes a change of state at a particular temperature.
Question 3. The latent heat of the fusion of ice is 80 cal/g. What do you mean by that?
Answer:
Given That
The latent heat of the fusion of ice is 80 cal/g.
This means the amount of heat required to effect the change of state of 1 g of ice at OX and 1 atmospheric pressure to lg of liquid water at OX and 1 atmospheric pressure is 80 calories.
Question 4. What do you mean by the melting point and boiling point & of a substance?
Answer:
Melting Point And Boiling Point Of A Substance
The melting point of a pure substance is the temperature at which the substance undergoes a change of state from solid to liquid under a particular pressure.
For example, at 1 atm pressure, the melting point of ice is 0°C. The ice melts at this condition and the temperature remains constant till all the ice melts.
The boiling point of a pure substance is the temperature at which the substance undergoes a change of state from liquid to gas at a particular pressure.
For example, at 1 atm pressure, the boiling point of pure water is 100°C. Water starts boiling at this condition and the temperature remains constant till all the water vaporizes.
Read And Learn More WBBSE Solutions For Class 8 School Science Short Answer Type Questions
Question 5. When two pieces of ice are pressed together for some time and then released, they form a single lump. Why?
Answer:
This is because when pressure is applied, then at the contact area between two pieces of ice, the melting point decreases.
So some ice melts. 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.
Question 6. Discuss briefly the effect of the presence of an impurity in the substance on its melting point. The presence of impurities or presence of any other substance decreases the melting point of the substance.
Answer:
For example, the melting point of ice is 0eC 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.
[For example The melting point of a mixture of 87% (by weight lead in antimony is 246X, while the melting point of pure lead is 327X and that of pure antimony is 631X.]
Question 7. In summer, when we perspire heavily, it feels comfortable if we stand beneath a fan. Why? In summer, when we perspire heavily, it feels comfortable if we stand beneath a fan.
Answer:
This is because the airflow produced by the revolving fan facilitates the process of evaporation. When sweat evaporates from our body it takes up heat from our body. As a result, the body cools down, making us feel comfortable.
Question 8. In summer dogs are very often seen panting. Why? The body of the dog is covered with furs. So, sweat is not produced on their body. They feel comfortable by hanging out their tongues and keep panting.
Answer:
Due to this the liquid on top of the tongue evaporates, taking up heat from the body, thus bringing down the body temperature.
Question 9. During cooking, the cooking utensil is generally covered with a lid. Why?
Answer: When covered with a lid, the vapor produced in the utensil cannot escape easily, and increases the pressure over the liquid surface.
We know that as pressure increases, the boiling point increases. So, water boils at a higher temperature (greater than 100X). So the food materials get properly boiled and cooked faster.
Question 10. What do you mean by “freezing mixture”? When salt is mixed with ice, its temperature decreases. This mixture is called a freezing mixture.
Answer:
When ice and common salt are mixed in a 3:1 weight ratio, the temperature becomes approximately – 23°C. This is a freezing mixture.
This is frequently used for the preservation of fish, meat, etc. and to carry medicine at low temperatures from one place to another place.
Question 11. How the cloud is formed?
Answer:
Formation Of Cloud:
The water that evaporates from sea, ocean, river, lakes, etc. mixes with hot air. Air containing water vapor is lighter than “dry” air (i.e., having much less water vapor).
So, it goes upwards towards a higher altitude. But with increasing altitude, the air pressure reduces. The air cools down and the water vapor of the air condenses as water droplets on the tiny dust particles floating in the air, forming clouds.
Question 12. What do you mean by good conductor and bad conductor of heat? Give examples.
Answer:
Good Conductor And Bad Conductor Of Heat
The substitute which heat can flow easily is called the educator of heat
The substance rough in which heat cannot flow easily is called a bad conductor of heat Generally metals are good conductors of heat. For example, copper, and steel.
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.
Question 13. Explain the reason(s).
Answer:
- Handles of cooking utensils are generally made up of bad conductors.
- In winter, wearing two shirts is more comfortable than wearing a single shirt made up of thick threads.
- Woolen clothes are more comfortable in winter.
- Ice blocks are covered with sawdust.
- Igloos are made of ice.
- In winter, birds fluff their feathers.
- table mats are made of poor conducting materials.
Question 2. In winter, a new cotton quilt is more comfortable than an old one.
Answer:
A thick glass tumbler cracks when hot water is poured on it.
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.
In winter, it feels warmer and more comfortable, if we wear two clothes made up of thin threads rather than one cloth made up of thick threads, the total thickness of the clothes being the same in both cases.
This is because when we wear two clothes, 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.
Question 3. Woolen 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.
Answer:
So wool and air pockets together prevent the outflow of heat from our body to the colder surroundings.
Question 4. Ice blocks are covered with sawdust. Saw dust is a bad conductor of heat.
Answer:
It prevents the easy flow of heat from the hotter surroundings to the ice blocks. Thus melting of ice is delayed.
Question 5. Ice is a bad conductor of heat. So igloos are built with ice blocks.
Answer:
When the temperature of the surroundings is much below zero degrees centigrade it is warmer inside the igloos.
Question 6. In winter, birds fluff their feathers.
Answer:
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.
Table mats are made of poor-conducting materials like rubber, spun jute, etc. This is because when hot utensils are placed on it,
the heat from the hot utensils cannot pass on easily through the mat, and the surface of the wooden table is not damaged.
Question 7. In winter, a new cotton quilt is more comfortable than an old one.
Answer:
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.
A thick glass tumbler is cracked when hot water is poured into 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 evenly 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 the uneven expansion of different portions of the same tumbler, it cracks.
Question 14. Explain why fuse wire is made up of lead and tin.
Answer:
The melting point of a mixture of lead and tin 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 at lower temperatures, thereby preventing further passage of electricity through it and thus protecting the costly and important electrical instruments and gadgets.
Question 15. During winter, we must not ignite a kerosene lamp or fire inside a confined room-explain why?
Answer:
During winter, if some kerosene lamp or fire is ignited inside a confined room, then it may prove fatal to sleep inside that room.
Since kerosene lamp or fire is burning, oxygen is continuously converted to carbon dioxide and poisonous carbon monoxide.
Since it is winter, doors, and windows are tightly shut, which prevents fresh air from entering the room.
Gradually the concentration of oxygen decreases and the concentration of poisonous carbon monoxide gas within the room increases and it may eventually kill a human being if exposed to this poisonous gas for a longer time.
Question 16. What is trade wind?
Answer:
Trade Wind:-
The equatorial regions which receive sun rays directly are very hot compared to the polar regions, where the sun rays fall obliquely.
The hot air of the equatorial zones being lighter rises up and the colder air from the polar regions fills this gap. This flow of air from the polar region to the equatorial region is called trade winds.
Question 17. Eagles can fly at a high altitude without flapping their wings. Why?
Answer:
This is possible because if they are flying over the rising convection currents of air, which is moving upwards, then this lends support to float and they can remain floated without flapping their wings.
Question 18. Explain with a reason(s).
Answer:
- In summer we prefer to wear white clothes.
- Cloudy nights are hotter than cloudless nights.
- The outer surface at bottom of the cooking utensils is coated black.
In summer we prefer to wear white clothes. Since the white surface is a good reflector and a poor absorber of heat,
the white clothes we wear reflect most of the radiant heat of the sun and so we are relieved of intense heat.
Cloudy nights are hotter than cloudless nights. The reason is, after sunset, the heated surface of the earth begins to cool down by radiating heat.
If there is a cloud, the heat radiated by the earth is reflected by the clouds back to the earth. So the earth remains warm.
On a cloudless night, the earth is cooled by radiating the absorbed heat (absorbed during day-time) during the night without any chance of being reflected back to the earth.
The outer surface at bottom of the cooking utensils is coated black. The reason is that a blackened surface absorbs heat very well and so cooking is done more quickly with such a utensil than with a new one with a polished outer surface.
Question 19. Give examples of two instances in our day-to-day life where heat is transferred by radiation through the air. Instances of radiation in our daily life
Answer:
When we sit in front of a room heater, we feel the heat radiated out by the heater. When we come out in the sunshine, we feel hot due to solar radiation.
When we stand next to a burning fire, we can feel the heat of the fire in our bodies. This heat is transferred from the fire to our body through radiation.
Question 20. One end of the objects like a steel spoon, a plastic scale, and a divider is put in a beaker of hot water. In which of these objects the other end will get hot?
Answer:
Steel spoons and dividers are made of metals which are good conductors of heat Hence they conduct heat quickly and as a result, the other ends get hot within a short span of time.
Since a plastic scale is a bad conductor of heat, it does not conduct heat rapidly and hence, it heats up very slowly.