Class 6 Science Chapter 11 Important Questions

• Have you ever seen air?
  We might not have seen air but its presence can be felt. As we notice it, when the leaves of the trees rustle or the clothes hanging on a clothes-line sway. Pages of an open book begin fluttering when the fan is switched on. The moving air makes it possible for you to fly your kite and so on.
• Why do you think mountaineers carry oxygen cylinders with them, while climbing high mountains?
  As we go higher up in the atmosphere the amount of air becomes less and less. The air at the top of very high mountains is so thin that it does not have enough oxygen for the people to breathe properly. So, the people who climb by mountains called mountaineers, carry cylinder containing “oxygen gas” with them. They breathe in oxygen from these cylinders, so as to survive on high mountains.

1. How can you say that animals and plants are dependent on each other for their survival?
   Both plants and animals are interdependent on each other for their survival. Plants cannot survive for long without animals, as they need CO2 for photosynthesis, which is exhaled by animals. Similarly, in the absence of plants, there will be no oxygen for animals to breathe in. Hence, we can say that both need each other and the balance of Oxygen and Carbon dioxide in atmosphere is maintained by their presence.
2. Give an experiment to show that one-fifth of air is Oxygen.
   Take a candle and fix it at the centre of a glass vessel. Add some quantity of water in the vessel. Now place an empty dry glass jar over it. Mark five lines at an equal distance above the water surface. Now, carefully light the candle and cover the glass jar. Wait for a few seconds and how the flame will be extinguished due to absence of oxygen to burn. Now, we can observe that the water level raised in the glass jar is 1/5 of the volume of air present in the glass jar.
   This experiment shows that one-fifth of air is Oxygen.

Take an empty glass bottle. Turn it, upside down. Now, dip the open mouth of the bottle into the bucket filled with water. Observe the bottle. Water does not enter the bottle. Now tilt the bottle slightly. We can see bubbles coming out of the bottle or hear any bubbly sound. It is “air”, that was present in the bottle. The bottle was not empty at all.
In fact, it was filled completely with air even when you turned it upside down. That is why we notice that water does not enter the bottle when it is in an inverted position, as there was no space for air to escape. When the bottle was tilted, the air was able to come out in the form of bubbles, and water filled up the empty space that the air has occupied.
This activity shows that air occupies space. It fills all the space in the bottle. It is present everywhere around us.

1. What do you mean by “atmosphere”?
   Our earth is surrounded by a thin layer of air. This layer extends up to many kilometres above the surface of the earth and is called atmosphere.
2. “Breathing through mouth may harm you”. How?
   The presence of dust particles in air varies from time to time, and from place to place. We inhale air when we breathe through our nostrils. Fine hair and mucus are present inside the nose to prevent dust particles from getting into the respiratory system.
   Breathing through mouth may harm us because we may inhale dust if present in air which may enter our respiratory system (lungs, etc.) and damage our health.
3. Why should you not sleep under the trees during the night?
   We should not sleep under the trees during the night because during night, trees release carbon dioxide and excess of carbon dioxide can cause suffocation.

Our earth is surrounded by a thin layer of air. This layer extends up to many kilometre above the surface of the earth and is called atmosphere.
Take an empty glass bottle. Turn it, upside down. Now, dip the open mouth of the bottle into the bucket filled with water. The water does not enter into the glass bottle because the bottle is filled with air. All the spaces in the bottle is occupied by air. The air present in empty glass bottle prevents the water from entering it.
If we tilt the bottle hold in water, we will find that the air present in the bottle goes out in the form of air bubbles. As the air form the bottle escapes, water starts entering the glass bottle. Actually the space vacated by the air leaving the bottle is now occupied by water. A glass bottle which appear to be empty to us is actually filled with air. This activity shows that air occupies space.

• Why does the transparent glass of windows, if not wiped out regularly appears hazy?
  The dust particles present in air stick to transparent glass of windows. These particles also attract other elements like pollen to collect on glass. That’s why, transparent glass of windows, if not wiped appears hazy.
• Earthworms comes out of the soil during heavy rains. Why?
  When it rains heavily, water fills up all the spaces occupied by the air in the soil. In this situation, animals living in the soil such as earthworms have to come out for respiration.
• How does weathercock (wind vane) work?
  It shows the direction, in which the air is moving at that place. Thus, if the wind is blowing from the north, the arrow will point towards the northern direction.

1. On a Sunday morning, Paheli’s friend visit her home. She wanted to see some flowering plant in the nearby garden. Both of them went to the garden. While returning from the garden they also observed some flowering plants on the roadside. But to their surprise they found that the leaves and flowers of these roadside plants were comparatively very dull. Can you help them to know why?
   The leaves and flowers of the roadside plants were comparatively very dull due to the presence of air pollutants in this region emitted from vehicles, industries etc. The air pollutants include dust particles, harmful gases, soot etc, which made the leaves and flowers appear dull.
2. Garima observed that when she left her tightly capped bottle full of water in the open in sunlight, tiny bubbles were formed all around inside the bottle. Help Garima to know why it so happened.
   It happened because air is dissolved in water in the form of oxygen. When water bottle is left in the open sunlight, tiny bubbles were formed all around inside the bottle because air dissolved in water starts escaping in the form of tiny bubbles due to the heat from the Sun.

We know that various gases are found in air. Thus, some organisms live on land, some in water and some under the soil. But all of the organisms need oxygen to respire. The terrestrial organisms take oxygen from the atmosphere. During respiration by living beings and burning of materials, carbon dioxide is produced along with some other gases. Also CO2 is used by plants in the process of photosynthesis in which oxygen is released in the atmosphere.
Thus, the concentration of oxygen in the air is maintained. It is very important to sustain life on the earth.
Carbon dioxide has the property of absorbing heat rays called infrared rays. Nature has a balance of carbon dioxide in the atmosphere and giving an optimum warmth to the atmosphere. The increase in the concentration of carbon dioxide in the air would cause increase in the temperature of the earth and ultimately lead to melting of glaciers causing flood etc. This phenomenon is called ‘Global Warming’. Hence, our nature in its own way has been maintaining balance of carbon dioxide and oxygen in the atmosphere.

• What is the role of long chimneys in factories?
  The burning of fuel produces smoke. Smoke contains a few gases and fine dust particles and it is often harmful. Long chimneys in factories take the harmful smoke and gases away from us.
• During an incident of fire, one is advised to wrap a woollen blanket over a burning. Why?
  During an incident of fire, one is advised to wrap a woollen blanket over a burning object because air gaps in woollen blanket, act as an insulating medium which cuts the supply of oxygen to the object that is burning thereby prevent it from further burning.
• How can you say that air contains dust particles?
  On a sunny day, go to a room which is a little bit dark. In this room, choose a thin opening through which light can enter the room. If there is no opening in the room, then make one. From this opening we will see that a sharp beam of Sunlight enters the room. In this beam, we will observe that there are some tiny solid-like particles moving freely. These particles are the dust particles and this shows the presence of dust particles in air.

Write down the uses of air. Discuss.
There are many different uses of air:

1. Air is used by human beings for breathing and respiration.
2. Air is used for burning fuels (like wood, coal and kerosene etc.) to make fire.
3. Compressed air is used to fill tyres of various kinds of vehicles.
4. Blowing air, called wind, is used to turn the blades of windmills.
5. Air helps in the movement of yachts or sailing boats, parachutes, gliders and aeroplanes.
6. Air helps the birds, bats and insects in flying. Air do not play an important role in the water cycle.

The oxygen of air being consumed by animals (and plants) in respiration, and in the burning of fuels is constantly being replaced by plants through the process of photosynthesis.
In this process, plants make their own food and oxygen is produced along with it. Plants also consume oxygen for respiration, but they produce more of it than they consume. That is why we say plants produce oxygen.
It is obvious that animals cannot live without plants. Similarly, plants cannot survive for long without animals. They would consume all the carbon dioxide in the atmosphere. We can see that both need each other, as the balance of oxygen and carbon dioxide in the atmosphere is thus maintained. This shows the interdependence of plants and animals.

What do you know about composition of air?
Air is a mixture of many gases. The gases in air are mainly nitrogen, oxygen, small amount of carbon dioxide, and many other gases. However, there may be some variations in the composition of air from place to place. We see that air contains mostly nitrogen and oxygen. In fact, these two gases together make up 99% of the air. The remaining 1% is constituted by carbon dioxide and a few other gases, water vapour and dust particles.