Properties of Air - Grade 8

• Both atmospheric pressure and  the density of air decrease with altitude.
  
• This is due to the fact that air molecules are constantly being pulled downwards (towards the center of the Earth) by Gravity.
•  So, air molecules are tightly packed near the surface of the earth, compared to some higher altitude.
  

At a
high altitude pressure is low as at higher elevations, there are fewer air molecules
above a given surface than a similar surface at lower levels.

Due to the intense heating from sun rays around the equator, the air in this region gets hot and hence ascends upwards, thus creating a low pressure in the region. Hence it is the equatorial low-pressure belt and is also called doldrums as it is the region of extreme calmness, not as much as a breeze.

As we go up the pressure decreases with altitude.

This is because at higher altitudes number of air molecules present are low when compared to those that are present in the lower atmosphere.

Moreover gravitational pull which is responsible for atmospheric pressure is low on the molecules that are away from its surface than those that are closer to it.

• Bars are the unit of pressure hence C is the correct option.
• Calorie is the unit of energy
• Newton is the unit of force.
• Watt is the unit of power.

Atmosphere,

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Near Earth's surface the pressure decreases with height at a rate of about 1 millibars for every 10 metres. However, over cold air the decrease in pressure can be much steeper because its density is greater than warmer air.

There exists a negative relation between pressure and temperature. When temperature increases pressure will fall and vice versa. That is why, pressure increases from equatorial region towards the polar region.

In the atmosphere, temperature decreases with altitude in the troposphere, but then increases in the stratosphere due to ozone absorption of UV radiation.

Fog is a thick cloud of tiny water droplets suspended in the atmosphere at or near the earth's surface which obscures or restricts visibility.

Air is a good conductor of heat.

Anemometer: Used to measure the speed of the wind.

The statement which is not true about air is that it has a chemical formula. Air has no chemical formula. It is a mixture of gases.

The method used to separate air into its components is by fractional distillation.

Air has a variable composition. The other options are true regarding air.

Oxygen is required for burning or combustion to take place. 

Air has mass, air occupies space and air expands when heated are all properties of air.

The experiment conducted using glass filled with water and card board indicates that air exerts pressure.

Carbon monoxide is an air pollutant whereas carbon dioxide is considered as a greenhouse gas.

A ball falling down to the ground is due to gravity and not due to air pressure. Hence, it doesn't show that air exerts pressure.

When winds blow over a week roof, it creates a low pressure over it and a high pressure inside causing the roof to be blown off. Opening the doors and windows reduces this pressure difference and avoids the roof from being blown off.

Because of the curved upper surface of the aeroplane more air flows in the same time as compared to the lower surface. Therefore, the pressure over the aeroplane is less when compared to the lower surface.

As we know air is a bad conductor (It consists of mostly nitrogen and oxygen which are electrically neutral and are not easily ionized) of electricity. So, Overhead cables need not be insulated.

On freezing water drops they converted in ice.

The humidity of a place varies from time to time as it also depends on the place we reside in. Coastal areas have more humidity or is more humid due to the pressure of the sea breeze.

Convection plays a major role in the formation of a thunderstorm. When warm air rises and meets the cold air, it condenses (releases latent heat). The heat helps fuel the thunderstorm. Then the downdraft forms, bringing cold air and precipitation down to the earth's surface.   

Air is used in generating electricity for example wind mill is used to generate electricity. 

As we go higher and higher the pressure and oxygen decreases, so as due to less oxygen content mountaineers carry oxygen cylinders with them.

The power of diffusion is max in gases because the inter-molecular space between the molecules is highest in gases,

During winter season land areas become cold and adjoining ocean surface becomes comparatively hot. So low-pressure centers develop in the ocean and the winds started blowing from surrounding high pressure to low pressure areas. As it comes to the low-pressure center, it gets warm and rises up. For this mechanism, devastating cyclones originated in the Bay of Bengal during October to early November.