Air and water around us have so many scientific facts to be observed. Following important points will be very helpful in solving multiple-choice tests for admission to class VI in Aligarh Muslim University and Jamia Millia Islamia. 

• The envelope of air around the earth is called the atmosphere.
• The atmosphere is held to the earth due to the force of gravity of the earth.
• Our atmosphere consists of four layers: troposphere, stratosphere, ionosphere and the exosphere.

• Troposphere

  • It is the layer closest to the earth surface.
  • About 80% of the air content in the atmosphere is present here.
  • All the weather changes take place here.

• Stratosphere

  • Aircraft fly here, as there are no weather changes here.
  • The ozone layer is present here, which absorbs the harmful radiation coming from the sun.

• Ionosphere

  • It helps in the transmission of radio waves around the world.

• Exosphere

  • It is the outermost layer of the atmosphere.
• Air is a mixture of gases, having the main content as nitrogen and oxygen.
• Air contains 78% nitrogen, 21% oxygen, 0.03% carbon dioxide and 0.97% other gases such as hydrogen, water vapour, and others.
• We breathe in oxygen and breathe out carbon dioxide.
• Plants prepare their food by the process of photosynthesis, they prepare their own food with the help of carbon dioxide, water, and sunlight.
• Air occupies space and it is due to this property of air the football bladder, tubes in cars, scooters, and cycles can be inflated.
• Since air has weight, it exerts pressure.
• When you suck the straw, you actually suck the air from the straw, so there is very little pressure inside the straw, hence the liquid from the bottle is pushed up into the straw.
• A common lift pump(hand pump) also works on the same principle as the straw.
• Water like all other liquid has the property to flow.
• Fountain pen also works due to the air pressure.

Water

• Water must be purified because it contains invisible impurities such as dissolved materials and germs.
• Water is the best solvent that is- it is able to dissolve a lot of materials.
• Soluble materials can be removed from water by:
  • Evaporation and 
  • Distillation
• Distilled water is pure water which does not contain any germs or impurities.
• Insoluble impurities can be removed by:
  • Decantation
  • Filtration
• The process of settling down of impurities is known sedimentation.
• After the impurities have been settled down the pure water is poured out leaving the sediments. This process is known as decantation.
• The process of filtering the impurities using filter paper is known as filtration.
• The impurities left behind on the filter paper is known as filtrate.
• Water can be kept purified by the following methods:
  • Keeping the animals and insects out of drinking water.
  • Keeping the source, where the water comes from, fully covered and clean.
  • Keeping water filtrate home to store drinking water.
  • Potassium permanganate crystals may be added to wells for killing germs.
  • Chlorine gas can be used to kill the germs.
• The best method for purification of water for drinking purpose is the boiling of water.
•  Any object when partially or entirely submerged into water, an upward force acts on it which is known as the upthrust or buoyant force.
• Objects float on water because buoyant force supports its weight.
• The objects which sink in water are partially supported by the buoyant force.
• Objects that float, displace water equal to their own weight.
• A ship floats on water as its weight is the same as the water it displaces.
•  The weight of an object in water is lesser than that in the air.
• The weight of an object in water is called its apparent weight. 
• The difference between the actual weight of an object and its apparent weight is equal to the weight of water displaced by it.