Q. Define planetary electrons?
Ans.
Rutherford compared his model of an atom with our solar system where the nucleus is like the sun and the electrons are like the planets. Thus, these electrons are also called planetary electrons.
Q. Alloy cannot be separated by physical means, though it is considered a mixture. Why?
Ans.
Alloys are homogeneous mixtures of metals and cannot be separated into their components by physical methods. But still, an alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition.
Q. How would you confirm that a colourless liquid given to you is pure water?
Ans.
Pure water has a fixed boiling point of 100oC under standard atmospheric pressure. This fact can be used to confirm that a colourless liquid given to us is water as follows : We take the colourless liquid in a distillation flask fitted with a thermometer. Let us heat the flask with a gas burner till the liquid starts boiling. If all the colourless liquid distils over (or boils off) at the same temperature of 100oC, without leaving behind any residue in the distillation flask, it will be pure water.
Q. Give three examples of the range of variations that you see in life forms around you.
Ans.
The following range of variations can be seen in different life forms which occurs on Earth.
(i) Variation in size: Some microscopic bacteria are few micrometre in size whereas blue whale
and redwood trees of California are approximately of 30 metres and 100 metres respectively.
and redwood trees of California are approximately of 30 metres and 100 metres respectively.
(ii) Variation in lifespan : Some pine trees live for thousands of years while insects like mosquitoes die within a few days.
(iii) Variation in body colours: Some life forms such as worms are colourless or even transparent whereas many of the birds, insects, flowers etc., are brightly coloured.
Q. (a) What happens to the buoyant force as more and more volume of a solid object is immersed in a liquid? When does the buoyant force become maximum?
(b)Define buoyant force. Name two factors on which buoyant force depends.
Ans.
(a) The buoyant or upthrust force increases as more and more volume of a solid object is immersed in a liquid. When object is fully immersed in water the buoyant force becomes maximum.
(b)Fluid applies an upward force on a solid which is partially or fully submerged in liquid. This phenomenon is called buoyancy (Upthrust) and applied upward force is called buoyant force.
Two factors on which buoyant force depends:
• Volume or shape of object.
• Density of fluid in which object is immersed.
Q. (a) A farmer found that xanthium and parthenium are also growing along with paddy in the field? What are such plants called? How the presence of these plants affect the crop yield?
(b) List any four methods for controlling and preventing the growth of such plants?
Ans.
Xanthium and Parthenium are called weeds.
Weeds grow simultaneously with the crop plants in a crop field. They grow very fast and take up all the necessary nutrients and water from the soil. As a result the crop plants do not get nutrition and they cannot grow as desired.
Methods of weed control:
- Application of herbicides control weeds very fast.
- Hand picking of weed plants are also done.
- Regular mowing of weeds is effective to suppress their growth.
- Biological control of weeds are possible by using insects, bacteria, fungi etc. as agents.
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