8 NCERT SCIENCE SUBJECTIVE QUESTIONS

Q1. Explain the effects of acid rain on: 1. Aquatic life 2. Statues and bridges Solution 1. Acid rain disrupts the reproductive cycle of aquatic species. It also leaches aluminum from the soil into the water, clogging the fish's gills and altering their blood chemistry. As a lake becomes acidified, one species after another disappears. 2. Acids present in the acid rain corrodes metals resulting in weakening of the bridges. Acid rain dissolves marble and stone and hence damages the statues. Q2. What is marble cancer? Solution The corrosion of the marble of monuments due to acids present in acid rain is called marble cancer. Q3. What happens when algae growing in water die? Solution Once the algae die, they serve as food for decomposers like bacteria. A lot of oxygen in the water body gets used up. This results in a decrease in the oxygen level which may kill aquatic organisms. Q4. What is an industrial effluent and what does it contain? Solution...

Q1. Name the first astronaut who landed on moon and when did he land on it. Solution On July 21, 1969 the American astronaut, Neil Armstrong, landed on the moon for the first time. Q2. Do all the stars in the sky move? Explain. Solution No. The Earth rotates from West to East on its axis. Hence, all stars in the sky appear to move from East to West. With reference to the Earth, the Pole star does not appear to move in the sky because it is located above the axis of rotation of the Earth in the north direction. It appears to remain stationary at a point in the sky.   Q3. Which star is nearest to earth? What does it provides us? Solution The sun is the nearest star. It is the main source of heat and light for all the planets. Q4. Why do the stars appear to us like point objects? Solution The stars are much bigger than the earth, yet they appear to us like point objects because they are millions of kilometres farther away from the earth. ...

Q1. Differentiate between luminous objects and non-luminous objects. Solution Luminous objects Non-luminous objects Luminous objects are the objects that emit light of their own. Non-luminous objects are the objects that reflect light. Luminous objects are seen on their own. Non-luminous objects are seen due to luminous objects. Example: Sun, candle Example: Moon, mirrors Q2. What is lateral inversion? Solution Lateral inversion is the phenomena when in an image formed by a mirror the left of the object appears on the right and the right appears on the left. Q3. Which is the most popular resource for visually challenged persons? Who developed it? Solution The most popular resource for visually challenged persons is Braille. Louis Braille developed this system and published it in 1821. Q4. Mention any three possible ways by which we can take care of our eyes. Solution (i) If advised, use suitable spectacles. (ii) Do not look at the sun or a p...

Q1. Which of the following cannot be charged by friction, if held by hand? (a) a plastic scale (b) a copper rod (c) an inflated balloon (d) a woolen cloth. Why? Solution Copper rod Except copper, the other three are insulators whereas copper is a conducting object. As soon as it gets charged by rubbing with another material, the electric charge produced on its surface flow through our hand and body into the earth and it remains uncharged. Q2. Explain the construction of an electroscope. Solution An electroscope consists of a large jar. A metal rod is fitted into the mouth of the jar with the help of the cork. At the lower end of the metal rod a pair of thin leaves of gold or aluminium is suspended. Q3. How would you use an electroscope to find out whether an object is charged or not? Solution Touch the body to be tested with the metal disc of an electroscope. If the leaves of an electroscope open up (diverge), the body is charged. If the leaves remain...

Q1. Why distilled water is a poor conductor of electricity? Solution Distilled water is a poor conductor of electricity because it does not contain any dissolved salts in it which can provide it ions to conduct electricity. Q2. Does pure water conduct electricity? If not, what can we do to make it conducting? Solution No, pure water does not conduct electricity. We can add a pinch of common salt in pure water to make it conducting. Q3. How will you identify the positive terminal of a battery if it is hidden in a box and you can only see two of its leads. Solution Both the leads of the battery can be dipped into a single piece of potato for long time. After that, it can be observed that there is green coloration near one of the terminals. This is the positive terminal and the other one is the negative terminal. Q4. Mention three common solutions that are good conductors of electric current. What is common in these solutions? Solution Good conductors a...

Q1. What are audible vibrations? Solution The vibrations whose frequencies are from 20 Hz to 20,000 Hz constitute the audible vibrations. Human ear can can only recognise these vibrations. Q2. Can sound travel in vacuum? Explain Solution Sound cannot travel in vacuum because sound travels by producing a vibration in the molecules of the medium surrounding it and there are no molecules in vacuum. Q3. How does the pitch of the sound depend on its frequency? Solution The pitch of sound depends on the frequency of vibration. The higher the frequency the more shrill is the sound and vice versa. Q4. Name the unit for the measurement of loudness of sound. Solution Loudness of sound is measured in decibels (dB). A sound of about 50 to 80 decibels is considered as audible sound. Q5. We cannot talk to one another on the surface of moon. Give reasons. Solution Sound requires a medium to propagate. It cannot travel in vacuum. Since there is vacuum on...

Q1. Why do we slip when we step on a banana peel? Solution We slip when we step on a banana peel because the inner side of banana peel being smooth and slippery reduces the friction between the sole of our shoe and the surface of road. Q2. Will it be possible to cut wood without the force of friction?  Solution We are able to cut wood because there is friction between the saw blade and log of wood. If there were no friction between the saw blade and log of wood, then cutting of wood would not be possible. Q3. Why do gymnasts apply a coarse substance to their hands? Solution Gymnasts apply a coarse substance to their hands for a better grip. Q4. State with reasons whether the following statement is true or false: Friction is a necessary evil Solution True, friction is a necessary evil.   Because:(a) Friction wears out the rubbing machine parts.(b) It produces heat which may damage machines.(c) It results in wastage of energy and fuel. Th...