8 NCERT SCIENCE TEXTBOOK SOLUTION

Chapter 18 - Pollution of Air and Water Exercise 250 Solution 1 Water gets contaminated by the addition of: (i) Agricultural chemicals: Farmers use excessive amounts of pesticides and fertilizers to increase crop production. These chemicals get carried away to the water bodies due to rains and floods which lead to water pollution. (ii) Industrial wastes: Industries release harmful chemical wastes into water sources, thereby polluting them. (iii) Sewage wastes: Waste materials from kitchens, toilets, and laundry sources are also responsible for contaminating water. Solution 2 An individual can reduce air pollution by: (i) By using public transport whenever possible. (ii) By not using vehicles for short distances. (iii) By using clean fuels such as LPG and CNG instead of diesel and petrol. (iv) Always disposing the garbage properly and not burning it. (v) Controlling the emissions from vehicles and household chimneys. Soluti...

Chapter 17 - Stars and Solar System Exercise 234 Solution 1 (c) A constellation Concept Insight: A constellation is not a member of the Solar System. Constellations are groups of stars that form recognizable shapes. Solution 2 (a) Sirius Sirius is a star and not a planet of the Sun. Solution 3 (a) Phases of the Moon occur because we can see only that part of the Moon which reflects light towards us. Solution 4 (a) The planet which is farthest from the Sun is __Neptune__ . (b) The planet which appears reddish in colour is __Mars__ . (c) A group of stars that appear to form a pattern in the sky is known as a __constellation__ . (d) A celestial body that revolves around a planet is known as a _satellite_ . (e) Shooting stars are actually not _ meteor_ . Concept Insight: Shooting stars are not stars, they are meteors. (f) Asteroids are found between the orbits of _Mars _ and __Jupiter_ . Concept Insight: Asteroids a...

Chapter 16 - Light Exercise 212 Solution 1 We can see objects only if light from any source enters our eyes.  If we are in a dark room, then it is not possible for us to see objects in the room. However, if there is light outside the room, then objects outside the room are visible to us. Concept insight: Light enables us to see things from which it comes or from which it is reflected. Solution 2 Regular reflection takes place from a smooth or a regular surface. In regular reflection, all reflected rays are parallel to each other for parallel incident rays. Irregular or diffused reflection takes place from a rough or an irregular surface. In diffused reflection, the reflected rays are not parallel to each other for parallel incident rays. This happens because of the presence of microscopic irregularities of the surface on which light rays are incident. Hence, parallel incident rays reflect in different directions. However, each ray obeys the laws of reflection. Theref...

Chapter 15 - Some Natural Phenomena Exercise 197 Solution 1 (b) A copper rod Concept Insight: Only non-conducting materials (bad conductors of electricity) can be easily charged by friction. Copper is a highly conducting material. Therefore, a copper rod cannot be charged easily by friction. Solution 2 (b) The rod becomes positively charged, while the cloth has a negative charge. Concept Insight: When an object is charged by rubbing it against another object, the two objects get oppositely charged. By convention, it is considered that the charged acquired by the glass rod is positive and charged acquired by the cloth is negative. Therefore, the rod becomes positively charged and the cloth becomes negatively charged. Solution 3 (a) False Concept Insight: Like charges repel each other, whereas unlike charges that attract each other. (b) True Concept Insight: A charged glass rod has positive charges on its surface while a char...

Chapter 14 - Chemical Effects of Electric Current Exercise 180 Solution 1 (a) acids, bases and salts. Concept insight: Solutions of acid, bases and salts conduct electricity because they produce ions when dissolved in water.  (b) chemical Concept insight: When an electric current passes through a solution, the solution decomposes into its positive and negative ions. This process of decomposition of the solution is a chemical effect of electricity. (c) negative Concept insight: When an electric current passes through a copper sulphate solution, the solution decomposes into positively charged copper ions and negatively charged sulphate ions. These positively charged copper ions get attracted towards the plate which is connected to the negative terminal of a battery. (d) electroplating Concept insight: By electroplating corrosion of metal objects can be prevented. Solution 2 The deflection in the compass needle shows that current is flowing through the wound up wir...