CBSE NCERT Solutions for Class 7 Science Chapter 18

In order to achieve the maximum marks in exams, all students need to read the NCERT books thoroughly and solve the exercise questions given at the end of every chapter. This will surely help to get the desired result. Here are some sample questions given below.

CBSE NCERT Solutions for Class 7 Science Chapter 18:

Question-1:

What is sewage? Explain why it is harmful to discharge untreated sewage into
rivers or seas.

Solution:

Sewage is the wastewater containing both liquid and solid wastes produced by
human activities from homes, industries, hospitals, offices, etc. Sewage contains various contaminants including disease-causing bacteria and other microbes. If untreated sewage is discharged into rivers or seas, then the water in the rivers or seas would also get contaminated. If this contaminated water is used for drinking, then it can cause diseases such as cholera, typhoid, dysentery, etc. which may lead to death. That is why it is harmful to discharge untreated sewage into rivers or seas.

Question-2:

Study the following statements about ozone:
(a) It is essential for breathing of living organisms.
(b) It is used to disinfect water.
(c) It absorbs ultraviolet rays
(d) Its proportion in air is about 3%.
Which of these statements are correct?
(i) (a), (b) and (c)
(ii) (b) and (c)
(iii) (a) and (d)
(iv) All four

Solution:

(ii) (b) and (c)

Question-3:

What is sludge? Explain how it is treated.

Solution:

Semi-solids such as faces that settle down during wastewater treatment are called
sludge. This sludge is removed using a skimmer and then transferred to a tank where it is decomposed by anaerobic bacteria to produce biogas. This biogas is used as a low-cost fuel for heating, cooking, etc. It is also used to produce electricity.

Question-4:

Untreated human excreta are a health hazard. Explain.

Solution:

Untreated human excretions are a health hazard. It causes pollution of soil and
water including the underground water. Thus, it contaminates the sources of water from which people collect water for drinking and household purposes. When this contaminated water is used for drinking, it can cause diseases such as cholera, typhoid, hepatitis, dysentery, etc., which may even lead to death.