NCERT Class 7 Science Model Paper 2 Questions and Answers Part 3 (For CBSE, ICSE, IAS, NET, NRA 2023)

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17. Define chemical equation with one example.

A chemical equation is the shorthand notation used to represent a chemical reaction


(a) Write the chemical name of washing soda.

(b) Give the common name of .

(c) Name two laboratory indicators.

Ans: (a) Sodium carbonate

(b) Milk of magnesia

(c) Methyl orange, phenolphthalein

19. Write short note on (i) The sewer system (ii) Manholes (iii) Sewage

Ans: (i) The sewer system is a channel of pipelines which carries out sewage from houses. The sewage leaves houses through drain pipes and flows to the sewer mains of the city.

(ii) A manhole is a hole, usually with a cover, through which a person may enter a sewer or drain to fix the faulty pipes.

(iii) Wastewater containing suspended impurities is called sewage. It is a complex mixture containing suspended solids, organic and inorganic impurities, nutrients and diseases- causing organism.

20. What is wastewater treatment plant? Draw its flow chart.

Ans: A wastewater treatment plant is a facility that treats wastewater from industrial source. There are basically three processes involved in treating wastewater: primary, secondary and tertiary.

Waste Water Treatment Plant

21. What is depletion of water table? Explain the factors in detail which promote the depletion of water table.

Ans: Depletion of water table

Factors responsible for depleting water table [lowering of water table] are many, like

(i) Excessive use due to increasing population.

(ii) Increase in industrial activities.

(iii) Agricultural needs.

(iv) Poor rainfall.

(v) Deforestation [cutting forests to create land for housing, industries, roads etc.]

Water Crisis and Rainfall

  • You all know that the water crisis is more serious when the monsoon fails, and drought hits most part of the country. Also, there is a problem of misuse of water by people and the problem of water pollution.
  • Pollution of water makes it unfit for drinking.
  • Let us now learn about the amount of rainfall in different parts of our country.
  • In our country, the annual rainfall varies in different parts. Some parts receive very little rainfall while others get very high rainfall every year. This explains why some parts in our country face droughts and others suffer from floods.
  • People living in low rainfall areas (arid region) spend a lot of their time and energy to obtain water for their domestic uses.

Scarcity of Water

  • Scarcity of water is defined as a situation where there is insufficient water to satisfy normal requirements.
  • Though water is a renewable resource, we humans are using it at a faster rate than its being replenished.
  • There are various factors contributing to the depletion of water table.
  • Increasing population: Growing population has resulted in a growing demand for houses, offices, shops, roads, etc. As a result, open areas like parks and playground are used for construction. This reduces the seepage of water into the ground. Moreover, construction work requires more water for which underground water is used. Since, the rate of consumption of underground water is more than the seepage; there is depletion of water table.
  • Increasing industries: Growing population has also resulted in an increase in the number of industries.
  • Water is used in almost every stage of production of things that we use. This has put a heavy load, especially on the freshwater ecosystem.
  • Agricultural activities: As we already know India is an agricultural country and farmers have to depend on rains for irrigating their fields. However, erratic monsoons result in excess use of groundwater thereby decreasing the underground water.
  • Deforestation: Trees have the ability to hold water. With deforestation, the number of plants decrease, hence the water keeps flowing towards the sea and the groundwater is not recharged.

In India causes of water scarcity are:

  • Uncontrolled use of bore well technology for extracting groundwater: Farmers have overexploited groundwater resource in our country due to uncertain monsoons and an unreliable state water supply.
  • Pollution of freshwater resources: This is due to the flow of untreated sewage from homes, toxic chemicals from industries, and of pesticides and insecticides used by farmer into water bodies.
  • No effective measure for water conservation: Very little has been done for groundwater recharge and for care of freshwater ecosystems.

Water Needed for Agriculture: Agricultural sector is the major user of water. At a global level, percentage of water used for different purposes is as follows.

Agriculture - 70 %

Industry - 25 %

Domestic use - 5 %

22. (a) Explain why rusting of iron object is faster is coastal areas than in deserts.

(3) (b) Explain the chemical reactions with help of examples.

(2) (i) Direct combination (ii) Single displacement reaction

Ans: (a) Percentage of moisture is high in coastal areas hence iron objects get both water and air for rusting.

In deserts the air is dry and there is no water or moisture in the air hence rusting does not take place as fast as in coastal areas.

(b) (i) Direct combination: In some reactions, new matter is added to the original substance and the substance gain mass. Such a reaction is called as synthesis or combination reaction.

For example,

(ii) Single displacement reaction: When one element displaces another element from a compound and takes its place, the reaction is called a displacement reaction.

For example,