Class 7 Science Notes Chapter 16 (Chapter 16) – Examplar Problems Book

Alright class, let's focus on Chapter 16, "Water: A Precious Resource," from your Science Exemplar book. This chapter is crucial, not just for your exams but for understanding a vital aspect of our lives and environment. Pay close attention as we break down the key concepts often targeted in competitive exams.

NCERT Class 7 Science Exemplar - Chapter 16: Water: A Precious Resource - Detailed Notes

1. Water Availability on Earth:

• About 71% of the Earth's surface is covered with water.
• However, most of this water (about 97%) is saline (salty) found in oceans and seas, unfit for direct human consumption, agriculture, or most industrial uses.
• Only about 3% is freshwater.
• A significant portion of this freshwater is locked up in glaciers and ice caps.
• A very small fraction (less than 1%, often cited around 0.006% of total Earth's water) is readily available as surface water (rivers, lakes) and groundwater for human use.
• Key takeaway: Usable freshwater is extremely scarce.

2. Forms of Water:

• Water exists naturally in three forms (states):
  • Solid: Ice, snow, glaciers, ice caps.
  • Liquid: Water in oceans, rivers, lakes, groundwater, rain.
  • Gas: Water vapour in the atmosphere.
• The continuous cycling between these forms maintains the total amount of water on Earth relatively constant.

3. The Water Cycle:

• The continuous circulation of water from the Earth's surface to the atmosphere and back again. Key processes include:
  • Evaporation: Liquid water turning into water vapour due to heat (mainly from the sun). Happens from oceans, rivers, lakes etc.
  • Transpiration: Release of water vapour by plants through tiny pores (stomata) in their leaves.
  • Condensation: Water vapour rising, cooling, and turning back into tiny liquid water droplets, forming clouds.
  • Precipitation: Water falling back to Earth from clouds in the form of rain, snow, sleet, or hail.
  • Infiltration: The process by which precipitation or surface water seeps into the ground, recharging groundwater.
  • Surface Runoff: Water flowing over the land surface into rivers, lakes, and eventually oceans.

4. Groundwater: A Vital Source:

• Water that seeps into the ground fills the spaces between soil particles and cracks in rocks. This stored underground water is called groundwater.
• Infiltration: The process responsible for replenishing groundwater.
• Water Table: The upper level of the zone where the ground is completely saturated with water.
  • It can rise or fall depending on rainfall (recharge) and extraction (usage).
  • It can be near the surface or many meters deep.
• Aquifer: A layer of permeable rock or sediment (like sand and gravel) that holds and transmits groundwater in significant quantities. Groundwater is typically extracted from aquifers using wells or tube wells.

5. Depletion of Water Table:

• Drawing out more groundwater than is replenished by natural processes (like infiltration from rainfall) leads to the lowering or depletion of the water table.
• Major Causes:
  • Increasing Population: Higher demand for drinking, sanitation, and domestic use.
  • Industrialization: Industries require large amounts of water for various processes.
  • Agricultural Activities: Irrigation consumes a vast amount of freshwater. Traditional flood irrigation is inefficient. Increased use of high-yielding crops often requires more water.
  • Deforestation: Trees help water seep into the ground. Cutting down forests reduces infiltration and increases surface runoff, leading to less groundwater recharge.
  • Uneven Distribution of Rainfall & Scanty Rainfall: Some areas naturally receive less rain, leading to lower recharge rates. Climate change can exacerbate this.
  • Decreasing Seepage Area: Construction of concrete roads, pavements, and buildings reduces the open soil area available for water infiltration.

6. Water Management & Conservation:

• Managing water resources wisely to ensure availability for present and future generations.
• Why Needed: Due to increasing demand, scarcity, and depletion of resources.
• Methods:
  • Rainwater Harvesting: Collecting rainwater where it falls (e.g., rooftops, open spaces) and storing it or directing it to recharge groundwater.
  • Bawris (Stepwells): Traditional water harvesting structures, particularly in arid/semi-arid regions, providing access to groundwater.
  • Drip Irrigation: A highly efficient irrigation technique where water is delivered directly to the base or roots of plants drop by drop, minimizing evaporation and wastage.
  • Water Conservation Habits (Individual Level):
    • Turning off taps while brushing or washing.
    • Checking for and repairing leaks in pipes and taps.
    • Reusing water (e.g., water used for washing vegetables can be used for watering plants).
    • Choosing water-efficient appliances.
    • Creating awareness about water scarcity.

7. Effect of Water Scarcity on Plants:

• Plants need water for photosynthesis, nutrient transport, and maintaining turgidity.
• Lack of water causes plants to wilt.
• Prolonged scarcity can lead to drying up and death of plants, affecting food chains and ecosystems.

8. World Water Day:

• Celebrated on March 22nd every year to raise awareness about the importance of freshwater and advocate for sustainable water management.

Multiple Choice Questions (MCQs):

1. The process by which water seeps into the ground to recharge groundwater is called:
   (a) Evaporation
   (b) Condensation
   (c) Infiltration
   (d) Transpiration

2. The upper level of groundwater is known as the:
   (a) Aquifer
   (b) Water Table
   (c) Surface Water
   (d) Water Cycle

3. Which of the following is NOT a primary cause for the depletion of the water table?
   (a) Increasing population
   (b) Afforestation (planting trees)
   (c) Increasing industrialisation
   (d) Agricultural activities

4. Drip irrigation is a technique used primarily for:
   (a) Generating electricity
   (b) Water conservation in agriculture
   (c) Purifying drinking water
   (d) Rainwater harvesting

5. Most of the water on Earth is found in:
   (a) Rivers and Lakes
   (b) Glaciers and Ice Caps
   (c) Groundwater
   (d) Oceans and Seas

6. Bawris are traditional structures primarily used for:
   (a) Flood control
   (b) Water harvesting and storage
   (c) Generating hydroelectric power
   (d) Decoration in palaces

7. Which process in the water cycle involves plants releasing water vapour?
   (a) Precipitation
   (b) Condensation
   (c) Evaporation
   (d) Transpiration

8. World Water Day is celebrated every year on:
   (a) June 5th
   (b) April 22nd
   (c) March 22nd
   (d) December 1st

9. An aquifer is:
   (a) The upper level of groundwater
   (b) A type of cloud formation
   (c) A layer of rock or soil that holds groundwater
   (d) A method for purifying water

10. Which human activity generally helps in increasing the groundwater level or recharge?
    (a) Deforestation
    (b) Constructing concrete roads
    (c) Rainwater harvesting
    (d) Over-extraction using tube wells

Answer Key for MCQs:

1. (c) Infiltration
2. (b) Water Table
3. (b) Afforestation (planting trees)
4. (b) Water conservation in agriculture
5. (d) Oceans and Seas
6. (b) Water harvesting and storage
7. (d) Transpiration
8. (c) March 22nd
9. (c) A layer of rock or soil that holds groundwater
10. (c) Rainwater harvesting

Make sure you understand these concepts thoroughly. Remember, water conservation is everyone's responsibility. Study well!