NCERT Solutions Curiosity Chapter 7 Chapter projects — Exploratory Projects

Book page 104 Updated on2026-09-05

Q1.
Society: Visit a site of water harvesting or a recharge pit. Find out from people how they are constructed and how they work. Prepare a report with illustrations.
Answer

This is a field project, so the report must be built out of what you see and hear. Here is how to do it and what a good report must contain.

How to go about it

  1. Find a site nearby — a rooftop rainwater harvesting system in a school or apartment block, a recharge pit or soak pit in a colony, a village tank or johad, a check dam, or a stepwell.
  2. Go with a teacher or an elder. Carry a notebook, a measuring tape and a camera or pencil for sketching.
  3. Ask the people who built it or look after it: Who made it, and when? What does it cost? How much water does it collect? Where does the water go? Who cleans it, and how often? Has the water level in the nearby well risen since it was built?
  4. Sketch or photograph the parts — catchment, gutter, down pipe, first-flush device, filter, pit or tank.

What a good report must contain

  • Name and location of the site, date of visit.
  • A labelled diagram of the structure.
  • How it is constructed — materials, depth, filter layers.
  • How it works — the path of the water from the roof or the ground to the pit.
  • Benefits reported by the people, and problems they face.
  • Your own conclusion linking it to the chapter — infiltration, aquifers, groundwater recharge.

Sample answer: “We visited the recharge pit in the corner of our school ground. It is a square pit about 1.5 m wide and 3 m deep. Rainwater from the school roof comes down a pipe into a small chamber that traps leaves and dust, and then enters the pit. Inside, the pit is filled in layers — large boulders at the bottom, then gravel, then coarse sand at the top. The caretaker told us the sand layer is cleaned before every monsoon. The water passing through these layers seeps into the ground by infiltration and reaches the aquifer below. Since the pit was built four years ago, the school borewell has not run dry in May, which it used to do earlier. We learnt that gravel and coarse sand are used because water seeps fastest through wide, open, connected spaces — exactly what we saw in Activity 7.5.”

Tip: a recharge pit is filled with gravel and sand, never clay. After Activity 7.5 you know exactly why.
Q2.
Activity: Tightly wrap a thin paper strip around a metallic rod. Try to burn the paper with a candle while rotating the rod continuously. Does the paper burn? Explain your observations.
Answer

No — the paper does not burn (it may darken a little, but it does not catch fire), as long as it is wrapped tightly around the metal rod and the rod is rotated continuously.

Why it happens: the metal rod is a good conductor of heat. Paper catches fire only when its own temperature reaches its ignition temperature. Here, the moment the flame heats the paper, the heat passes straight through the thin paper into the metal touching it, and the metal carries it away rapidly along the rod by conduction. The paper therefore never gets hot enough to burn. Rotating the rod keeps moving fresh, cooler metal under the flame and stops any one spot from becoming too hot.
Flame gives heat to the paper
Paper is thin and pressed tightly against the metal
⇒ heat is conducted away into the rod almost at once
⇒ paper stays below its ignition temperature ⇒ it does not burn
Check it yourself: try the same thing with a wooden or a glass rod, or leave the paper loosely wrapped so that a layer of air lies between paper and metal. Now the paper burns — because wood, glass and air are poor conductors and cannot carry the heat away.
Caution: do this only under the supervision of a teacher or an adult, and hold the rod at its far end.
Q3.
Activity: Take a sheet of paper. Draw a spiral on it, as shown in Fig. 7.17a. Cut the paper along the spiral. Suspend the paper as shown in the Fig. 7.17b above a burning candle. Observe what happens. Provide an explanation for your observation.
Answer

Observation: the paper spiral begins to rotate steadily above the candle, and it keeps turning as long as the candle burns below it. Take the candle away and it slows down and stops.

Candle heats the air above it
Hot air expands → becomes lighterrises
Cooler, heavier air moves in from the sides to take its place
= a steady upward stream of air, a convection current
The rising air pushes against the slanting turns of the spiral
⇒ the spiral spins
Why it happens: the spiral is cut so that each turn is tilted, like the blades of a fan or a windmill. When the rising column of hot air strikes these tilted surfaces, it pushes each one sideways as it passes, and the paper turns on its thread. The spiral is, in effect, a tiny turbine driven by convection.
Try This: hold the spiral over a hot cup of tea, or over a room heater. It turns there too — proof that any source of rising warm air will do. Keep the paper well above the flame so that it does not catch fire.
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