NCERT Solutions for Class 9th Science Chapter 10 Let us investigate — Activity 10.4

Book page 187 Updated on2026-09-08

Q1.

Now, submerge the two metal spoons in water without touching the sides or bottom of the bucket and tap them against one another again (Fig. 10.6b). Do you again hear the sound produced?

Answer

Yes, the clink is clearly heard — a little duller and different in quality from the clink in air, but unmistakably there.

The tap sets the spoons vibrating. The vibrating metal squeezes and releases the water around it, so compressions and rarefactions travel out through the water, cross the water surface into the air, and reach your ear.

Why the spoons must not touch the bucket: if they touched the sides or bottom, the vibration could travel to your ear through the solid bucket and the table. By keeping them clear, the only path left is water → air, so the experiment really is a test of whether liquids carry sound.

Tip: the sound changes in quality because water and air carry the different frequencies in the note unequally, and some of the sound is reflected back at the water surface instead of crossing into the air.

Q2.

If sound did not travel through liquids, would you have heard this sound?

Answer

No. With the spoons held clear of the bucket, water is the only medium touching them.

Path taken by the sound: vibrating spoons → water → water surface → air → ear

If sound could not propagate through liquids, the first link of that chain would be broken. The vibration would die out at the surface of the metal, no compression would ever leave the spoons, and you would hear nothing at all.

Why the logic is sound: this is how a scientist tests a claim — remove every alternative path, and see whether the effect survives. Because the clink is heard, the only remaining explanation is that water carried it. Sound therefore propagates through liquids as well as through solids and gases.

Did you know? Sound travels about 4 – 5 times faster in water than in air, and far further before fading. That is exactly why sonar, and not light or radio, is used to find submarines and map the ocean floor.

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