Chapter 15: Blow Hot, Blow Cold
उसी से ठंडा उसी से गर्म (डॉ. जाकिर हुसैन एवं सांस के प्रयोग)
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- Story Author: Written by Dr. Zakir Hussain, the third President of India and prominent educator who wrote inspiring children's stories.
- Dual function of human breath: Breath can be used to cool down something hotter than body temperature (e.g., blowing over hot tea/potatoes) OR to warm up something colder than body temperature (e.g., blowing into cold hands in winter).
- Stethoscope principle: A doctor listens to internal chest heartbeats using a stethoscope, which amplifies sound vibrations.
- Spectacles cleaning: Blowing warm breath onto spectacle glass creates condensation moisture (water vapor) because warm exhaled breath meets colder glass.
- Musical wind instruments: Whistle, Flute (बांसुरी), Shehnai, Been, and Mouth organ operate by vibrating air columns blown from the mouth.
- Trap: CBSE asks who wrote 'Blow Hot, Blow Cold' and options include APJ Abdul Kalam or Nehru. Correct: Written by Dr. Zakir Hussain (former President of India).
- Trap: CBSE states breath only cools things down. Correct: Breath cools things that are hotter than breath, and warms things that are colder than breath.
NCERT Core Facts: Dr. Zakir Hussain's Science Parable
Chapter 15 features the famous folk story of a woodcutter and a tiny magical creature named Mian Balishtiye (मियां बालिश्तिये), written by Dr. Zakir Hussain (the former President of India).
1. The Author: Dr. Zakir Hussain
- Dr. Zakir Hussain was the third President of India (1967–1969) and a pioneer of basic national education (Nai Talim).
- He authored numerous didactic stories for young children explaining physical science concepts through village folklore.
2. The Science of "Blow Hot, Blow Cold"
- When Breath Cools (उसी से ठंडा):
- When a woodcutter was boiling potatoes in the forest, he picked a burning hot potato and blew on it:
- Why? Because the potato was much hotter than his breath. The cooler air from his mouth accelerated convective heat loss, cooling the potato.
- When Breath Warms (उसी से गर्म):
- In freezing winter morning air, the woodcutter's fingers grew numb. He brought his hands close to his mouth and blew hard:
- Why? Because his hands were colder than his internal body temperature. The warm exhaled air transferred heat to his frozen fingers!
3. Breathing Moisture & Cleaning Glasses
- When you blow warm breath onto a mirror or eye glasses, the glass turns foggy and moist.
- Scientific Explanation: Warm air exhaled from human lungs contains dissolved water vapor (moisture). When this warm vapor collides with the cooler surface of the glass, it condenses into microscopic water droplets!
Test Your Mastery: Blow Hot, Blow Cold
Authentic CBSE exam questions tested directly from this NCERT chapter.
The story 'Blow Hot, Blow Cold' in Class 5 EVS was written by which former President of India?
कक्षा 5 की पुस्तक में दी गई कहानी 'उसी से ठंडा उसी से गर्म' भारत के किस पूर्व राष्ट्रपति द्वारा लिखी गई है?
We blow on hot tea to cool it down because:
हम गर्म चाय पर फूंक मारकर उसे ठंडा करते हैं क्योंकि:
Who wrote the NCERT Class 5 EVS story 'Blow Hot, Blow Cold'?
The story 'Blow Hot, Blow Cold' was written by Dr. Zakir Hussain, the third President of India, to explain how human breath can both cool down hot objects and warm up cold objects.
Why do spectacles fog up when you blow on them according to NCERT?
Exhaled human breath contains warm water vapor. When it touches the cooler spectacle glass, the vapor immediately condenses into tiny liquid water droplets, making the glass foggy.