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Understanding why pen caps prevent drying involves physics of evaporation and latent heat. Option A is CORRECT. The image states: पेन का टुकड़ा ठंडा नहीं होने देता क्योंकि पेन से गुप्त ऊष्मा प्राप्त कर बर्फ पिघलने लगती है तथा ऊष्मा का हास होने से पेन ठंडा हो जाता है. Evaporation is an endothermic process. For ink to evaporate it must absorb the latent heat of vaporization from its surroundings (the pen and nearby air). This causes the pen to feel cool. When the cap is placed on the pen the small enclosed air space quickly becomes saturated with ink vapour (reaches 100% relative humidity with respect to ink). At saturation evaporation stops because the air cannot hold any more vapour. With no evaporation the ink does not dry out. Option B is wrong. Ink does not freeze inside a pen at normal room or even outdoor temperatures. Ink has antifreeze properties and remains liquid well below 0 degrees C. Warmth from cap is not the mechanism as caps are not heated. Option C is wrong. The cap does not create significant pressure inside. The tiny air gap sealed by a cap does not build up measurable pressure. If pressure were the mechanism uncapped pens would dry immediately which happens slowly. Option D is wrong. While oxygen can eventually react with some ink components this chemical oxidation is not the primary mechanism of pen drying. The primary cause is evaporation of the ink solvent.
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Common questions and clear answers for this topic.
Heat is a form of energy that transfers between objects or systems due to a temperature difference, moving from a hotter body to a cooler one until thermal equilibrium is reached.
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