Q: A calm , cloudless night is more favourable for the formation of dew . Why ?
Sol: Wind promotes evaporation and hot condensation . Hence , a calm night is favourable for the formation of dew . A cloudless night promotes radiation from object such as grass , green , leaves etc. So dew formation is favoured when the night is cloudless .
Q: Is fire extinguished more effectively by hot water or cold water ?
Sol: Fire is extinguished by the vaporation of water which lowers the temperature of the burning body . Further the water vapour envelops the body , keeping oxygen away . Hot water evaporates more than cold water as it is closer to its boiling point , that is hot water is more volatile than cold water . The latent heat of evaporation being large , hot water will extinguish a fire more effectively than cold water .
Q: Why does a weighted metallic wire and not a nylon wire cut through ice ?
Sol: Due to pressure , the melting point of the ice below the wire is lowered . Heat is conducted through the metallic wire from above to melt the ice below the wire . The wire passes through the water formed which is reconverted to ice after being relieved of the pressure . This is possible only when the wire is a good conductor of heat . Nylon is a bad conductor .
Q: Why is the temperature of an ice block lowered when common salt is added to it ?
Sol: When common salt is added , it melts in the water on the surface of the ice block . The latent heat of fusion is supplied by the ice and so its temperature is lowered .
Q: Is temperature a microscopic or macroscopic concept ?
Sol: Temperature is a macroscopic concept . This means that temperature is an average property of the large number of molecules which constitute a system . We can not define the temperature of a single molecule .