NCERT Solution for Class 9 Science Chapter 1
Matter in Our Surroundings
NCERT Books for Session 2022-2023
CBSE Board and UP Board
Intext Questions
Page No-3
Questions No-2
The smell of hot sizzling food reaches you several meters away, but to get the smell from cold food you have to go close.
Solids diffuse at a very slow rate. But, if the temperature of the solid is increased, then the rate of diffusion of the solid particles into air increases. This is due to an increase in the kinetic energy of solid particles. Hence, the smell of hot sizzling food reaches us even at a distance, but to get the smell from cold food we have to go close.
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The smell of hot sizzling food reaches severed meters away, as the particles of hot food have more kinetic energy and hence the rate of diffusion is more than the particles of cold food.
Evaporation is directly proportional to temperature, means hot food evaporates easily.
Diffusion of hot food vapors with air becomes very fast and can reach to a distant place within a very short time.
The observation that the smell of hot sizzling food travels further than the smell of cold food can be explained by several factors related to the nature of molecules and their behavior at different temperatures:
1. Rate of diffusion: During the process of cooking, the molecules in the food become energetic due to heating and start moving at a faster pace. The increase in molecular motion increases the rate at which these aroma-carrying molecules diffuse outwards into the surrounding air. Consequently, the smell passes and affects the nose at a greater distance.
2. Vapor pressure: Vapor pressure increases with higher temperature for most substances of interest. Volatile compounds that give the hot food its aroma have high vapour pressure at a higher temperature. These molecules have high vapor pressures, which make it easy for them to diffuse away from the food. Therefore, the smell is noticeable even from a distance because of this elevated vapor pressure.
3. Convection currents: When hot plates or pans are placed on a table, they create convective currents in the hot air resulting from them. They convey the odor molecules away upwards and away from the food source and help distribute the aroma over huge distances.
4. Sensitivity of our olfactory system: A stronger sense of smell towards some volatile compounds which are emitted could also mean that they were emitted at a relatively higher temperature. The aroma from hot food might therefore be detectable at a longer distance than the aroma of cold food due to this sensitivity.