15.Process of evaporation takes place
(a)Only at room temperature (b) at 100°C
(c) below its boiling points (d) at boiling point
only correct please
Answers
Answer:
All substances are composed of molecules. For a given body—say, a glass of water—the temperature of the whole is really just the average molecular energy of the molecules it comprises. But the average is only an average, and at a given moment, the speeds of individual molecules vary, with a generally bell-shaped distribution. That means some are travelling much slower, and others much faster. Since the thermal energy is in the form of random molecular movements, with everything smashing and crashing around, for any given molecule, the speed with which it travels changes with each new collision.
Now water is held together not only by the simple weight of gravity holding it down, but by a collection of weak bonds called hydrogen bonds. This is because of the uniquely "bent" shape of the water molecule.The effect is that a hydrogen atom from one molecule has a tendency to "stick" to the oxygen atom of an adjacent molecule. This is what makes water remain in a liquid state at a much higher temperature than you would expect by its molecular weight alone.
Now what happens is that at the surface of the liquid, a given molecule might get enough energy to simply break free of the hydrogen bonds that keep it in the liquid. When it does this, it becomes a vapor, and it starts behaving as a gas—the molecules with which it now interacts are traveling with a much higher velocity, and they do not collide at speeds low enough (in general) for hydrogen bonding to resume. The exception to that last point is in the formation of fog, clouds, and ultimately rain.
All the while, however, there are other water molecules in the gas phase (by phase here, I mean the designation of either liquid, solid, gas, etc.) directly above the surface of the liquid phase. For the converse reason, the distribution of speeds in the gas phase will include a few molecules with such a *low* speed, that when it collides with the surface of the liquid, it loses the energy it would need to remain as a vapor, and succumbs to the hydrogen bonds that make it now a part of the liquid.
The net effect here is that the liquid will gain some, and it will lose some. The net direction depends on whether the air is saturated with H2O or not.
Usually, it is not saturated, so you expect a puddle to dry up over time, even if the temperatre of the water never gets near boiling. But the drier the air is—that is to say, more arid, less humid—the less the loss of water molecules from liquid to gas will be offset by transfers from gas to liquid. So after a rainstorm, the air is close to saturated with water, and puddles dry very slowly. This is also why spilled water will dry much quicker in Phonix or Las Vegas, even if it's not a hot day. And it's the reason why blowing on things dries it out—you're forcing the saturated vapor at the surface away, and pushing in air that's much drier and can remove more moisture.
The converse can also happen, but we don't normally notice it in the liquid itself. If you have a cup of iced beverage with an open top, water from the air around it will crash into the surface, lose the energy needed to escape, and increase the overal amount of liquid in the cup. But it's a samll amount generally not noticed. What we *do* tend to notice is when this happens to the cold, dry exterior of the cup itself. This condenses onto the cup an makes the outside of it wet; that water on the outside of the cup is from the air around it condensing, rather than from the beverage in the cup itself.
You can also have a situation where the rate of change from liquid to gas is exactly the same as the rate of change from gas to liquid. This is called equilibrium, and there's no net change in the liquid volume. The best example of this would be on a closed bottle or jar. The air in the container soon becomes saturated, and so long as the jar remains closed and there's no great change in temperature for the jar, the liquid volume remains fixed.
Explanation:
Answer:
Evaporation happens when a liquid substance becomes a gas. When water is heated, it evaporates. The molecules move and vibrate so quickly that they escape into the atmosphere as molecules of water vapor. ... Heat from the sun, or solar energy, powers the evaporation process.