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Oil evaporation

In Science Corner on March 9, 2008 at 4:56 pm

Oil evaporation


Unlike water, why do coconut oil and other edible oils not evaporate?


C. S. Tamil Selvan

Pothanur, Tamil Nadu A liquid consists of molecules (or atoms) which perpetually move around. The freedom with which they move depends on the size, weight and shape of the molecules and more importantly on the strength of attraction between the molecules. These characteristic features differ from one liquid to another and decide the rate of evaporation.

At any temperature, the different molecules constituting the liquid perpetually collide against each other and exchange energy so as to possess a distribution of energies.

Most of the molecules have much smaller energy than the intermolecular bond energy and so do not have the ability to leave the liquid surface, but very few molecules at the liquid surface have sufficient energy to break the intermolecular bond and come out of the liquid surface; this is the process of evaporation.

At higher temperatures, the fraction of higher energy molecules is higher and so the evaporation rate is higher at higher temperature. Similarly, if the liquid is spread out to have larger free surface area, the evaporation rate is proportionally higher.

As a general rule, while comparing liquids of similar materials, the ones with higher molecular size and weight evaporate slowly. However, comparison of different classes of liquids needs to be based on careful examination.

Water molecule is made up of one oxygen atom bonded to two hydrogen atoms; but each molecule is not totally free of the neighbouring molecules but they are bonded to each other through a kind of bond called the hydrogen bond.

These bonds are stronger than the dipolar bonds between relatively passive molecules but are weaker than the bonds between atoms inside the molecule itself. This accounts for lower evaporation rate of water as compared to alcohols.

The edible oils, on the other hand, have rather large molecules of long carbonaceous chains consisting of more than about 15 carbon atoms.

The bonding between these molecules is weaker than that in the case of water, but still the evaporation rate is very much lower. This is because the size and anisotropic shape of the oil molecules do not allow free movement past each other.

Another effect is specifically important in the case of such long chain molecules. Often the molecular collision can take place from the lateral side of the chain and the energy transferred in these events gets used up in bending the chain rather than in moving it.

This effect shifts the energy distribution to lower energy side and the energy available for molecular translational motion is reduced while the kinetics of removal of the massive oil molecules require much larger amount of energy to be possessed by the molecules.

Dr. H. K. Sahu

IGCAR, Kalpakkam, Tamil Nadu


In Science Corner on January 31, 2008 at 8:40 pm


Q: If a pendulum can swing to and fro endlessly, why does a ball thrown from a height not keep bouncing the same way?

A:  Any moving body continues to move unless it is opposed by a force like that of friction. A pendulum, in principle, should perpetually oscillate to and fro if the air resistance in the form of frictional drag were absent.However, such a drag force exists and it exerts a mild opposing force on the pendulum. Thus, although it appears to be moving to and fro endlessly, actually a pendulum comes to rest after a sufficiently long time.

This is known as the damping of the oscillation. The case of a ball dropped from a height on to the floor is different. It bounces from the floor, during which the ball loses a fraction of its energy by deforming itself and the floor at the point where it hits.

The ideal case, where no energy is lost during bouncing, is unachievable in practice. On its rising track it is acted on by two forces, one, the downward force of gravity and two, the air resistance. The air resistance reduced the energy further. Hence, the ball does not rise to as much height as it was originally dropped from.

The ratio of the height of a bounce to that of the previous bounce is known as the coefficient of restitution and is a property of the combination of the materials of the ball and the floor. This process repeats in the subsequent bounces and the height to which it can rise keeps reducing. This leads eventually to stop the ball from further rising and bouncing.

Q.Pricking rain drop

In Science Corner on January 26, 2008 at 7:21 pm


Pricking rain drop

Why does a rain drop prick us when we are moving on bike but does not when we walk?


The rain-drops arrive at the surface of earth after they condense from the cloud at a fairly high altitude. As they move down under the action of the earth’s gravity, their velocity increases.

However, friction or drag force offered by the atmospheric air tends to slow it down. Under the concurrent action of these two opposing forces, the drops reach the earth with some velocity depending on the size of the drop and the wind velocity.

Any moving object carries a momentum which is the product of its mass and its velocity. This bears a relative velocity with respect to another moving object.

The relative velocity of a moving body with respect to another is the sum of the two velocities if the two objects are moving against each other; it is the difference in the velocities if they are moving in the same direction.

When we move perpendicular to the vertically falling rain drops, the relative velocity is the velocity at which we move towards the falling drops.

The product of the relative velocity and the mass of the rain drop is the relative momentum. This momentum gets transferred on to our body when a drop hits. A localized application of such a force causes the sensation of getting pricked by the drop.

When we are walking, our motion is slow and the force exerted by the drop on our body is low and we feel no pricking.

But when we are moving on a bike at a high velocity and rain drops fall at an angle (on the front side), the relative velocity of the drop with respect to us is the velocity of our motion which is high. The pricking force is experienced due to the high relative momentum that gets transferred onto us.

It may be noted that this prick like experience is only on the forward side of the body rather than on the back side although rain drops do fall on the back side of our body while we are moving forward on a bike. This is so, because the relative momentum in such an event is practically zero and no force is experienced.

Courtesy:The Hindu