What is required for surface particles of a liquid to break away to form gas?

Picture 2.4 The particles in a liquid can move around. Some of them have enough KE to escape from the surface and form a

vapour

. This is in dynamic equilibrium with the liquid.

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2. The kinetic theory of matter

Quiz 2

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As the temperature of a liquid is increased, the particles gain more energy and move faster and faster. Jostling about and colliding increases until eventually the particles at the surface gain enough energy to overcome the attractive forces from their neighbors and break away into the surrounding space. At this point, the liquid is becoming a gas (also called a vapor). The temperature at which this happens depends on what the substance is. This temperature, known as the boiling point, remains constant during the entire process of boiling because the added heat is being used up to break the attraction between the particles. The reverse process, condensation, occurs at the same temperature as boiling. Like the melting point, the boiling point is unique for each pure substance, and can be used as an analytical tool for determining the identities of unknown substances.

The amount of energy required for a given amount of a liquid to vaporize or become a gas is called the heat of vaporization (or condensation). It varies from substance to substance because the particle of different substances may be heavier or lighter and may exert different attractive forces. The amount of energy absorbed when 1 gram of water completely changes to a vapor is 540 calories. Conversely, 540 calories are released when 1 gram of water vapor changes back to liquid.

When a liquid reaches the boiling point, particles on the surface actually gain enough energy to break away from the surface. But as heating continues, particles Gallium melts at 86°F (30° C). © Yoav Levy/Phototake NYC. Reproduced with permission.
throughout the liquid are also increasing in energy and moving faster. In a body of the liquid, however, the particles cannot escape into the air, as those on the surface can. That is not only because they happen to be buried deep down below the surface. It is also because the atmosphere is pushing down on the entire liquid and all the particles within it, and, in order to break away, these particles deep within the liquid must acquire enough energy to overcome this additional pressure. (The surface particles can just fly off into the spaces between the air molecules.) When a group of interior particles finally do get enough energy-get hot enough-to overcome the atmospheric pressure, they can push each other away, leaving a hollow space within the liquid. This is a bubble. It is not entirely empty, however, because it contains many trapped particles, flying around inside. The light-weight bubble then rises through the liquid and breaks at the surface, releasing its trapped particles as vapor. We then say the liquid is boiling.

Since the pressure inside the bubbles must overcome atmospheric pressure in order for the bubbles to form, the boiling point of a substance depends on atmospheric pressure. Liquids will boil at lower temperatures if the atmospheric pressure is lower, as it is on a mountain. At the top of Mount Everest, 29,000 ft (8,839 m) above sea level, where the pressure is only about one-third that at sea level, water boils at 158°F (70°C). At 10,000 ft (3,048 m) above sea level, water boils at 192°F (89°C). It would take longer to cook an egg where the boiling point is 192°F (89°C) than at sea level where the boiling point is 212°F (100°C). The normal boiling point of a liquid is defined as its boiling point when the atmospheric pressure is exactly 760 mm Hg, or 1 atmosphere.

With the diminishing supplies of fresh water today, it is increasingly important to find ways of desalinating-removing the salt from sea water in order to make it useful for human consumption, agriculture, and industry. Changes in state, both boiling and freezing, are useful for this purpose. When salt water is heated to boiling and the vapors cooled, they condense to form water again, but the salt stays behind in a very salty residue called brine. By this process, called distillation, freshwater has been recovered from salt water. Similarly, when salt water freezes, much of the salt stays behind as a very salty slush. The ice is removed from the brine and melted to produce relatively fresh water.

Learning Objectives:

Explain the changes in state and their properties with the help of the KPT.
Describe and explain evidence for the movement of particles in liquids and gases. Some examples are Brownian motion and diffusion.

The KPT can be used to explain how conversion of the states of a material, between solids, liquids and gases can occur as a result of increase or decrease in heat energy.

Recall that the KPT assumes that all matter are made up of tiny particles with tiny spaces in between each other and these particles are in continuous random motion.

In the solid, the strong attractions between the particles hold them tightly together. Even though the particles are in constant random motion, the amount of energy produced by the motion is not enough to disrupt the structure.

Microscopic view of a solid when represented by the KPT

When a solid is heated, the particles gain energy and start to vibrate faster. Further heating of the material results in an increase in amount of energy absorbed and the increase in movement of the particles. Eventually the particles gain enough energy and movement to break free of the solid structure, resulting in the melting of the material to form a liquid.

The temperature in which the particles of a material is able to have enough energy to break free of the solid structure is known as the melting point of the material.

Microscopic view of a liquid when represented by the KPT

The particles in the liquid is the same as that found in the original solid which have melted. The only difference is that the liquid particles carries more energy than the solid particles.

Within the liquid, there are some particles which have more energy than the other. These more energetic particles may have sufficient energy to escape from the surface of the liquid to give gas or vapour without further heating of the liquid.

As the temperature of the liquid increases, the rate of evaporation increases as there are more particles with sufficient energy to break free of the liquid structure. However, if a liquid is heated further, more and more particles have enough energy to overcome the attractive forces between each other and break free of the surface of the liquid together. At this point, we can say that the liquid is boiling and is converted into a gas.

The specific temperature at which a liquid starts to boil and is converted to a gas is known as the liquid’s boiling point. Eventually, all of the liquid particles will have enough energy to leave the liquid, resulting in the conversion of all of the liquid into a gas (hence the disappearance of the liquid).

Microscopic view of a gas when represented by the KPT

While the conversion of a solid to liquid or liquid to gas involves the gain of energy sufficient for the particles to break free of their original structure, the conversion of a gas to liquid (condensation) and a liquid to solid (freezing) is a result of the loss of energy of the particles due to the decrease in temperature of the material.

Summary video of the change in state of water, explained with the help of the KPT.

The KPT is also able to explain the movement of more than one type of particles with respect to each other. Two examples which we will be looking at are Brownian motion and diffusion.

Brownian motion is the random motion of particles suspended in a fluid (liquid or gas), resulting from the collision with the fast moving particles in the fluid.

The direction of the force of atomic bombardment is constantly changing, and at different times the particle is hit more on one side than another, leading to the seemingly random nature of the motion.

This phenomenon is named after the botanist Robert Brown, who observed the movement of particles, which are trapped in cavities inside pollen grains, through water. At that point in time he was not able to determine the mechanisms that caused this motion. Brownian motion is later explained in detail by Albert Einstein and subsequently served as convincing evidence that atoms and molecules exist.

Diffusion is the net movement of molecules or atoms from a region of higher concentration to a region of lower concentration, aka down a concentration gradient, without the exertion of additional forces. Diffusion can be seen when we drop a drop of food dye into water and given some time, the food dye is spread evenly in the water to give a coloured solution.

The microscopic movement movement of the particles during diffusion can be observed in the animation below.

Initially, there are solute molecules on the left side of a barrier, demarcated by the purple line, and none on the right. After the barrier is removed, and the solute diffuses to fill the whole container. In the top figure, we see a single molecule which is moving around randomly. With more molecules, the solute fills the container more and more uniformly with time (Middle). With an enormous number of solute molecules, the solute appears to move smoothly and deterministically from high-concentration areas to low-concentration areas (Bottom). There is no microscopic force pushing molecules rightward.