Lecture 8

Atoms in Combination: States of Matter

Electrostatic attraction among the atomic constituents is responsible not only for the chemical bonds forming molecules, but also for the interactions between molecules that determine the physical state of a substance.

Depending upon the relative strength of the attraction between molecules versus the kinetic energy ("state of agitation") of the individual molecules, a given substance will find itself in a solid, liquid, gaseous or plasma state.

SOLIDS

When in the solid state, the chemical bonds are sufficiently strong to give a fixed shape and volume to the body. In a solid, individual molecule oscillate around a position of equilibrium, without straying too far from it. According to the regularity of the molecules arrangement within the solid, one deals with either crystals or amorphous materials.

In crystals, a basic elementary cell is repeated over and over with great regularity, forming very impressive geometrical shapes of macroscopic dimensions. Even though the underlying structure is not always appearing ("a diamond in the rough"), expert cutting can bring into evidence the crystalline structure.
Modern technology can produce large scale crystals, by patiently "growing" them in the appropriate environment.

In amorphous materials (e.g. glasses), the regularity of the basic molecule (or cluster of molecules) is not replicated over large distances, but becomes irregular and unpredictable over distances exceeding a few atoms.

[Notice that a substance often referred to as "crystal" (e.g. "a crystal chandelier") is in reality a heavy glass (often a glass containing Lead in its molecule), cut and polished to mimic the appearance of real crystals]

In between glaases and crystals, we find polycrystalline materials, consisting of a conglomeration of minute individual crystals. Ceramics, porcelains (and also teeth and bones) belong to this category.

Another very important family of solids is represented by the wide variety of organic materials. Originally only found in living organisms, modern technology is now making greater and greater progress in replicating nature, and is capable of producing a very vast array of plastic materials. A relatively new product (could you imagine a world without plastic? your parents to some extent, and definitely your grand-parents, can), plastics can now be produced in an incredible variety of products with different properties. The basic chemical structure of plastics is the one of polymers, extremely long chains of intertwined molecular groups, typically centerd around a Carbon atom.

Some food for thought.

In spite of the technological advances, modern technologies cannot yet compete with millions of years of evolution: can one yet find a plastic that can take a nail or a screw the way wood does? On the other side, it takes many years for a tree to build a nice trunk. Such a slow process, if repeated industrially, could never become cost effective.

Another point in favour of Nature is that all natural substances are bio-degradable, while, unfortunately, plastics are not yet (would you like a world without plastic garbage? your grand-parents had it...).

Another term in the equation is the depletion of natural resources. Plastic are more often produced as petroleum derivates and our plastic production is using, in a few decades, the oil reserves that were formed over millions of years of early life on earth......there is no easy answer

LIQUIDS

When the attractive forces between molecules are not strong enough to keep them in a fixed position, but still sufficient to hold the whole body together, we have a liquid. An immediate proof of the force that keeps a liquid together is the surface tension.

Within the body of the liquid, the forces acting on a given molecule are, on the average, balanced, since surrounding molecules are pulling equally from all directions. For a molecule near the surface on the contrary, the pull from the body of the liquid is not balanced in the outside direction, therefore the surface molecules will feel a sizeable attraction towards the body of the liquid.

Surface tension is responsible for many phenomena, like the curvature of a liquid surface near the walls of the container, the capability of small insects to walk on water, the rolling up of small liquid volumes (e.g. dew) into beads, etc.

In spite of surface tension, tending keep molecules within the liquid, some of them will always escape from the surface.More in detail, one has a continuous exchange, with some molecules escaping into the surrounding medium and some others being re-captured from it. The amount of vapour (i.e. of the gaseous phase of the substance) that can be contained in the surrounding medium depends on the temperature. In the case of water and air for instance, at a given temperature only so much water vapour can be present in the air.

The actual water vapour content of the air (the humidity) is expressed in percent : this figure represents the fraction of the maximum allowed (for the given temperature) water vapour that is actually present in the air. When the temperature decreases, less water vapour can be accomodated, and the excess will condense.

Among the (not too numerous) substances that are liquid under normal conditions, water has a variety of very important properties. Apart from having a rather large surface tension, water can dissolve many substances (but obviously not all, water is no good for dissolving fats), it can store large amount of thermal energy (more on this below), and it has an anomalous behaviour with respect to temperature changes, that plays a fundamental role in the development of life on earth.

Let us remember the definition of density : density of a given substance is the weight of a unit volume of it (e.g. grams per cubic centimeter). It is an easily explained fact that the density of any substance (solid or liquid or gas free to expand) decreases with increasing temperature (or, which is the same, increases with decreasing temperature). The reason for this is straightforward : since increasing temperature causes a body to expand, but does not change the total number of atoms (or molecules) in the body, a fixed volume of an expanded body will contain less molecules, therefore it will weigh less.

This rule applies to water too, but with one notable anomaly: when cooled, water density will continue to increase until reaching 4⁰ C (about 40 F), at which point further cooling will cause the water to increase its volume, i.e its density will decrease.

You might think of this volume increase with decreasing temperature as a nuisance, since it causes your frozen pipes to burst (ice occupies more volume than the water it came from), but on the other side it allows the sea, where life originated, to remain unfrozen. If near freezing water, and ice, were heavier than slightly warmer water, they would sink to the bottom, causing the sea and the lakes to freeze from the bottom up, and so preventing the maintenance of life as we know it....

GASES

When the forces between molecules are not strong enough to hold them thogether, then we are in the presence of a gas. Notice that, contrary to some naive pre-conception, gases are not necessarily light : Radon, the heaviest member of the rightmost column in the Periodic Table, is heavier than Lead !! And by now we should know why the members of that family are all gases : because of their full outer shell, these elements do not attach easily to anything else, including themselves... The reason why a gas is light, is because of its density or, which is the same, the number of molecules in a given volume. Given that the gas, if left to itself, will expand to occupy all the free space around it, the number of molecules in a given volume will always be at a minimum. Heavily compressed, or, even better, "liquid" gas will be as dense as any other material.

When dealing with gases, an important quantity is the pressure. Our understading of a gas as an assembly of molecules in continuous random motion allows us to understand immediately the origin of gas pressure: if we have a certain amount of gas enclosed in a container, pressure is a the macroscopic manifestation of the energy transferred by the innumerable collisions of the gas molecules against the container's wall. This picture in turn allows us to understand some of the basic features of pressure:

• adding more gas to a container, the pressure increases. This is because, with more molecules present, the average number of collisions per unit time will increase
• if a gas is contained in a fixed volume, increasing its temperature will increase the pressure. To explain this, we should remember that the temperature of a body is a measure of the average kinetic energy of its molecules. Increasing the temperature, the energy transferred to the container wall by the molecules will increase accordingly.

Macroscopically, pressure is defined as the force acting on a unit surface, p = F/S, and its standard, albeit not widely used, units are then Newton/m² = pascal. You probably are more familiar with "pounds per square inch" (PSI), or maybe some other unit traditionally used to measure atmospheric pressure.

What is atmospheric pressure ? Effectively, it is the weight of the air column sitting above us, and this weight is not negligible. Expressed in familiar units, atmospheric pressure is about 14.7 PSI, i.e. the atmosphere exerts, on every square inch of your body, a weight of about 15 pounds!! In reality, you don't feel this effect since the pressure is the same inside and outside your body. If your body was to be sealed, and air extracted from it, you would be immediately crushed. As we will explore later, variations of atmospheric pressure affect in a very direct way the weather.

In other traditional units, atmospheric pressure is equivalent to the weight of a 760 mm (30 in) column of mercury or a 10 m (30 ft) column of water.


QUESTION : which of the following phenomena are not caused by variations in atmospheric pressure ?


A operation of barometers

B ocean tides

C changes in weather

D wind

E they all are

PLASMA

In addition to the traditional states of matter -solid, liquid and gas- more recently a new state was recognized, the Plasma state. Plasma is typically encountered when a gas is brought to such a high temperature that the inter-molecular collisions have enough energy to strip off the electrons from the nuclei. A substance is therefore in a plasma state when it is fully ionized.

Plasma, which is the state of matter found in the stars, can also be produced, on a small scale, in the laboratory, where attempts are made to control the process of nuclear fusion. Being electrically charged, plasma is affected by electric and magnetic fields. In the (hot) fusion research, "magnetic bottles" (i.e. suitably shaped, very strong magnetic fields) are employed to contain plasma at high enough densities and temperature, so as to trigger the fusion reactions.

Plasma state can also be present at low temperatures, provided the substance is in a state of extremely low density, so that stripped off electrons have a very low probability of recombining. This is what happens in the higher levels of the atmosphere, where a layer of highly ionized gases, called the ionosphere, is found. Before the advent of artificial satellites, the ionosphere played a very useful role in the long distance transmission of radio signals. Electromagnetic waves of the right wavelength (the so called "short waves", $\lambda\sim 50 m$ or less) could bounce a few times between earth and ionosphere, and in doing so could travel around the earth.