CHAPTER 4


WHAT ARE SOME PROPERTIES OF


?

The most obvious property of
water is its ability to flow, to
escape, to slide through cracks,
to slip through the fingers.
Another obvious property is that
water easily fills its
containers--from a bottle to a
river bed.
--Paul Caro
Water


Aristotle believed that all substances in nature consisted of some combination of four elements: earth, water, fire and air. Water fit neatly into this Aristotelian model of nature. Evaporated water became air, and the invisible water vapor upon condensing became water again. Aristotle’s method of doing science did not involve experiments, but was instead based purely on reason. Democritus was another ancient Greek who espoused a different model for matter which included water. Democritus’ model, also based on "pure reason" rather than experimentation, was that all matter consisted of "atoms" smaller than could be discerned by the eye.

Aristotelian "science" without experimentation prevailed western thought for 2000 years, and the Aristotelian "four element model" was generally accepted (not Democritus' "atomic model" of matter) until experimental science emerged during the sixteenth and seventeenth centuries in Europe.

Ancient Asians also believed that water was a fundamental entity representing the source of everything on the planet. Water has also been central to many spiritual rituals throughout the ages in many other early cultures. Not until the mid-sixteenth century did English chemist Robert Boyle (1627-91) revive Democritus' "atomic model" of matter when Boyle defined a chemical element. Boyle also came to the important conclusion that water should be excluded from among the most fundamental entities into which substances could be divided.

The scientific community, however, was slow to make the transition from the four Aristotelian "elements" to accepting the scientific evidence that atoms are the basic constituents of all matter. In fact, another century passed before Boyle’s revolutionary revival of the "atomic" concept of matter was generally accepted. In 1783 Antoine-Laurent de Lavoisier (1743-1794) produced convincing experimental evidence that water is a compound of other more fundamental chemical elements (or atoms), namely, hydrogen and oxygen. A molecule is a compound consisting of two or more atoms physically associated by chemical bonds. Thus Lavoisier showed through indirect evidence that water was a molecule consisting of two atoms of hydrogen, and one of oxygen.

Most recently, with the development of powerful modern microscopes developed in the 1970's and 1980's, it has become possible to image for the first time individual atoms and molecules. The image at the right was obtained by Dr. B.L. Ramakrishna using a scanning probe microscope from ASU's Center for Solid State Science. The blue peaks in the image represent individual molecules of graphite. With images like this one and ones with even higher resolution, we now have direct evidence to support Democritus' "atomic" theory of matter".

As we approach the 21st century, we continue to add details which improve our model for atomic structure. Scientific research is also still probing the basic structure of the water molecule, one of the most prevalent and most important chemical compounds on our planet. In fact, many details of the molecular model for water are still under development. Scientific enlightenment is generally regarded to have appeared during the sixteenth and seventeenth centuries in Europe because controlled experiments prevailed as the way to understand the laws of nature. The ancient Greeks never gained sufficient respect for scientific experimentation that we can today attribute the beginnings of science to that culture, even though, in retrospect, we know that Democritus had proposed the correct model for matter. He did not perform scientific experiments to test his model. We do know that there were a few exceptions of scientifically enlightened ancient Greek scholars. Archimedes was one of them, and he explored what caused objects to sink and float in water.

Personal Lab 4: What Factors Determine Whether Objects Float or Sink?
[Chapter4]

Your experiments with water involved the use of materials have similar volumes, but different masses, and therefore different weights. You found that the net weight of an object depended on the net effects of gravity and the buoyant force of water. You also found that different materials have properties called densities. Density is a fundamental property of matter that provides a measure of the mass in a unit volume, where a unit volume is generally taken to be a cubic meter (m3). Therefore density is expressed in units of kilograms/m3. Density of an object is its weight divided by its volume.

An atom, until late in the 20th century was regarded as the fundamental building block of all materials, and consists of a nucleus surrounded by one or more electrons. The nuclei of atoms are comprised of protons and neutrons. The neutrons have no electric charge, and so are "electrically neutral". A single proton has "one unit" of positive electric charge. An electron has "one unit" of negative electric charge. An atom in its neutral or unionized state has equal numbers of positive and negative charges (i.e. equal numbers of protons and electrons). Even though balanced numerically, the electrons are constantly redistributing their positions around the nucleus of an atom, so that the charge distribution may not on average be symmetrical. In the next chapter you will perform an experiment where you encountered the effects of an imbalance in the distribution of positive and negative charges around the water molecules. This uneven distribution of charge in a water molecule can cause a stream of water to respond to an electric force. Above we noted that molecules are combinations of atoms held together by chemical bonds. What is the source of these chemical bonds? The force of attraction that constitutes the chemical bond that holds atoms together to form molecules is the electrostatic force. This electrical force of attraction arises because there are unbalanced electric charges in atoms.

We wish to consider substances on such small scales that we must develop a model of the smallest structures that cannot be broken down further without destroying the chemical character of the substance, namely, atoms. We will explore the atomic model of the material world using water as an example of a molecular compound consisting of two hydrogen atoms and one oxygen atom. If the overall electric charge distribution of any atom is positive, then the atom can attract and bond to another atom if a net negative charge is sensed. Since atomic nuclei are surrounded by electrons in our model, there are negative charge distributions. In your experiments you compared the strengths of the gravitational and the electric forces, and found that the electric forces are very powerful. In fact we know that the strength of the electric force is 1040times stronger than that of the gravitational force! Therefore the forces at the atomic scales are very strong.

Water, one of the simpler molecules, consists of one oxygen atom and two hydrogen atoms. An oxygen atom has 16 protons and 16 neutrons in its nucleus, and 16 electrons normally comprise the electron distributions that surrounds the nucleus. Because of the way the electrons are distributed, the oxygen atom is said to have a valence of two, or has an affinity or attraction for two units of negative charge. The simplest chemical element, and by far the most abundant element in the universe is the hydrogen atom, consisting of a unique nucleus, which contains only one proton, and no neutron. An electron is bound to the proton due to the force of attraction between the electrostatic charges between the proton (positive charge) and electron (negative charge) in the hydrogen atom. The electron in a hydrogen atom is available to form chemical bonds with oxygen atoms, if they come sufficiently close, with relative speeds conducive for forming the chemical bond induced by the unlike electric charges and resulting electrostatic force of attraction. The interaction needed for a water molecule to form from two H atoms and an O atom, is for a low speed collision or interaction to occur among these three atoms. Usually the interactions would happen sequentially with, perhaps, an O and an H atom interacting to form OH. If another H is on collision course with the newly formed OH molecule, then there is a good chance that the collision of OH with H will succeed in forming water, or H2O. Chemists abbreviate this process of successive low speed collisions, called a chemical reactions, in a shortened form which is written as

O + H = OH

OH + H = H20


Because both oxygen and hydrogen are very abundant on our planet, and the temperature and pressure at the surface are in the appropriate ranges, liquid water covers most of the surface of the Earth. Interestingly the water molecule has very peculiar characteristics compared to other molecules.

Water is liquid at room temperatures. Water is peculiar because it is more dense in liquid form than in the solid form, called ice. What happened to the volume of the liquid water that you froze in the film canister? What did you observe when you tried to submerge the frozen water for the experiment on buoyant forces? All forms of water are familiar to us, water, ice and steam or water vapor. The chemical compound water is unusual in that the temperature and atmospheric pressure which determine when water makes the transition between liquid, vapor and solid happen to be values typically found on the surface of our planet. Could it be that life evolved on Earth easily because the prevailing conditions on the planet favored water in liquid form? No other planet in the solar system presently has conditions favoring water existing in a liquid state. Billions of years ago Mars may have had a thicker atmosphere, and therefore a higher atmospheric pressure than today. With a slightly higher atmospheric pressure and the current temperatures we observe on the planet Mars (maximum daytime temperatures reach about 20 degrees C (70 degrees F) in the Martian summer), planetary geologists have constructed models which predict that Mars could have been covered by oceans of water several billion years ago. If this model is correct, then what happened in the interim? If true, then how could Mars have lost these hypothetical oceans? Very recently a group of scientists reported that they had found circumstantial evidence for primitive life forms in meteorites known to have originated on Mars when there could have been oceans of water. If these preliminary scientific results are confirmed by additional experimental evidence, it is very intriguing to consider the profound impact of living in a planetary system in which two (not one) of the planets developed life.

We are all familiar with the three phases of water: 1) a solid phase called ice, 2) a liquid phase, and 3) a gas phase called water vapor. What determines when water takes on these very different forms? Is it only the temperature that matters, or are there additional variables that determine the transitions between the various phases of substances, including water?

The liquid phase means that a substance flows into a container and assumes the inside shape of the container. Assisted by gravity, a cup of water with its free, exposed surface assuming a horizontal position is recognizable to all. Is the surface of a glass of water actually flat? Examine the surface of the water in a glass carefully.

What if you were in a zero gravity environment, such as a space craft orbiting the Earth? What would happen to a "glass of water"? How might you simulate a weightless glass of water without getting launched into space? When we apply our molecular model to liquid water, we think of the molecules being free to move individually, but also influencing neighboring molecules, so when we pour water from a glass, we see a stream flowing down, rather than the individual molecules dispersing in a large volume, like the water vapor rising from the tea pot. This property of water that makes it flow is viscosity. Liquids such as honey and molasses have very large viscosities. All fluids (gases and liquids) actually have some measurable viscosity, but that for liquid water and water vapor are very small.

Liquids have a kind of outer layer or skin which arises because of a phenomenon known as surface tension. Surface tension is characteristic of liquids at the interfaces with boundaries, such as where water and air meet, or water and the glass that contains it meet. You will perform an experiment using a drop of water on plastic wrap in one of your Personal Labs. Examine a drop of water on a piece of plastic wrap. What do you notice? What happens to a drop of water on a piece of newspaper? What do you think causes the difference? The reason that the water drop on the plastic wrap was rounded rather than flat, is found in the combined phenomena of surface tension and atmospheric air pressure. The individual molecules of water in liquid form are free to assume a lowest energy configuration. This happens when the H atoms in the water molecules of liquid water are oriented preferentially toward the air-liquid interface. When water molecules all have their H atoms extending "outwards", as in the case of a drop of water, the drop tends to pull itself together into a round shape. In addition air molecules exert a pressure on the liquid surface of a drop of water and help contain the water in its rounded shape. The electrostatic forces dominate the structure of a tiny drop of water. However, when the liquid has a larger mass, such as in a glass of water, then weight of the liquid (i.e. the force of gravity) dominates the surface tension forces and determines the shape of the "free surface" of the liquid in the glass.

Generally we refer to both liquids and gases as fluids. However, these to difference phases of the same chemical compound respond very differently to pressure. For example, water vapor can be compressed in a pressure cooker, whereas once we have filled a container with liquid water, we cannot make room for more by applying pressure to the water. Liquids are very difficult to compress because their molecules are essentially in contact with one another. For this reason we refer to liquids as incompressible fluids. Gases, on the other hand, have relatively large distances between their molecules. Consequently gases can easily be compressed to fill smaller volumes, or add more gas to a given volume, as in the case of the pressure cooker which continually produces more steam while being heated. We refer to gases as compressible fluids.

Even though water and other liquids are incompressible, they can transfer pressure very efficiently. Hydraulic pressure is produced by applying pressure on one part of a liquid, and having its affect applied throughout the entire liquid. In fact, this is the principle underlying a hydraulic lift and the brakes in your automobile.

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