Chem1 General Chemistry Virtual Textbook → Preliminaries
Classification and properties of matter
Matter is “anything that has mass and occupies space”, we were taught in school. True enough, but not very satisfying. A more complete answer is unfortunately far beyond the scope of this course, but we will offer a hint of it in the later section on atomic structure. For the moment, let’s side-step definition of matter and focus on the chemist’s view: matter is what chemical substances are composed of. But what do we mean by chemical substances? How do we organize our view of matter and its properties? These will be the subjects of this lesson.
The science of chemistry developed from observations made about the nature and behavior of different kinds of matter, which we refer to collectively as the properties of matter.
By observing a sample of matter and measuring its various properties, we gradually acquire enough information to characterize it; to distinguish it from other kinds of matter. This is the first step in the development of chemical science, in which interest is focussed on specific kinds of matter and the transformations between them.
If you think about the various observable properties of matter, it will become apparent that these fall into two classes. Some properties, such as mass and volume, depend on the quantity of matter in the sample we are studying. Clearly, these properties, as important as they may be, cannot by themselves be used to characterize a kind of matter; to say that “water has a mass of 2 kg” is nonsense, although it may be quite true in a particular instance. Properties of this kind are called extensive properties of matter.
This definition of the density illustrates an important general rule: the ratio of two extensive properties is always an intensive property.
Suppose we make further measurements, and find that the same quantity of water whose mass is 2.0 kg also occupies a volume of 2.0 litres. We have measured two extensive properties (mass and volume) of the same sample of matter. This allows us to define a new quantity, the quotient m/V which defines another property of water which we call the density. Unlike the mass and the volume, which by themselves refer only to individual samples of water, the density (mass per unit volume) is a property of all samples of pure water at the same temperature. Density is an example of an intensive property of matter.
Intensive properties are extremely important, because every possible kind of matter possesses a unique set of intensive properties that distinguishes it from every other kind of matter. Some intensive properies can be determined by simple observations: color (absorption spectrum), melting point, density, solubility, acidic or alkaline nature, and density are common examples. Even more fundamental, but less directly observable, is chemical composition.
Intensive properties are extremely important, because every possible kind of matter possesses a unique set of intensive properties that distinguishes it from every other kind of matter. In other words, intensive properties serve to characterize matter. Many of the intensive properties depend on such variables as the temperature and pressure, but the ways in which these properties change with such variables can themselves be regarded as intensive properties.
Classify each of the following as an extensive or intensive property.
|The volume of beer in a mug||ext; depends on size of the mug.|
|The percentage of alcohol in the beer||int; same for any same-sized sample.|
|The number of calories of energy you derive from eating a banana||ext; depends on size and sugar content of the banana.|
|The number of calories of energy made available to your body when you consume 10.0 g of sugar||int; same for any 10-g portion of sugar.|
|The mass of iron present in your blood||ext; depends on volume of blood in the body.|
|The mass of iron present in 5 mL of your blool||int; the same for any 5-mL sample.|
|The electrical resistance of a piece of 22-gauge copper wire.||ext; depends on length of the wire.|
|The electrical resistance of a 1-km length of 22-gauge copper wire||int; same for any 1-km length of the same wire.|
|The pressure of air in a bicycle tire||pressure itself is intensive, but is also dependent on the quantity of air in the tire.|
The last example shows that not everything is black or white!
But we often encounter matter whose different parts exhibit different sets of intensive properties. This brings up another distinction that we address immediately below.
|One useful way of organizing our understanding of matter is to think of a hierarchy that extends down from the most general and complex to the simplest and most fundamental. The orange-colored boxes represent the central realm of chemistry, which deals ultimately with specific chemical substances, but as a practical matter, chemical science extends both above and below this region.|
Alternatively, it is sometimes more useful to cast our classification into two dimensions:
Both dimensions are defined in terms of intensive properties, so if you are not sure what these are, be sure to re-read the material in the preceding section. We will begin by looking at the distinction represented in the top line of the diagram.
Homogeneous matter (from the Greek homo = same) can be thought of as being uniform and continuous, whereas heterogeneous matter (hetero = different) implies non-uniformity and discontinuity. To take this further, we first need to define "uniformity" in a more precise way, and this takes us to the concept of phases.
A phase is a region of matter that possesses uniform intensive properties throughout its volume. A volume of water, a chunk of ice, a grain of sand, a piece of copper— each of these constitutes a single phase, and by the above definition, is said to be homogeneous.
A sample of matter can contain more than a single phase; a cool drink with ice floating in it consists of at least two phases, the liquid and the ice. If it is a carbonated beverage, you can probably see gas bubbles in it that make up a third phase.
Each phase in a multiphase system is separated from its neighbors by a phase boundary, a thin region in which the intensive properties change discontinuously. Have you ever wondered why you can easily see the ice floating in a glass of water although both the water and the ice are transparent? The answer is that when light crosses a phase boundary, its direction of travel is slightly bent, and a portion of the light gets reflected back; it is these reflected and distorted light rays emerging from that reveal the chunks of ice floating in the liquid.
If, instead of visible chunks of material, the second phase is broken into tiny particles, the light rays usually bounce off the surfaces of many of these particles in random directions before they emerge from the medium and are detected by the eye. This phenomenon, known as scattering, gives multiphase systems of this kind a cloudy appearance, rendering them translucent instead of transparent. Two very common examples are ordinary fog, in which water droplets are suspended in the air, and milk, which consists of butterfat globules suspended in an aqueous solution.
Getting back to our classification, we can say that
Dichotomies ("either-or" classifications) often tend to break down when closely examined, and the distinction between homogeneous and heterogeneous matter is a good example; this really a matter of degree, since at the microscopic level all matter is made up of atoms or molecules separated by empty space! For most practical purposes, we consider matter as homogeneous when any discontinuities it contains are too small to affect its visual appearance.
How large must a molecule or an agglomeration of molecules be before it begins to exhibit properties of a being a separate phase? Such particles span the gap between the micro and macro worlds, and have been known as colloids since they began to be studied around 1900. But with the development of nanotechnology in the 1990s, this distinction has become even more fuzzy.
The air around us, most of the liquids and solids we encounter, and all too much of the water we drink consists not of pure substances, but of mixtures. You probably have a general idea of what a mixture is, and how it differs from a pure substance; what is the scientific criterion for making this distinction?
To a chemist, a pure substance usually refers to a sample of matter that has a distinct set of properties that are common to all other samples of that substance. A good example would be ordinary salt, sodium chloride. No matter what its source (from a mine, evaporated from seawater, or made in the laboratory), all samples of this substance, once they have been purified, possess the same unique set of properties.
A mixture, in contrast, is composed of two or more substances, and it can exhibit a wide range of properties depending on the relative amounts of the components present in the mixture. For example, you can dissolve up to 357 g of salt in one litre of water at room temperature, making possible an infinite variety of "salt water" solutions. For each of these concentrations, properties such as the density, boiling and freezing points, and the vapor pressure of the resulting solution will be different.
"9944100% Pure: It Floats"
This description of Ivory Soap is a classic example of junk science from the 19th century. Not only is the term "pure" meaningless when applied to an undefined mixture such as hand soap, but the implication that its ability to float is evidence of this purity is deceptive. The low density is achieved by beating air bubbles into it, actually reducing the "purity" of the product and in a sense cheating the consumer.
We all prefer to drink "pure" water, but we don't usually concern ourselves with the dissolved atmospheric gases and ions that are present in most drinking waters. These same substances could seriously interfere with certain uses to which we put water in the laboratory, were we customarily use distilled or de-ionized water. But even this still contains some dissolved gases and occasionally some silica, but their small amounts and relative inertness make these impurities insignificant for most purposes. When water of the highest obtainable purity is required for certain types of exacting measurements, it is commonly filtered, de-ionized, and triple-vacuum distilled. But even this "chemically pure" water is a mixture of isotopic species: there are two stable isotopes of both hydrogen (H1 and H2, often denoted by D) and oxygen (O16 and O18) which give rise to combinations such as H2O18, HDO16, etc., all of which are readily identifiable in the infrared spectra of water vapor. (Interestingly, the ratio of O18/O16 in water varies enough from place to place that it is now possible to determine the source of a particular water sample with some precision.) And to top this off, the two hydrogen atoms in water contain protons whose magnetic moments can be parallel or antiparallel, giving rise to ortho- and para-water, respectively.
Since chemistry is an experimental science, we need a set of experimental criteria for placing a given sample of matter in one of these categories. There is no single experiment that will always succeed in unambiguously deciding this kind of question. However, there is one principle that will always work in theory, if not in practice. This is based on the fact that the various components of a mixture can, in principle, always be separated into pure substances.
Consider a heterogeneous mixture of salt water and sand. The sand can be separated from the salt water by the mechanical process of filtration. Similarly, the butterfat contained in milk may be separated from the water by a mechanical process known as centrifugation, which depends on differences in density between the two components. These examples illustrate the general principle that heterogeneous matter may be separated into homogeneous matter by mechanical means. Turning this around, we have an operational definition of heterogeneous matter: if, by some mechanical operation we can separate a sample of matter into two or more other kinds of matter, then our original sample was heterogeneous.
To find a similar operational defnition for homogeneous mixtures, consider how we might separate the two components of a solution of salt water. The most obvious way would be to evaporate off the water, leaving the salt as a solid residue. Thus a homogeneous mixture can be separated into pure substances by undergoing appropriate changes of state— that is, by evaporation, freezing, etc. If a sample of matter remains unchanged by carrying out operations of this kind, then it could be a pure substance.
Some common methods of separating homogeneous mixtures into their components are outlined below.
Distillation. A liquid is partly boiled away; the first portions of the condensed vapor will be enriched in the lower-boiling component.
Fractional crystallization. A hot saturated solution of a solid in a liquid is allowed to cool slowly; the first solid that crystallizes out tends to be of higher purity.
Liquid-liquid extraction. Two mutually-insoluble liquids, one containing two or more solutes (dissolved substances), are shaken together. Each solute will concentrate in the liquid in which it is more soluble.
Chromatography. As a liquid or gaseous mixture flows along a column containing an adsorbant material, the more strongly-adsorbed components tend to move more slowly and emerge later than the less-strongly adsorbed components.
Since chemistry is partly the study of the transformations that matter can undergo, we can also assign to any substance a set of chemical properties that express the various changes of composition the substance is known to undergo. Chemical properties also include the conditions of temperature, etc., required to bring about the change, and the amount of energy released or absorbed as the change takes place.
The properties that we described above are traditionally known as physical properties, and are to be distinguished from chemical properties that usually refer to changes in composition that a substance can undergo. For example, we can state some of the more distinctive physical and chemical properties of sodium:
physical properties (25°C)
appearance: a soft, shiny metal
density: 0.97 g cm3
melting point: 97.5°C
boiling point: 960°C
forms an oxide Na2O and a hydride NaH
burns in air to form sodium peroxide Na2O2
reacts violently with water to release hydrogen gas
dissolves in liquid ammonia to form a deep blue solution
Classify each of the statements as a physical or chemical property, and explain the basis for your answer.
|Chlorine is a greenish-yellow gas at room temperature.||This is another way of stating that the boiling point (a physical property) is below 20°C.|
|Liquid oxygen is attracted by a magnet.||Even under the influence of the magnet, the oxygen is still the same substance, O2, so the effect is purely a physical property.|
|Gold is highly resistant to corrosion.||Corrosion involves the reaction of a metal with oxygen and water, so corrosion (and by extension, resistance to corrosion) is definitely a chemical property.|
|Hydrogen cyanide is an extremely poisonous gas.||Most poisonous substances act by combining chemically with substances that interfere with some aspect of cellular biochemistry, so we can consider this to be a chemical property of HCN.|
|Sugar is a high-energy food.||The chemical energy contained in a food or fuel can be released only through a chemical reaction leading to lower-energy products. The “high-energy” part might be considered a physical property, since this depends on the quantity of energy obtainable from a given mass of the substance.|
Another dubious dichotomy
The more closely one looks at the distinction between physical and chemical properties, the more blurred this distinction becomes. For example, the high boiling point of water compared to that of methane, CH4, is a consequence of the electrostatic attractions between O-H bonds in adjacent molecules, in contrast to those between C-H bonds; at this level, we are really getting into chemistry! So although you will likely be expected to "distinguish between" physical and chemical properties on an exam, don't take it too seriously.
Make sure you thoroughly understand the following essential ideas which have been presented above. It is especially imortant that you know the precise meanings of all the highlighted terms in the context of this topic.
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