What is Chemistry? (more detailed)

At one time it was easy to define chemistry. The traditional definition goes something like this: Chemistry is the study of the nature, properties, and composition of matter, and how these undergo changes. That served as a perfectly adequate definition as late as the 1930s, when natural science (the systematic knowledge of nature) seemed quite clearly divisible into the physical and biological sciences, with the former being comprised of physics, chemistry, geology and astronomy and the latter consisting of botany and zoology. This classification is still used, but the emergence of important important fields to study such as oceanography, paleobotany, meteorology, pharmacy and biochemistry, for example, have made it increasingly clear that the dividing lines between the sciences are no longer at all sharp. Chemistry, for instance, now overlaps so much with geology (thus we have geochemistry), astronomy (astrochemistry), and physics (physical and analytical chemistry) that it is probably impossible to devise a really good modern definition of chemistry, except, perhaps, to fall back on the operational definition: Chemistry is what chemists do!

Chemistry plays an important part in all of the other natural sciences, basic and applied. Plant growth and metabolism, the formation of igneous rocks, the role played by ozone in the atmosphere, the degradation of environmental pollutants, the properties of lunar soil, the medical action of drugs, establishment of forensic evidence: none of these can be understood without the knowledge and perspective provided by chemistry. Indeed, many people study chemistry so that they can apply it to their own particular field of interest. Of course, chemistry itself is the field of interest for many people, too. Many study chemistry not to apply it to another field, but simply to learn more about the physical world and the behaviour of matter from a chemical viewpoint. Some simply like "what chemists do" and so decide to "do it" themselves.  

Chemistry is a way of studying matter. What is matter? As is true with many of those words which are really basic to science, matter is hard to define. It is often said that matter is anything which has mass and occupies space. But then what are "mass" and "space"? Although we can define these, the process yields very little insight into what matter is. So let us just say that matter is anything which has real physical existence; in a word matter is just stuff. Iron, air, wool, gold, milk, aspirin, monkeys, rubber, and pizza - these are all matter. Some things which are not matter are heat, cold, colours, dreams hopes, ideas, sunlight, beauty, fear, and x-rays. None of these is "stuff"; none is matter.

A sample of matter can be either a pure substance or a mixture. A pure substance has a fixed, characteristic composition an a fixed, definite set of properties. Pure substances are for example copper, salt, diamond, water, table sugar, oxygen, mercury, vitamin C, and ozone. A pure substance may be a single element, such as copper or oxygen, or a compound of two or more elements in a fixed ratio, such as salt (39.34 % sodium and 60.66 % chlorine) or table sugar (42.11 % carbon, 6.48 % hydrogen, and 51.41 % oxygen).

A mixture is a collection of pure substances simply mixed together. Its composition is variable, as are its properties. Examples of mixtures are milk, wood, concrete, saltwater, air, granite, motor oil, chocolate, and elephants.

A pure substance can be a solid, a liquid, or a gas; these are the three states of matter A solid maintains its volume and shape; a liquid, its volume only; and a gas, neither. Solids tend to be hard and unyielding; liquids maintain their volumes and flow to adopt the shapes of their containers. The ability to flow is called fluidity, and so gases and liquids are called fluids.

One of the goals of chemistry is to be able to describe the properties of matter in terms of its internal structure, the arrangement and interrelationship of its parts. This word, structure, sometimes refers to the physical arrangement of particles, such as atoms or molecules in space. At other times it is used to indicate some other arrangement, such as the arrangement of energy levels of an electron in an atom. The structure of matter determines its properties. Properties can be classed as either physical or chemical. A physical property of a substance can be characterized without specific reference to any other substance and usually describes the response of the substance to some external influence, such as heat, light, force, electricity, etc. Physical properties include boiling point, melting point, thermal (heat) conductivity, colour, refractive index, viscosity, reflectivity, hardness, tensile strength, and electrical conductivity.

A chemical property, on the other hand, describes a chemical change: the interaction of one substance with another , or the change of one substance into another. Iron rusts in a moist environment, unrefrigerated milk turns sour, wood burns in air, photographs bleach when exposed to sunlight for a long time, dynamite explodes - each of these is a chemical property because each involves chemical change. During chemical changes, substances are actually changed into other substances. The simultaneous disappearance of some substances (called the reactants) and appearance of others (the products) is characteristic in chemical change (chemical reaction). Chemical changes are generally characterized by pronounced internal structural rearrangements.

Physical changes are not characterized by the transformation of one substance into another, but rather by the change of the form of a given substance. The bending of a piece of copper wire fails to change the property of copper into another substance; crushing a block of ice leaves only crushed ice; melting an iron nail yields a substance still called iron: These are all usually accepted as physical changes.

Properties of matter may also be categorized as either macroscopic or microscopic. A macroscopic property describes characteristics or behaviour of a sample which is large enough to see, handle, manipulate, weigh, etc. A microscopic property describes the behaviour of a much smaller sample of matter, an atom or molecule for instance.. Macroscopic and microscopic properties are often different. A banana is yellow, but we do not use colour to describe an atom. Some properties, on the other hand, can be either microscopic or macroscopic; mass is one of these.

Another word that is often used is system. A system is a portion of the universe which we wish to observe or consider. The size of the portion is usually small and a system may be a real one (in a test tube or flask, for example), or an imaginary one which this text is just referring to.  

   Viewed from an historical point of view, it is clear that scientific knowledge has been obtained and that therefore  science has "advanced" in a series of fairly logical steps. On the other hand, counterparts to these steps are difficult to identify in the day-to-day professional activities of a scientist. The way in which science and in particular chemistry advances can be describes in terms of a series of cycles (see diagram below). Observations and data (and laws) lead to the proposal of theories that, in turn, suggest predictions which can be tested by designing new experiments, and the whole process starts all over again.