Matter and Measurement

According to a tale told by the Roman writer Vitruvius, the king of Syracuse suspected that a goldsmith had cheated him by replacing some of the gold in a votive wreath with a cheaper metal. He asked Archimedes to find out without damaging the wreath. Archimedes is said to have noticed, while lowering himself into a bath, that the water level rose, realised that the displaced water could give the wreath's volume, and run through the streets shouting "Eureka!". Wikipedia's Density article calls the story "well-known but probably apocryphal", and says it first appears in Vitruvius about two centuries later. Whether or not it happened, the idea in it is the starting point of chemistry: different materials have measurable properties, and you can tell them apart by measuring them. This chapter sets out what chemistry studies and the vocabulary the rest of the course uses.

What Chemistry Is

Wikipedia's Chemistry article opens with a short definition: "Chemistry is the scientific study of the properties and behavior of matter." It adds that chemistry is a physical science covering the composition and structure of matter and the changes matter undergoes in reactions, and that it addresses the nature of chemical bonds. It lists five main branches: analytical, biochemistry, organic, inorganic and physical chemistry (the picture caption adds applied chemistry as a sixth). This course is about the ideas that all of them share.

Matter and Its States

Matter is "anything that has mass and occupies space by having volume", in the wording of Wikipedia's Matter article, which also says physics has no single agreed definition and that the answer depends on the scale and the field. For chemistry the everyday version is enough. The same article describes the "classical everyday phases such as solid, liquid, and gas", and lists others: plasma, Bose-Einstein condensates, fermionic condensates and quark-gluon plasma.

The descriptions of the particles in the three cards are the standard textbook picture; I have written them from general knowledge, and Chapter 2 supplies the particle model they rest on (check against your own textbook). A change between states, such as ice melting or water boiling, rearranges the particles but does not make a different substance.

Substances and Mixtures

Wikipedia's Chemical substance article defines a substance as "a unique form of matter with constant chemical composition and characteristic properties". Compare:

The examples in the right-hand column are mine, chosen as standard cases. The article states the law of constant composition as "All samples of a compound have the same composition", and says there are 118 known elements, about 80 of them stable. It notes that a mixture's parts are combined physically, with no chemical reaction, so they can be separated by "mechanical processes, such as chromatography, distillation, or evaporation"; its example of a magnet pulling iron away from sulfur shows the idea. When a mixture is separated so that one substance is isolated, the Matter article says the result is "chemically pure".

Physical and Chemical Change

Melting ice and rusting iron are both changes, but they differ. In a physical change the same substance is still there afterwards, perhaps in a different state or shape. In a chemical change (a chemical reaction) one or more new substances form. The Chemistry article describes reactions as transformations that rearrange electrons in bonds to form different substances; it does not state the physical-versus-chemical distinction in so many words, so the two-way split above is the usual textbook one, given here from general knowledge. Clues that a chemical change may have happened include a colour change, a gas given off, a solid forming in a liquid, or heat or light released, though none of them is proof on its own (check; a boiling kettle gives off a gas, and no new substance forms).

Measuring Matter: Mass, Volume and Density

Two measurements describe how much matter there is: mass (how much matter) and volume (how much space it takes). Neither identifies a material, since a kilogram of lead and a kilogram of feathers have the same mass. Their ratio does. The Density article defines density as "mass per unit volume", written ρ = m / V, with the SI unit kilogram per cubic metre.

Where to Read More

• Encyclopaedias. Wikipedia's Chemistry, Matter, Chemical substance and Density articles are the sources used in this chapter. I read the Chemistry and Matter articles only partly (each to the first 100,000 characters of about 200,000), the Chemical substance and Density articles in full.
• A school textbook. A GCSE or equivalent chemistry textbook covers this material with diagrams and practice questions.
• Practical. Weighing a measured volume of water, then of cooking oil, shows density directly with a kitchen scale and a measuring jug.

Exercises