The Handy Chemistry Answer Book. Justin P. Lomont

that air has weight. His experiment to prove this fact was prompted by the observation that water from a mineshaft could only be pumped upward to reach a certain height. Torricelli thought that the air pushing down on the surface of the water must play a role. To test this theory, in 1643 he placed a sealed tube of mercury upside down in a bowl of mercury. He observed that the weight of the air would keep the mercury in the tube at a certain level, and on different days he observed that the mercury would rise to different levels. We now know this is because the air pressure varies from day to day, and Torricelli’s experiment was the first barometer.

      Who first realized that oxygen gas (O₂) was required for fire?

      Philo of Byzantium in the second century B.C.E. was the first to observe (or at least the first to record such an observation) that if you placed a jar on top of a candle with water around its base, some water would be drawn up into the jar as the candle burned and eventually went out once all the oxygen was consumed. Although the experiment was well-designed, he ended up with an incorrect conclusion about the process. Robert Boyle repeated the experiment but replaced the candle with a mouse (seriously), and noticed the water also rose up the container. From this experiment he correctly inferred that whatever the component in air was (he called it nitroaerues), it was needed for both combustion and respiration. Robert Hooke, and others, likely produced oxygen gas in the seventeenth century, but didn’t realize it was an element as the phlogiston theory (see below) was in vogue at the time. So to really realize that oxygen gas was required for fire, it first had to be, well, discovered.

      What is the theory of phlogiston?

      In 1667, a scientist named Johann Joachim Becher introduced the theory of phlogiston as an explanation for the various observations scientists had made regarding combustion. These observations include the fact that some objects can burn while others cannot, and that a flame in a sealed container can go out before the combustible material is consumed. Becher proposed that a weightless (or almost weightless) substance called phlogiston was present in all materials that could burn and that this phlogiston was the substance being given off during combustion. If a candle placed in a closed container went out, Becher said this was because the phlogiston from the candle was moving into the air and that the air could only absorb a certain concentration of phlogiston before it became saturated and could no longer absorb more phlogiston from the candle. Another tenet of this theory was that the purpose of breathing was to remove phlogiston from the body. Air that had been used for combustion couldn’t be used to breathe then because it was already saturated with phlogiston.

      How was the theory of phlogiston disproved?

      Antoine Lavoisier, an eighteenth-century French chemist, disproved the theory of phlogiston by showing that combustion required a gas (oxygen) and that that gas has weight. Lavoisier did this by burning elements in closed containers. These solids gained mass, but the total weight of the containers did not change—what did change was the pressure inside the vessel. When Lavoisier opened the vessel up, air rushed in, and the total weight of the vessel increased. So Becher had it backward: oxygen was being used up by the candle instead of phlogiston being given off by the flame.

      How was oxygen gas first discovered?

      Well, to answer that question, you would first want to know who first discovered oxygen, and there is no simple answer to that question! There are three people to whom discovery of this can be ascribed: Carl Wilhelm Scheel, Joseph Priestley, and Antoine Lavoisier. Scheele produced O₂ (he called it “fire aire”) from mercuric oxide (HgO) in 1772, but the result wasn’t published until 1777. Meanwhile, in 1774 Priestley produced O₂ (he called it “dephlogisticated air”) using a similar experiment, which was published in 1775. Lavoisier claimed to have independently discovered the gas, and was in fact the first to explain how combustion worked via quantitative experiments, leading to the principle of Conservation of Mass, and ultimately disproving the entire idea of phlogiston. Whew. So Scheel found it first, but didn’t report it; Priestley reported it first, but didn’t have the explanation correct; and Lavoisier was last, but nailed it. Who would you give credit to?

      What is electrochemistry and how was it discovered?

      Modern electrochemistry studies reactions that take place at the interface of an electronic conductor and a source of charged ions (possibly a liquid). The development of electrochemistry began with studies on magnetism, electric charge, and conductivity. The earliest experiments typically focused on questions surrounding properties of materials; for example, which materials can be magnetized and which materials can be charged? As early as the 1750s scientists had discovered that electrical signals were important to human life and were using them to treat medical issues such as muscle spasms. In the late 1700s, Charles Coulomb developed laws describing the interactions of charged bodies, which are still used widely today and taught in any introductory course on electricity and magnetism.

      The first electrochemical cells were developed during the 1800s. Electrochemical cells are arrangements of electrodes and sources of ions that either generate electric current from a chemical reaction, or alternatively, use electricity to drive a chemical reaction. Today these cells find applications in daily life, such as in the batteries that power your car or cell phone. Today electrochemistry still constitutes an important field of research and is one that will likely continue to lead to the development of new products and technologies.

      What is the law of definite proportions?

      The law of definite proportions says that a substance always contains the same proportions of each element of which it’s composed. For example, a molecule of water (H₂O) always contains two hydrogen atoms for every oxygen atom. This is commonly understood among modern chemists, but it was an important step in working toward a microscopic understanding of the composition of matter. The first to make such claims, in the early 1800s, was the French chemist Joseph Proust. It was a controversial idea at that time, and other chemists believed that elements could be combined in any proportion.

      What is Avogadro’s constant?

      Avogadro’s constant is a large number used to discuss large quantities of atoms or molecules, usually when chemists talk about quantities they can actually see or measure out. The number itself (rounded at three decimal places) is 6.022 × 10²³. It’s just a big number that relates an atomic or molecular mass to the mass of a collection of many atoms or molecules. Avogadro’s number of atoms of an element is called a mole of that element, and, similarly, Avogadro’s number of molecules of a compound is a mole of that compound. For example, the atomic mass of oxygen is about 16 grams per mole, and 6.022 × 10²³ atoms (1 mole) of oxygen weigh(s) about 16 grams. The most recent (and accurate) definition of this constant was 6.02214078(18) × 10²³, which was calculated by careful measurements of the mass and volume of 1-kilogram (about 2.2 lbs.) spheres of silicon-28, a particular isotope of silicon (see next chapter concerning isotopes).

      When was Avogadro’s constant discovered?

      Amedeo Carlo Avogadro published a paper in 1811 describing his theory that a volume of gas (at a given temperature and pressure) contains a certain number of atoms or molecules regardless of what gas it is. Avogadro didn’t actually determine what that number was, however. It took just over fifty years for someone to make progress on that: Johann Josef Loschmidt, in 1865, estimated the average size of molecules in air. It’s nothing short of amazing that he ended up being off by only a factor of two. Jean Perrin, a French physicist, accurately determined the constant using a few different techniques. He was awarded the Nobel Prize in Physics in 1926 for the work, but Perrin proposed that the constant be named for Avogadro—and the name stuck. (For more on the use of the constant, see “Atoms and Molecules.”)

      Why is chemistry “the central science”?

      Chemistry is called the central science because it’s related to everything! It connects and draws from topics in biology, physics, materials science, mathematics, engineering, and other fields. Chemistry is important to how our body functions, to the food we eat, to