Hypotheses, theories and laws
When we find that an idea explains or correlates a number of facts, we call this idea a hypothesis. We can subject it to further tests and to experimental checking of deductions. If the hypothesis continues to agree with the results of experiment, we call it a theory or a law.
A theory, such as the atomic theory, usually involves some idea about the nature of some part of the Universe, a law represents a summarizing statement about observed experimental facts. For example, there is a law of the constancy of the angles between the faces of crystals. The law states that whenever we measure the angles between corresponding faces of various crystals of a pure substance, they will have the same value. It does not explain the fact. We find an explanation of the fact in the atomic theory of crystals, the theory that in crystals the atoms are in a regular order.
Chemists and other scientists use the word theory in two different senses. The first meaning of the word is the meaning described above namely a hypothesis that has been verified. The second use of the word theory is to represent a systematic body of knowledge, compounded of facts, laws, theories, deductive arguments and so on.
Thus, by the atomic theory we mean not only the idea that substances consist of atoms, but also all the facts about substances that can be explained and interpreted in terms of atoms and the arguments that explain the properties of substances in terms of their atomic structure.
Answer the following questions:
1. What is a theory?
2. What theories do chemists use in their work?
3. What do we mean by the atomic theory?
The worlds greatest chemist
The periodic system of the chemical elements by Mendeleyev has long since served as the greatest history-making contribution to the study of nature. As any work of genius it shows two characteristic features: it adds more to the present knowledge, and it fruitfully develops along different directions in future.
It allowed to predict in advance the existence and properties of yet undiscovered elements. Many outstanding researchers owe to it, to a considerable degree, the ideas of their experiments, calculations, hypotheses and theories. Take, for example, the German Otto Hahn, who discovered the fission of the uranium nucleus. Or the American Glenn Seaborg who led a group of researchers that obtained, in laboratory conditions, a number of elements, including mendelevium, named in honour of Mendeleyev. That element bears the name of the great Russian scientist not only because Mendeleyev laid the foundation of the modern science of atom, but also because he drew his colleagues special attention to uranium ( 92), which at the time had closed his periodic table. A long train of transuraniums followed the once final uranium.
The periodic system hasnt crumbled with time; on the contrary, its structure has expanded. At present it is the basis of modern teaching on substances, the structure of matter, atoms and nuclear energy.
|
|
|
Answer the following questions:
1. Why is the periodic system by Mendeleyev valued so much?
2. Why does the element 101 bear Mendeleyevs name?
3. Has the periodic table changed with time? In what way has it changed?
The atomic theory
In 1805 the English chemist and physicist John Dalton put forward the hypothesis according to which all substances were stated to consist of small particles of matter, of several different kinds, corresponding to the different elements. He called these particles atoms, from the Greek word atoms,meaning indivisible. The hypothesis gave a simple explanation or picture of previously observed but unsatisfactorily explained relations among the weights of substances taking part in chemical reactions with one another. As it was verified by further work in chemistry and physics, Daltons atomic hypothesis became the atomic theory.
The rapid progress of science during the twentieth century is well illustrated by the increase in our knowledge about atoms. In a popular textbook of chemistry written in the early years of the twentieth century, atoms were defined to be the imaginary units of which bodies are aggregates. The article in Atom in the 11th edition of the Encyclopaedia Britannica, published in 1910, ends with the words The atomic theory has been of priceless value to chemists, but it has more than once happened in the history of science that a hypothesis, after having been useful in the discovery and the coordination of knowledge, has been abandoned and replaced by one more in harmony with later discoveries.
Answer the following questions:
1. What was the main idea of this hypothesis?
2. In what way was the hypothesis verified?
3. What does sometimes happen to a hypothesis in the course of history?
Molecules in gases and liquids
According to Avogadros principle, equal volumes of gases regardless of composition, contain the same number of molecules at the same temperature and pressure. As a consequence of the principle, the gram-molecular weight of any gaseous substance occupies 22.4 litres at standard temperature (0 C) and pressure (760 mm of mercury). The number of molecules per gram-mole has been calculated by different methods of increasing refinement through the years, and is now considered to be 6.023 × 1023 atoms per gram-atom, or molecules per gram-mole), and it is accurate within 0.1%. For example, one mole of ammonia gas (NH3 weighs 17.073 grams, occupies a volume of 22.4 litre at standard temperature and pressure, and contains 6.023 × 1023 molecules).
At the same temperature, molecules of a liquid move at the same rate at those in a gas. In a liquid, however, the extent of motion must be restricted. Liquids flow as a stream and tend to form drops to a greater or less extent, thus giving evidence of the importance of the force of cohesion between the molecules in a liquid. Heating liquids, as a rule, results in their expansion, an effect explained by the tendency of the molecules to occupy more space when they move at a faster rate. Also, increase in pressure has but slight effect on the volume compressible.
Answer the following questions:
1. What does Avogadros principle state?
2. In what way can the number of molecules per mole be determined?
|
|
|
3. What forces act between the molecules in a gas and in a liquid?
The nature of a liquid
When iodine crystals are heated to 114C, they melt forming liquid iodine. The temperature at which the crystals and the liquid are in equilibrium that is, at which crystals have no tendency to melt or the liquid has no tendency to freeze is called the melting point of the crystals, and the freezing point of the liquid. This temperature is 114C for iodine.
Liquid iodine differs from solid iodine (crystals) mainly in its fluidity. Like the solid, and unlike the gas, it has a definite volume (1g occupies about 0.2 cm3), but it does not have a definite shape: instead, it fits itself to the shape of the bottom part of its container.
From the molecular viewpoint the process of melting can be described in the following way. As a crystal is heated, its molecules are increasingly agitated, and move about more and more vigorously, but at lower temperature, this thermal agitation does not carry any one molecule any significant distance away from the position fixed for it by the arrangement of its neighbours in the crystal. At the melting point the agitation finally becomes so great that it causes the molecules to slip by one another and to change somewhat their location relative to one another. They continue to stay close together, but do not continue to retain a regular fixed arrangement.
Answer the following questions:
1. What temperature is called the melting point?
2. What is the difference between liquid and crystalline iodine?
3. In what way is it possible to explain the change from the solid to the liquid?
