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Different materials may be distinguished by their properties, the most obvious of which is the physical state, or state of aggregation, on the basis of which all materials are classified as solids, liquids, and gases. The characteristic feature of gas is that its molecules are not held together, but move around freely. Because of this freedom of molecular motion, a gas does not possess either definite shape or definite size, it shapes itself to its container. A liquid, on the other hand, has a definite volume, but does not have a definite shape. Only a solid is characterized both by a definite shape and definite size. What is the characteristic feature of a gas?

• Its molecules are not held together and move around freely

• It has a definite volume but not a definite shape

• It has a definite shape and size

• It takes the shape of its container

By the word substance, a chemist means an essentially pure substance. Actually, all substances are more or less impure. When referring to very impure substances, solutions, and mixtures, the word material should be used instead. All substances can be divided into two classes: elementary substances and compounds. What are elementary substances and compounds?

• Substances with definite shape and size

• Substances with atoms of only one kind and substances with atoms of two or more different kinds

• Substances that are impure and substances that are pure

• Substances that take the shape of their container and substances that have a definite volume but not a definite shape

The interaction of atoms and molecules is called chemistry. The metabolic activities of microorganisms involve complex chemical reactions. Nutrients are broken down by microbes to obtain energy and to make new cells. What is the interaction of atoms and molecules called?

• Biology

• Physics

• Chemistry

• Geology

Atoms are the smallest units of chemical elements that enter into chemical reactions. Atoms consist of a nucleus, which contains protons and neutrons, and electrons that move around the nucleus. The atomic number is the number of protons in the nucleus; the total number of protons and neutrons is the atomic weight. What are atoms?

• The smallest units of chemical elements that enter into chemical reactions

• The smallest units of compounds that enter into chemical reactions

• The smallest units of mixtures that enter into chemical reactions

• The smallest units of substances that enter into chemical reactions

Suppose we wish to determine whether air is a pure substance or a solution, one method would be to liquefy a certain amount and then observe what happens to it as it slowly evaporates. As the evaporation proceeds one may observe that: a- The light blue color gradually becomes deeper b- The temperature of the liquid slowly rises c- The densities of both liquid and gas change. Any one of these as well as other possible observations show that air must contain two or more components whose relative amounts change during the evaporation, causing the observed changes in properties due to differences between the components in color, volatility, density, chemical behavior. Still other properties might have been used.

If liquid air is distilled in a scientifically constructed still, it is possible to separate it into two nearly pure constituents. One of these constituents, nitrogen, is found to be slightly lighter than air; it can be condensed to a colorless liquid boiling at -194oC; it is very inert chemically, reacting with few other substances. The other constituent, oxygen, is slightly heavier than air; it gives, when condensed at low temperatures, a blue liquid boiling at -182.5oC, and it reacts readily with many substances.

Every chemical laboratory should be provided with running water,gas and electricity. The water supply is conducted from the mains by means of pipes, the piping terminating in taps, under which there are sinks to take away waste-water and other non-objectionable liquids. When one needs water, one turns the tap on and stops it flowing by turning the tap off. Similarly, a system of pipes is attached to the gas main from where gas reaches the various kinds of burners. They serve for producing flames of different intensity, the Bunsen burner being the most common type used. Apart from a gas supply there is electricity which serves for lighting and as a driving power. For operating electricity, switches or switch buttons are employed. That is why we talk about switching on the light or switching it off.

The laboratory is also equipped with a large variety of apparatus and devices. One of them, a desiccator, is used for drying materials. Ovens, furnaces or kilns serve for generating high temperatures. Where harmful vapors and undesirable odors develop during the operation, a hood with suitable ventilation has to be provided for their escape.

Of primary importance are glass and porcelain vessels. Glass vessels for chemical processes are made of special materials. They have to resist sudden changes in temperature, to withstand very high temperature: refractory glass, and be affected by a few substances as possible. The necessary assortment of laboratory glassware includes test tubes, beakers, various flasks, watch glasses, funnels, bottles, and cylinders. Porcelain articles consist of various kinds of dishes, basins and crucibles of various diameters. A grinding mortar with a pestle, desiccating dishes and stirrers are also generally made of porcelain.

At present, also plastic materials are finding increasing use in laboratories, many of them being chemically resistant, unattacked by alkalis or acids /acid-or alkali-proof, and unbreakable. Containers made of them are especially suitable for storing stock solutions.