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Encyclopedia about Chemistry

 

Atoms and compounds

Every material is composed of a great number of atoms. The type and number of atoms determines the physical and chemical characteristics of that material. A compound, a combination made up of a number of different atoms, is called a molecule. Chemistry is the study of combinations of atoms, and especially, of molecules. With the help of already gained knowledge, chemists attempt to produce synthetic materials, which may work better than the materials that are being used at the present time.

Chemistry is able to transform raw materials into new materials and compounds with different and new characteristics, products which can often be used for a variety of new purposes. A few examples of what has been produced in the chemical industry: Varnishes and sprays, paints, plastics, cosmetics, medicines and cleaning products or even modern materials for the textile industry. Metals and alloyed steel are also produced in their present form thanks to the reactions and processes of industrial chemistry.

Another very important example of the uses of chemistry is the production of pure materials for various uses. For example, integrated circuits and photo cells can be produced from highly purified silicon. Chemistry is also gaining importance in environmental matters. Industrial processes which produce various materials, as well as simple consumption of raw materials, can cause harmful pollutants to be released into the atmosphere. With the help of special, highly developed processes, chemistry can transform those harmful pollutants into less dangerous materials. These materials, which are no longer useful to heavy industry, can then be returned to their natural place in nature. Or, they can be recycled. With the help of advances in chemistry, more and more useful materials are manufactured for day-to-day life. At the same time, our environment is protected by the removal of some of the more harmful byproducts of chemical reactions in heavy industry.

Characteristics of materials

Our environment is composed of innumerable objects, things in combination with one another, depending on one another. Physics would call them objects, or bodies. Biology would classify those that are living as organisms, or living matter. Chemistry concentrates on the composition of matter. In order to safely distinguish and differentiate one object from another, the characteristics of both must be well known. So in chemistry, all materials have their own original and distinguishable, identifiable criteria. Some characteristics are able to be distinguished from others using our senses. To distinguish others, it is necessary to carry out more thorough chemical and physical analyses.

The characteristics of materials can be determined by human sense organs, like sight. The colour of an object, or its texture, or its crystalline shape, or other shape, can be seen. Taste and odour can also be used to determine the nature of an object. Of course, it should be mentioned that professional chemists have long given up on senses like taste and smell, citing safety concerns.

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Periodic table of the elements, chemical symbols

Atoms of various elements differ in the number of individual particles they contain (neutrons, protons and electrons). These three are the constituent particles which atoms are made of.

The periodic table of the elements contains the elements in systematic relation to one another. Their order is determined by the amount of protons each contains, and at the same time the amount of electrons each element contains in its electron cloud.

The amount of protons of an element in the periodic table is seen as its defining factor, its atomic number.

Vertically, the periodic table is arranged in groups. Elements in the same group have the same number of electrons in their outermost shell. There are eight main groups of elements, denoted with the Roman numerals from I to VIIII. Some groups are better known by their familiar names. The number of electrons in an element’s most outer shell increases from left to right across the periodic table. Group I is called alkaline metals. Group II is called alkaline earth metals. Elements of Group VII are called halogens, with Group VIII the noble gases.

Besides these eight groups, there are other groups whose elements differ from Groups I-VIIIA. These groups’ characteristics are so different that they are denoted with Roman numerals plus the letter B at the end: IB-VIIIB. These are the transition metals.

Elements of the first main group have one electron in their outer shells. Elements in the second group have two, and so forth, up to Group VIIIA (the noble gases), which have eight electrons in their outer orbital. Eight electrons, for A Group elements, means a full electron shell, at which time another more outer electron shell begins to fill.

Using more simplified atomic models, one of the basic concepts is that individual elements try to fill their outer shells completely. This basic characteristic of elements can help us to understand why chemical reactions take place. As long as elements in the main groups do not have full outer shells, that is, that they have a small amount of electrons, they will try to donate those electrons to other groups (elements from Groups I through IV). If, on the other hand, elements have nearly full shells, that is, a lot of electrons, and they are lacking only one or a few electrons to complete their outer shell, they will try to attract electrons (elements in Groups V-VII). Because elements in Group VIII (the noble gases) already have filled outer shells, they do not enter into reactions with other elements. They are said to be unreactive, or inert.

In addition to vertical groups, elements in the periodic table are arranged horizontally, from left to right – in so-called periods. The difference between periods is found in the number of electron shells (from up to down in the table, the number of outer shells increases by one from one period to the next). In the first period, elements (hydrogen (H) and helium (He)) have only one shell. Elements in the second period have two shells, and so on. Various characteristics of the individual elements are more predictable thanks to their placement in the periodic table. In addition, colours are used to make the elements even more noticeable.

Hydrogen is given a white colour, because it cannot be placed in the same category with any other element.
The first column (Group IA) is called the alkaline metals and is given the color yellow.
The second column (Group IIA) is called the alkaline earth metals and are purple.
The transition metals are dark yellow, and they belong to other groups, B groups.
In addition, lanthanoids and actinoids (brown and pink) are part of these other B groups.
In groups IIIA-VIA, a number of metals are found. These are emphasised with light blue.
Light green is the colour given metalloids, in the same groups as some other metals.
Non-metals are dark-green and can be found in Groups IVA to VIIA.
The noble gases, in Group VIIIA have been given a dark blue colour.
Besides non-metals, metalloids and rare metals, all other elements are considered to be metallic, or metals.
Meaning of some of the abbreviations given chemical symbols of elements in the periodic table:
The top number in coloured letters in the period table is the atomic number, also an indication of the amount of proteins in the nucleus.
Abbreviations (for example H,Cl,Fe) are chemical symbols of elements, often, but not always, shortened from their names (Hydrogen, Chlorine, Iron).
The number at the bottom of the box for each element indicates that element’s relative atomic mass (number of protons plus number of neutrons in the nucleus, together).
Further information:
- Atomic masses of elements 104 and higher are approximations.
- The names of elements 101-109 were internationally recognised August 30, 1997.
- Uranium, with atomic number 92, is the heaviest naturally-occurring element. Elements with higher atomic numbers are so unstable that they are not found to exist in nature. They are therefore synthesised in the laboratory.
- Lanthanoids and actinoids are separated from the main body of the periodic table to save space, because the sixth and seventh periods each have 32 elements.

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States of matter and the gas laws

The state of matter of a material is the state in which it is found under a certain set of conditions, specifically at a certain temperature and pressure. All substances can be classified into three states of matter: solid, liquid or gas.

The state of matter of a substance is dependent on its temperature. Every substance has its own individual point, or limit, when it transforms from one state of matter to another. This change is dependent on temperature. The state of matter of a substance can be determined immediately. In general, a substance name or chemical formula is given followed by a letter in parentheses: (s), (l) or (g). These letters, of course, stand for solid, liquid and gas, respectively. If a solution is present, that is, if a material is dissolved in a liquid, the abbreviation (aq), for aqueous, is used.

Water in its solid form is called ice. Its melting point, the point where ice changes to liquid water, is 0° C. Liquid water vaporises, or changes to gas, at around 100 ° C. The word "around" is used because under certain conditions, both melting point and boiling point can vary. The air pressure and humidity (air moisture) can affect these temperatures. Generally, in chemistry, we use usual air moisture conditions and a standard air pressure of 1014 HPa.

Without any further information on temperature, room temperature of 20° C is assumed.

The temperature at which a liquid material changes to its gaseous state is called its boiling point. Both boiling point and melting point are dependent on air pressure. Knowing these temperatures, chemists are well on their way to determining what an unknown substance is, by measuring that substance’s melting and/or boiling points. This is done with the help of a thermometer.

Some materials change their state of matter directly from the solid state to the gaseous state. That process is called sublimation (this occurs with so-called dry ice, which does not melt but rather vaporises directly into the air, as does carbon dioxide CO2). The opposite process, from gas to solid, is called resublimation.

There are some materials which are soluble, or able to dissolve, in liquids. When they do dissolve, a solution is the result. Liquids in which other materials can dissolve are called solvents. Not only solid materials, but also liquids and gases, can dissolve in other materials. In whatever case, each material has a different solubility in a solvent, or amount of material which can dissolve in a certain amount of solvent. Soluble materials, therefore, are no longer soluble once a certain limit has been reached. Once this limit has been reached, adding more solute is like adding sand to water. In this case, when no more material (called solute) can be dissolved, a solution is said to be saturated.

Let’s take a closer look at an example, of regular everyday table salt (NaCl): If we begin adding salt to water, it does dissolve, and the grains of salt that were visible become hidden by the water. But if we continue to add salt to the water, at some point no more salt will dissolve in the water. The rest will fall to the bottom of the glass or beaker of water, forming a visible sediment.

The mass of a material which can dissolve in a certain amount of solvent can be determined experimentally, and is one of the characteristics of a material (solubility). Solubility depends on temperature. It is measured in grams of dissolved material (solute) to 1 litre of solvent. (If other conditions are not given, it is assumed that room temperature is 20 ° C and air pressure is 1014 HPa .

There are some materials that do not ignite when heated, and are therefore considered inflammable, or unable to burn. Heavier materials, like metals and glass, begin to glow at high temperatures. After they cool down, these materials are the same as they were before the process was begun. A certain amount of the material does catch fire, however, if we place the material in an open flame. In this case, the material, or part of it, combines with oxygen in the air and can exhibit phenomena typical for that material. For a number of materials, this type of burning causes the flame to become coloured. Others can give off a certain odour. Some naturally-occurring materials, such as coal and wood burn and leave ashes. On the other hand, alcohol and gasoline do not leave any traces after they have been burned.

In gases, molecules are relatively distant from one another. They move fast and are independent of one another. Their movements are not uniform. The volume, then, that a gas takes up is much greater than the sum of the volume of its particles. In general, the characteristics of elements in the gaseous state are defined using the gas laws.

The Boyle-Mariott Law says that at constant temperature the volume of a gas is inversely proportional to a given pressure (the lower the pressure, the greater the volume of a gas).

The Gay-Lussac Law says that at constant pressure, the volume of a gas is proportional to its temperature.

Relationship between temperature and pressure

At constant volume, the pressure of a certain volume of gas rises with rising temperature.

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Mixtures and pure materials

Materials are considered pure when they are composed of atoms or molecules of only one type, when they are not mixed with any other materials. The characteristics of pure substances can be determined experimentally. For a given substance, certain properties are characteristic.

Of all of the known naturally-occurring substances, only a few can be considered to be completely pure. Gold and sulfur are natural substances with a high degree of purity. Because, however, they still contain a certain degree of impurity, they are not, in reality, pure substances. With the help of chemical procedures, though, it is possible to purify substances.

Many of the substances around us are impure. Though they may sometimes look pure, they are actually mixtures. A mixture is composed of a number of pure substances, which can be solid, liquid or gas. We can determine whether a substance is pure with the naked eye, that is, with no external aid, or with the help of a microscope (heterogeneous mixture), or by using a chemical experiment (homogeneous mixture).

Homogeneous mixtures:

steel (solid phase/solid phase) bronze
solution (solid/liquid) sugar water
ideal mixture of two liquids (liquid/liquid) ethanol-water
ideal mixture of gases (gas/gas)
air

Heterogeneous mixtures:

mixture of solid substances (solid/solid) ores
paste (solid/liquid) toothpaste
hard foam (solid/gas) polystyrene
suspension (solid/liquid)
paints
colloidal solution (solid/liquid) protein solutions
emulsion (liquid/liquid) milk
foam (liquid/gas) bubble shampoo
smoke (solid/gas) smoke from a chimney
fog (liquid/gas)
clouds

In all of these mixtures, the characteristics of the individual pure substances remain unchanged, but according to the nature of each mixture, the characteristics of each of the parts can be either magnified or reduced.

One of the goals of chemistry is to regain, or separate, a pure substance from the mix it is in. There are a number of separation procedures which work to extract pure substances from their mixes. Usually, some of the defining characteristics of the substance to be extracted are used in the separation process.

Separation of solid particles from a solution of mixtures:

If we allow a saturated solution or colloidal solution to sit for a while, the solid particles that are not dissolved will settle to the bottom of the glass to form a sediment. This process is called sedimentation, or settling. The solution above the sediment can be poured off, or decanted. Separation of the undissolved portion of the solid from the solution itself can be achieved using a filter. We simply pour the mixture through a filter, allowing the liquid material to pass through the filter to a beaker or glass below, while the solid portion of the mixture is caught up on the filter – if, that is, its particles are larger than the holes, or pores, of the filter.

Separation of a dissolved substance from a solution:

Salts dissolved in saltwater can be regained by heating up, then boiling the water out of the solution. As this process occurs, the salt begins to concentrate in what is left of the solution, crystallising out as the maximum solubility of a certain lower volume is reached. This process is therefore called crystallisation. Solid materials are obtained in a high degree of purity using this method. While salts in seawater are dissolved in that seawater, normal rock salt (NaCl or table salt) contrary to salt from the ocean, can be found on dry ground and can be mined as a mineral from quarries and mines). The process of dissolving one or more materials in solution, only to retrieve them in a more pure form, is called extraction. Once a solution is made, it is heated, the water boils off, and the constituent salts contained in the solution crystallise as pure materials.

Separation of a liquid mixture:

A mixture of two liquid substances can be separated using distillation. The mixture is heated. As the boiling points of two or more individual liquids are reached, one begins to boil off. This is the liquid with the lower boiling point. The other, or others, begin to boil when their own boiling points are reached. Cooling of the apparatus leads to condensation of the gas which was released from the boiling, condensation being the change from gas back to liquid. As long as we change the glass beaker or vessel in which the gas condenses, the individual liquids can be separated. It is enough to watch the thermometer – if it is constant, only particles of one liquid are being changed to gas at one time (boiling, or vaporizing).

There are other special processes for separation of more complex mixtures and small amounts of substances. Using extraction, for example, we can obtain individual materials by dissolving them first. Another separation method is called chromatography. A drop of a mixture of materials is placed on chromatographic paper. After that, the paper is submerged on one end in a solvent. The individual components of the mixture move with the solvent, at different rates, separating from one another according to their solubility in the specific solvent, or, as the case may be, according to their absorption qualities.

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