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| Pure Substances
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| The distinctive features of pure substances are:
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A pure substance is composed of the same kind of particles e.g. hydrogen, oxygen, water, sodium chloride, etc.
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A pure substance is homogeneous, irrespective of its origin or method of preparation.
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A pure substance has definite properties, characteristic of itself.
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A pure substance has the same composition throughout. For example, different samples of water, prepared by different methods, by different people at different places always consist of hydrogen and oxygen in the ratio 1:8 by mass and 2:1 by volume. If any sample has a different ratio of these elements, then it certainly is not water.
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| A solution of salt in water or sugar solution being homogeneous appears to consist of one type of particles. But it is made up of more than one kind of particles. Hence it is not a pure substance. It is a mixture.
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| Pure substances can be classified into elements and compounds. Lavoisier, a French chemist, was the first to do this on the basis of quantitative studies. He showed that when we heat mercuric oxide it changes into mercury and oxygen.
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| Mercuric oxide is a compound because it decomposes into simpler substances, whereas mercury and oxygen cannot be further decomposed into anything simpler as they are elements.
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| Element
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| An element is defined as a pure substance as it is made up of only one kind of atoms having the same atomic number. The smallest particle of an element is the atom, which has all the properties of that element. It cannot be further reduced to simpler substances by ordinary physical or chemical processes. Example: Hydrogen and oxygen
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| Elements can be divided into two groups, namely, naturally occurring and artificially prepared. Three hundred years ago scientists all over the world knew of only twelve elements, but today, many more are known. There are about 114 known elements. Out of these, about ninety-five are naturally occurring elements and of the rest, nearly 14 elements have been artificially prepared in the laboratories. These artificially prepared elements, however, have a very short life, as they break up into more stable lighter elements. Molecules of elements may consist of single atom, two atoms, or even more. All the hundreds of thousands of compounds existing, and produced by man, are formed as a result of combination of two or more of these elements in a fixed proportion.
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| Noble gases, some metals, carbon, silicon etc. have only one atom in their molecules. These are called monatomic molecules. Some elements have two atoms in their molecules. These are called diatomic molecules e.g., hydrogen (H2), oxygen (O2), nitrogen (N2), chlorine (Cl2), etc. Those elements that have more than two atoms in their molecules are said to be polyatomic. For example a molecule of phosphorus consist four atoms (P4), and that of sulphur contains eight atoms (S8).
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| Most of the elements are solids, while eleven of them are gases and only two are liquids. The two liquids are mercury and bromine. However, two other metals can also exist in the liquid state at around 300C. They are gallium and caesium (gallium melting point = 29.90C and caesium melting point = 28.60C). Elements can be broadly divided into four categories: Metals, non-metals, metalloids and noble gases.
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Metals
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| These are generally solids with characteristics such as hardness, malleability, ductility, high tensile strength, lustre and ability to conduct heat and electricity. Example: Copper, iron, zinc etc.
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Non-metals
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| These are generally non-lustrous, brittle, poor conductors of heat and electricity. Example: Sulphur, phosphorus, nitrogen etc.
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Metalloids
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| Metalloids are those elements that have properties, which lie in-between those of metals and non-metals. Example: Arsenic, tin, bismuth etc.
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Noble gases
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| Are a group of six elements that do not combine with other elements and tend to exist by themselves. They are characterized by extreme chemical inactivity.. Example: Neon, helium, argon etc.
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| Compound
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| A compound is a pure substance that is formed by the combination of atoms of two or more elements by either transfer or by sharing of electrons. The atoms of the different elements in a compound are chemically combined in a fixed and constant proportion. If this proportion is different, the same elements produce entirely a different compound. Example: Example: Water is a compound of hydrogen and oxygen present in the ratio of 2:1 by volume and 1:8 by weight; if the ratio by volume changes to 1:1 and by weight to 1:16, it forms an entirely new compound called hydrogen peroxide. Thus, compounds are represented by a definite formula with its constituents combined in fixed proportions.
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| While the combination of two or more elements forms a compound, similarly, two or more simple compounds can form a complex compound by a process called synthesis. A compound as a pure substance can be decomposed into simpler substances by some suitable chemical technique. However, the properties of compounds are absolutely different from the properties of the elements that constitute the compound. For instance, hydrogen is combustible and oxygen is a supporter of combustion. But their product, water, neither burns, nor helps in burning. It actually extinguishes fire. Another familiar example is that of sodium and chlorine. Sodium is a violently reactive metal while chlorine is a highly poisonous gas with a choking and irritating smell. One cannot even think of consuming a piece of sodium or breathing in chlorine as such. But their product, i.e., sodium chloride, is consumed by all of us daily in the form of common salt.
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| Formation of a compound is a chemical process and always involves exchange of energy. For example, hydrogen burns in oxygen to form water, liberating heat. Nitrogen combines with oxygen to form nitric oxide by absorbing heat. A candle burns in air to produce water (vapour) and carbon dioxide, liberating heat and light. During the process of photosynthesis, carbon dioxide and water combine to form carbohydrate with absorption of light energy. The elements present in a compound can be re-obtained only by chemical processes. Water can be decomposed to hydrogen and oxygen only by electrolysis. If we bring a magnet near a sample of iron sulphide, the iron present in the iron sulphide cannot be separated.
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| We can summarize the properties of compounds as follows:
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A compound cannot be separated into its constituents by mechanical or physical means.
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Properties of a compound differ entirely from those of its constituent elements.
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Energy changes are involved in the formation of a compound.
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The constituent elements in a compound are in a fixed proportion by weight.
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A compound is a homogenous substance. That is it is same throughout in properties and composition.
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A compound has a fixed melting point and boiling point. For example, ice melts at 0oC.
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