(Online Cours) CAPF Assistant Commandant: General Science - Chemistry

Online Course for Central Armed Police Forces (CAPF) Exam (Assistant Commandant)

General Science


Acid, Base & Salts


  • The word ‘acid’ is derived from a Latin word, which means “sour”. The sour taste of most of the fruits and vegetables is due to various types of acids present in them. The digestive fluids of most of the animals and humans also contain acids.

  • An acid is a compound, which on dissolving in water yields hydronium ions (H3O+) as the only positive ions. The characteristic property of an acid is due to the presence of these hydronium ions.

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    Acids are compounds that contain Hydrogen (Hydrochloric, HCl; Sulphuric, H2SO4; Nitric, HNO3). However, not all compounds that contain Hydrogen are acids (Water, H2O; Methane, CH4). Acids are usually compounds of non metals with Hydrogen and sometimes Oxygen.

  • Acids can be classified in various ways, depending on the factors mentioned below:
    1. Classification Based on the Strength of the acid.
    2. Classification Based on the Basicity of the Acid.
    3. Classification Based on the Concentration of the acid.
    4. Classification Based on the presence of Oxygen.

Atomic Structure

  • An atom is the smallest particle of the element that can exist independently and retain all its chemical properties. Atoms are made up of fundamental particles: electrons, protons and neutrons.

  • Dalton’s Atomic Theory: John Dalton provided a simple theory of matter to provide theoretical justification to the laws of chemical combinations in 1805. The basic postulates of the theory are:

  • All substances are made up of tiny, indivisible particles called atoms.

  • Atoms of the same element are identical in shape, size, mass and other properties.

  • Each element is composed of its own kind of atoms. Atoms of different elements are different in all respects.

  • Atom is the smallest unit that takes part in chemical combinations.

  • Atoms combine with each other in simple whole number ratios to form compound atoms called molecules.

Chemical Bonding

  • Atoms are made up of three smaller particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus of the atom. Protons have a single positive charge. This is called the AtomicNumber of an atom. The Atomic Number tells us the number of electrons that the atom contains. It is these electrons that determine the chemical properties of the atom and the way it combines with other atoms to form specific compounds. Electrons have a single negative charge. Normally, atoms are electrically neutral so that the number of electrons is equal to the number of protons.

  • Electrons orbit around the nucleus. Electrons cannot orbit the nucleus of an atom in any orbit. The electrons are restricted to specific paths called orbitals or shells. Each shell can only hold a certain number of electrons.When a shell is full, no more electrons can go into that shell. The key to the properties of atoms is the electrons in the outer shell. A complete outer shell of electrons is a very stable condition for an atom.

  • Valency: Hydrogen is the simplest element. It has one electron. Its outer shell only holds two electrons. Valency can be simply defined as the number of Hydrogen atoms that an element can combine with. The atoms with full electron shells (Helium, Neon, Argon) are chemically inert forming few compounds. The atoms don’t even interact with each other very much. These elements are gases with very low boiling points. The atoms with a single outer electron or a single missing electron are all highly reactive. Sodium is more reactive than Magnesium. Chlorine is more reactive than Oxygen. Generally speaking, the closer an atom is to having a full electron shell, the more reactive it is. Atoms with one outer electron are more reactive than those with two outer electrons, etc. Atoms that are one electron short of a full shell are more reactive than those that are two short.

Chemical Reactions & Equations

  • Atoms and Molecules, Elements and Compounds: There are about a hundred different types of atoms in the Universe. Substances made up of a single type of atom are called Elements. Some elements are made up of single atoms: Carbon©, Helium(He), Sodium(Na), Iron(Fe) etc. He, Fe, and Na are the Chemical Symbols of the elements.

  • Some elements are made up of groups of atoms: Oxygen(O2), Ozone(O3), Chlorine(Cl3) etc. These groups of atoms are called molecules.

  • Molecules can also be made up of combinations of different types of atoms. These substances are called compounds: Common Salt(NaCl), Methane(CH4), Ammonia(NH3) etc. O2, CH4, NH3 are the Chemical Formulas of Oxygen, Methane and Ammonia respectively. CH4 means that a single molecule of methane contains one atom of Carbon and four atoms of Hydrogen. This chemical formula could have been written but the C1 H4 is never written. Similarly, a molecule of Ammonia (NH3) contains one atom of Nitrogen and three atoms of Hydrogen.

  • A change in which one or more new substances are formed is called a chemical change. A chemical change is also called a chemical reaction. The change may conveniently be represented by a chemical equation.

Organic Chemistry

Organic chemistry is that branch of chemistry which deals with the study of compounds of carbon with hydrogen (hydrocarbons), and their derivatives. Presently about five million organic compounds are known. Organic compounds were found to contain mainly hydrogen and carbon. Therefore, organic chemistry is defined as the study of hydrocarbons and their derivatives. Most atoms are only capable of forming small molecules. However one or two can form larger molecules. By far and away the best atom for making large molecules with is Carbon. Carbon can make molecules that have tens, hundreds, thousands even millions of atoms! The huge number of possible combinations means that there are more Carbon compounds that those of all the other elements put together! A single Carbon atom is capable of combining with up to four other atoms. We say it has a valency of 4. Sometimes a Carbon atom will combine with fewer atoms. The Carbon atom is one of the few that will combine with itself. In other words Carbon combines with other Carbon atoms. This means that Carbon atoms can form chains and rings onto which other atoms can be attached. This leads to a huge number of different compounds. Organic Chemistry is essentially the chemistry of Carbon. Carbon compounds are classified according to how the Carbon atoms are arranged and what other groups of atoms are attached.

Classification of Elements

  • The grouping of elements with similar properties together and the separation of elements with dissimilar properties is known as classification of elements. The table, which classifies elements on the basis of their properties, is called the periodic table. Döbereiner grouped the elements into triads and Newlands gave the Law of Octaves. Mendeléev arranged the elements in increasing order of their atomic masses and according to their chemical properties.

  • Dobereiner’s Triads arranged elements in an increasing order of atomic mass, in groups of three. The atomic mass of the middle element was the arithmetic mean of the other two elements of the triad.

  • Newland’s law of octaves states that on arranging elements in increasing order of their atomic mass, the eighth element resembles the first in physical and chemical properties, just like the eighth node on a musical scale resembles the first note.

Properties of Gases

  • First, we know that a gas has no definite volume or shape; a gas will fill whatever volume is available to it. Contrast this to the behavior of a liquid, which always has a distinct upper surface when its volume is less than that of the space it occupies.

  • The other outstanding characteristic of gases is their low densities, compared with those of liquids and solids. The most remarkable property of gases, however, is that to a very good approximation, they all behave the same way in response to changes in temperature and pressure, expanding or contracting by predictable amounts. This is very different from the behavior of liquids or solids, in which the properties of each particular substance must be determined individually.

Common Elements and Compounds

Hydrogen: Symbol H, formula H2. The first element in the periodic table and the most basic and common of all elements in the universe. Over ninety percent of all the atoms in the universe are hydrogen atoms and they are the lightest of all elements. The name hydrogen comes from the Latin word “hydro” which means water. Scientists use the letter “H” to represent hydrogen in all chemical equations and descriptions.

  • Hydrogen atom has one electron in its valence shell like alkali metals.

  • Hydrogen generally shows + 1 valency like alkali metals.

  • Hydrogen is a good reducing agent like other alkali metals.

  • The isotopes of hydrogen: Protium has an atomic number 1, and mass number 1, Deuterium, has an atomic number 1, and mass number 2 and Tritium has an atomic number 1, and mass number 3.

  • It has a vapour density of 1, which is 14.4 times lighter than air.

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