(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

  • 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

  • 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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