Physical changes alter a substance's physical properties or state of matter without producing new substances. Chemical changes form new substances through chemical reactions. Some key differences are that physical changes are often reversible while chemical changes are usually not, and chemical changes involve greater energy transfers. Common examples of physical changes include changes of state like boiling or melting, while chemical changes include combustion reactions and electrolysis.

1. CHEMICAL REACTIONS

2. Physical Changes

3. Physical Changes
Physical changes are changes in physical properties such as volume or density, or
changes in state such as changes from solid to liquid or from liquid to gas.
• Physical changes do not involve the production of new substances.
• Physical changes are often readily reversible.
• When compared with chemical changes, only small energy changes are involved in
physical processes.
Following are the examples of physical changes:
Changes of state
Changes in shape and size
Dissolve a solute in a solvent
Filtration

4. Chemical Changes

5. Chemical Change
Chemical changes are those in which new substances with different compositions and
properties are formed.
• In a chemical change reactants are converted into products.
• Chemical changes are often very difficult to reverse.
Examples:
• The combustion of petrol or diesel in car engines and methane in Bunsen burner are
examples of chemical changes. In both cases, new substances (carbon dioxide and
water) are produced, and heat and light energy are released.
• When an electric current is passed through water, two new substances, hydrogen gas
and oxygen gas, are produced.

6. Chemical Changes
Which of the following processes is a chemical change?

7. Physical and Chemical Change
Energy Changes Associated with Physical and Chemical Change
In general, the energy changes associated with physical processes such as boiling,
melting and dissolving are much smaller than those associated with chemical changes.
Boiling
When liquid water is boiled the water molecules gain sufficient kinetic energy to escape
from the liquid surface and enter the air as a vapour. The molecules of water vapour are
identical to those in the liquid water. This process is an example of a physical change.
Energy measurements show that 44 kJ of energy is required to boil 18 g of water at
100oC. This physical change is easy to reverse by cooling the water vapour.
Electrolysis
When water is electrolysed the covalent bonds holding the atoms together are broken and
new products form. This chemical change requires much more energy than the physical
change involved in boiling the water. Measurements show that 286 kJ of energy is
required to electrically decompose 18 g of water. This is much greater than the energy
required for boiling. This chemical change is very difficult to reverse compared with the
physical change involved in boiling.

8. Electron Dot Diagram
Representation of some ionic compounds by Lewis dot formulae or electron dot
diagram
Formation of magnesium oxide
Electronic configuration of Magnesium (12) → 2, 8, 2
Electronic configuration of Oxygen (8) → 2, 6
Formation of calcium fluoride
Electronic configuration of Calcium (20) → 2, 8, 8, 2
Electronic configuration of Fluorine (9) → 2, 7

9. Few More Examples of Electron Dot Diagram
Hydrogen(H2)
Oxygen(O2)
Nitrogen(N2)
Water(H2O)
Methane(CH4)

10. Chemical Formulae and Compounds
Compound: Compounds consist of two or more elements that are chemically combined.
The formulas of compounds contain the symbols of the elements involved, and the
subscripts indicate the relative numbers of atoms of each element in the compound.
Since atoms of elements combine to form molecules, it should be possible to represent
the molecule in terms of symbols of the constituent atoms. This symbolic expression for a
molecule is called a Formula.
Example of Formulae of Few Elements and Compounds
Nitrogen, Chlorine, Carbon Dioxide, Carbon Monoxide

11. Chemical Formulae and Compounds
Compound Formula Compound Formula
Hydrochloric acid HCl Carbon monoxide CO
Sulphuric acid H2SO4 Carbon dioxide CO2
Sulphurous acid H2SO3 Silica (sand) SiO2
Nitric acid HNO3 Caustic soda
(Sodium hydroxide)
NaOH
Nitrous acid HNO2 Caustic potash
(Potassium hydroxide)
KOH
Phosphoric acid H3PO4 Washing soda
(Sodium carbonate)
Na2CO3
Boric acid H3BO3 Baking soda
(Sodium bicarbonate)
NaHCO3
Sulphur dioxide SO2 Limestone or marble
(Calcium carbonate)
CaCO3
Sulphur trioxide SO3 Water H2O
Nitrous oxide N2O Sulphuretted hydrogen
(Hydrogen Sulphide)
H2S
Nitric oxide NO Ammonia NH3
Nitrogen trioxide N2O3 Phosphine PH3
Nitrogen dioxide NO2 Methane CH4
Nitrogen pentoxide N2O5

12. Chemical Formulae and Compounds
VALANCY
Consider the following molecules: HCl - Hydrogen chloride (Hydrochloric acid) H2O
(Water) NH3 (Ammonia) CH4 (Methane)
Looking at these molecules, we find that while one chlorine atom combines with only one
hydrogen atom, one oxygen atom combines with two, one nitrogen atom combines with
three while one carbon atom combines with four hydrogen atoms, i.e., the atoms of
chlorine, oxygen, nitrogen and carbon show different combining capacities.
This combining capacity of an atom or a radical is called its valency. It is measured in
terms of hydrogen atoms or oxygen atoms. Valency of an atom or a radical is the
number of hydrogen atoms or double the number of oxygen atoms which will
combine with it.
Examples: HCl, H2SO4, SO2 , H2S, NH3

13. Chemical Formulae and Compounds
HOW TO WRITE A FORMULA?
In the formation of a chemical compound, combination takes place between the positive and the
negative radicals in such a way that the product of valency and the number of radicals is the
same for both the radicals. Thus to write the formula of a compound we proceed as follows:
(i) Write the symbols of the two radicals side by side with valencies at the top, the positive
radical to the left and negative radical to the right.
(ii) Cross the valencies after removing the common factor, if any. The numbers are to be placed
to the lower right of each symbol. The compound radical must be enclosed within a bracket and
the number placed outside the bracket to the lower right.
Example 1: For writing the formula of sodium sulphate, proceeding as above we have:
(i) Writing symbols side by side with valencies at the top.
Na1+SO4
2-
(ii) There being nothing common in 1 and 2, crossing the valencies we get the desired formula:
Na2SO4
Example 2: Similarly for writing the formula of barium carbonate, we have the two steps:
(i) Writing the symbols side by side:
Ba2+CO3
2-
(ii) Removing the common factor 2, and crossing the valencies we get the desired formula:
BaCO3

14. Chemical Formulae and Compounds
Identify the correct formulae for the following compounds.
(a) chromium(III) oxide (b) potassium nitrate
(c) magnesium carbonate (d) mercury(II) chloride

15. Chemical Formulae and Compounds
TYPES OF COMPOUNDS
Compounds are classified in following three groups on basis of their bonding and
structure:
1. Covalent Molecular Compounds
2. Covalent Network Compounds
3. Ionic Compounds

16. Chemical Formulae and Compounds
Covalent Molecular Compounds
In covalent molecular compounds the formula represents the number of atoms of each
element in one molecule of the compound. Covalent molecular compounds are formed by
covalent bonding between different atoms.
Covalent bond is a bond formed when two atoms share one or more electron pairs. Each
atom contributes equal number of electron(s) towards the bond formation.
Examples:
NH3 – ammonia
CH4 – methane or carbon tetrahydride
CCl4 – tetrachloromethane (carbon tetrachloride)
CO2 – carbon dioxide
H2O – dihydrogen oxide (water)

17. Chemical Formulae and Compounds
Covalent Network Compounds
Covalent network compounds do not occur as simple molecules. For example, silicon
dioxide contains silicon and oxygen atoms covalently bonded together in an extended
three-dimensional network structure as shown in the figure below.

18. Chemical Formulae and Compounds
Ionic Compounds
Ionic compounds consist of oppositely charged ions held together by electrostatic
attraction to form a crystal lattice. The electrostatic attraction between the oppositely
charged ions is called ionic bonding.
The ions in an ionic solid are arranged in a regular three-dimensional lattice. The
structure of sodium chloride is shown in the figure below.