Solution-Class 8-Science-Chapter-13-Chemical Change and Chemical Bond-Maharashtra Board

a. Anarrowis drawn in between the reactants and products while writing the equation for a chemical reaction.

b. Rusting of iron is a slow chemical change.

c. The spoiling of food is a chemical change which is recognized from the generation of certain  smell due to it.

d. A colourless solution of calcium hydroxide in a test tube turns  milky on blowing in it through a  blow tube for some time.

e. The white particles of baking soda disappear when put in lemon juice. This means that it is a chemical change.

f. Oxygen is a reactant  in respiration.

g. Sodium chloride is ionic compound while hydrogen chloride is covalent compound.

h. Electron duplet is complete in each hydrogen in a hydrogen molecule.

i. Chlorine (Cl₂) molecule is formed by sharing of electrons between two chlorine atoms.

Chemical change is a process in which a new substance is formed.

• Respiration is irreversible biological process
• During respiration, air is inhaled, oxygen present in this inhaled air reacts with glucose present in the cells of the body forming carbon dioxide and water.
• Moreover, we cannot obtain glucose and oxygen from carbon dioxide and water.
• During respiration, absorption and evolution of energy takes place.

These are characteristics of chemical reaction. Hence, respiration is a chemical reaction.

Word equation :

Glucose + Oxygen  Carbon dioxide + Water

Hard water does not form lather with soap and is brackish to taste. This is because hard water contains the chloride and sulphate salts of calcium and magnesium in dissolved state.

When a solution of washing soda is added to hard water, if forms a precipitate of calcium carbonate and magnesium carbonate which is removed by filtration thus water is softened

Equation :

Calcium chloride + Sodium Carbonate à Calcium Carbonate + Sodium chloride

Limestone is predominantly Calcium carbonate (CaCo₃). When calcium carbonate reacts with 10% solution of HCl. Then Calcium chloride is formed as a salt along with water and carbon dioxide liberates slowly.

Equation :

2HCl + CaCO3 → CaCl2 + CO2 +H2O

Bubbles are seen on adding lemon juice to baking soda because lemon juice is citric acid and baking soda is sodium bicarbonate.

When baking soda is added to lemon juice a chemical change takes place in citric acid present in the lemon juice and carbon dioxide gas is formed. This CO₂ is released in the form of bubbles.

Equation :

Calcium carbonate + Dilute hydrochloric acid à Calcium Chloride + Water + Carbon dioxide

This is a neutralization reaction.

Sodium chloride :

• Sodium has atomic number 11 and electronic configuration 2, 8, 1.
• Sodium atom has 1 electron in its outermost shell.
• It loses one electron from its outermost shell, i.e., M shell. Then its L shell becomes the outermost shell with a stable octet.
• The nucleus of sodium atom has 11 protons but the number of electrons in the atom has become 10. So, there is a net unit positive charge giving a sodium cation (Na⁺).
• On the other hand, chlorine has electronic configuration 2, 8, 7. Chlorine atom has 7 electrons in its outermost shell and requires one electron to complete its octet.
• Thus, the electron lost by sodium is taken up by chlorine.
• When chlorine atom gains one electron, octet of chlorine is completed and its K, L, M shells have together 18 electrons and the nucleus has 17 protons. This leads to the formation of an ion (Cl^-).
• Thus, a chlorine atom accepts one electron from a sodium atom and consequently a chloride ion with one unit negative charge and a sodium ion with one unit positive charge are formed.
• Sodium and chloride ions, being oppositely charged, attract each other due to the electrostatic force of attraction. An ionic bond is formed and this results in the formation of sodium chloride (NaCl) molecule.

Potassium fluoride :

• Potassium has atomic number 19 and electronic configuration 2, 8, 8, 1.
• Potassium atom has 1 electron in its outermost shell.
• It loses one electron from its outermost shell, i.e. N shell. Then its M shell becomes the outermost shell with a stable octet.
• The nucleus of potassium atom has 19 protons but the number of electrons in the atom has become 18. So there is a net unit positive charge giving a potassium cation (K⁺).
• On the other hand, fluorine has electronic configuration 2, 7. Fluorine has 7 electrons in the outermost shell and requires one electron to complete its octet.
• Thus, the electron lost by potassium is taken up by chlorine.
  (6) When fluorine atom gains one electron, octet of fluorine is completed, its K and L shells have together 10 electrons and the nucleus has 9 protons. This leads to the formation of an ion (F^-).
• Thus, a fluorine atom accepts one electron from a potassium atom and consequently a fluoride ion with one unit negative charge and a potassium ion with one unit positive charge are formed.
• Potassium and fluoride ions, being oppositely charged, attract each other due to electrostatic force of attraction. An ionic bond is formed and this results in the formation of potassium fluoride (KF) molecule.

Water :

• Hydrogen has atomic number 1 and electronic configuration 1.
• Hydrogen has 1 electron in its K shell
• Oxygen has atomic 8 and electronic configuration 2, 6.
• See fig. There are six electrons in the valence shell of oxygen atom. It means that the electron octet in oxygen is short of two electrons and the valency of oxygen is ‘2’.
• In the H2O molecule the oxygen atom completes its octet by forming two covalent bonds, one each with the two hydrogen atoms.
• While this happens, the duplets of the two hydrogen atoms also are completed.

Hydrogen chloride :

• Hydrogen has atomic number 1 and electronic configuration 1, that means it has 1 electron in its K shell and its duplet is short of one electron therefore, the valency of hydrogen is one.
• On the other hand, chlorine has electronic configuration 2, 8, 7. Chlorine atom has 7 electrons in its outermost shell and requires one electron to complete its octet.
• The two atoms, hydrogen and chlorine share one electron with each other. As a result, the electron duplet of hydrogen and octet of chlorine is complete and a covalent band is formed between them.