Chapter 13 - Chemical Change and Chemical Bond
1. Complete the statement by filling the gaps using appropriate term from the terms given in the bracket.
(slow, coloured, arrow, fast, smell, milky, physical, product, chemical, reactant, covalent, ionic, octet, duplet, exchange, sharing, equality sign)
a. An ……………. is drawn in between the reactants and products while writing the equation for a chemical reaction.
Ans: arrow
b. Rusting of iron is a ………………… chemical change.
Ans: slow
c. The spoiling of food is a chemical change which is recognized from the generation of certain ………… due to
Ans: smell
d. A colourless solution of calcium hydroxide in a test tube turns ….. on blowing in it through a blow tube for some time.
Ans: milky
e. The white particles of baking soda disappear when put in lemon juice. This means that it is a ………. change.
Ans: chemical
f. Oxygen is a …………….. in respiration.
Ans: reactant
g. Sodium chloride is ……….. compound while hydrogen chloride is ……… compound.
Ans: ionic, covalent
h. Electron ……… is complete in each hydrogen in a hydrogn molecule.
Ans: duplet
i. Chlorine (Cl₂) molecule is formed by …………… of electrons between two chlorine atoms.
Ans: sharing
2. Explain by writing a word equation.
a. Respiration is a chemical change.
Ans:
(i) Respiration is a biological process that occurs continuously.
(ii) In this process, living organisms inhale air and exhale carbon dioxide.
(iii) Through detailed study, it is learned that glucose in the cells reacts with oxygen from the inhaled air to form carbon dioxide and water. Hence, respiration is a chemical change.
(iv) The word equation for a chemical change occurring in respiration is as follows:
(v) The word equation is as follows:
Glucose + Oxygen → Carbon dioxide + Water
b. Hard water gets softened on mixing with a solution of washing soda.
Ans:
(i) Hard water contains the chloride and sulphate salts of calcium and magnesium in dissolved form.
(ii) When washing soda (sodium carbonate) is added to hard water, a chemical reaction occurs.
(iii) In this reaction, a precipitate of insoluble carbonate salts of calcium and magnesium is formed.
(iv) Since the dissolved salts of calcium and magnesium are removed in the form of insoluble carbonate salts as precipitate, the water gets softened.
Hence, hard water gets softened by mixing with a solution of washing soda.
(v) The word equation for the removal of calcium from hard water is,
Calcium chloride + Sodium carbonate → Calcium carbonate + Sodium chloride
CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl
c. Lime stone powder disappears on adding to dilute hydrochloric acids.
Ans:
(i) Limestone powder is calcium carbonate.
(ii) Limestone powder reacts with diluted hydrochloric acid to produce calcium chloride, carbon dioxide gas, and water.
(iii) Calcium chloride is water soluble, and therefore, calcium chloride and water mix up to give a clear solution.
(iv) The other product, carbon dioxide, forms bubbles and mixes up in the air. Hence, lime stone powder disappears when added to diluted hydrochloric acids.
(v) The word equation is as follows:
Calcium carbonate + dilute Hydrochloric acid → Calcium chloride + Carbon dioxide + Water
d. Bubbles are seen on adding lemon juice to baking soda.
Ans:
(i) When baking soda is added to lemon juice, a chemical change takes place.
(ii) The citric acid present in the lemon juice reacts with baking soda to form carbon dioxide gas and sodium citrate.
(iii) The released carbon dioxide gas produces bubbles.
(iv) Hence, bubbles are seen when adding lemon juice to baking soda.
(v) The word equation is as follows:
Citric acid + Sodium bicarbonate → Carbon dioxide + Sodium citrate
3. Match the pairs.
| A | B |
|---|---|
a. Photosynthesis | i. Tendency to lose electrons |
b. Water | ii. Reactant in combustion process |
c. Sodium chloride | iii. Chemical change |
d. Dissolution of salt in water | iv. Covalent bond |
e. Carbon | v. Ionic bond |
f. Fluorine | vi. Physical change |
g. Magnesium | vii. Tendency to form anion |
Ans:
| A | B |
|---|---|
a. Photosynthesis | iii. Chemical change |
b. Water | iv. Covalent bond |
c. Sodium chloride | v. Ionic bond |
d. Dissolution of salt in water | vi. Physical change |
e. Carbon | ii. Reactant in combustion process |
f. Fluorine | vii. Tendency to form anion |
g. Magnesium | i. Tendency to lose electrons |
4. Show with the help of diagram of electronic configuration how the following compound are formed from the constituent atoms.
a. Sodium chloride
Ans:
(i) Sodium chloride is formed from its constituent elements, sodium (Na) and chlorine (17CI).
(ii) The electronic configuration of sodium is (2, 8, 1) and that of chlorine is (2, 8, 7).
(iii) The valency of sodium is I, as it has one electron in the valence shell (M-shell). The valency of chlorine is 1, as it has 7 electrons in its valence shell (M-shell), and it just needs one electron to complete the octet.
Step 1 : Write the symbols of the radicals.
Na Cl
Step 2 : Write the valency below the respective radical.
Na Cl
1 1
Step 3 : Cross-multiply symbols of radicals with their respective valency.
Na → 1
Cl → 1
Step 4 : Write down the chemical formula of the compound.
NaCl
b. Potassium fluoride
Ans:
(i) Potassium fluoride is formed from its constituent elements, potassium (K) and fluorine (F).
(ii) The electronic configuration of potassium is (2, 8, 8, 1), and that of fluorine is (2, 7).
(iii) The valency of potassium is 1, as it has one electron in the valence shell (N-shell). The valency of fluorine is 1, as it has 7 electrons in its valence shell (L-shell), and it just needs one electron to complete the octet.
Step 1 : Write the symbols of the radicals.
K F
Step 2 : Write the valency below the respective radical.
K F
1 1
Step 3 : Cross-multiply symbols of radicals with their respective valency.
K → 1
F → 1
Step 4 : Write down the chemical formula of the compound.
KF
c. Water
Ans:
(i) Water is formed from its constituent elements, hydrogen (H) and oxygen (O).
(ii) The valency of hydrogen is 1, as it contains only one electron and needs only one electron to complete its electron duplex. The valency of oxygen is two, and it needs two electrons to complete the electron octet.
Step 1 : Write the symbols of the radicals.
H O
Step 2 : Write the valency below the respective radical.
H O
1 2
Step 3 : Cross-multiply symbols of radicals with their respective valency.
H → 2
O → 1
Step 4 : Write down the chemical formula of the compound.
H₂O
d. Hydrogen chloride
Ans:
(i) Hydrogen chloride is formed from its constituent elements, hydrogen (H) and chloride (Cl).
(ii) An atom of chlorine contains seven electrons in its valence shell, and it needs one electron to complete the electron octet. Therefore, the valency of the chlorine atom is one.
(iii) An atom of hydrogen contains only one electron, and it needs only one electron to complete its electron duplex. Therefore, the valency of the hydrogen atom is one.
Step 1 : Write the symbols of the radicals.
H Cl
Step 2 : Write the valency below the respective radical.
H Cl
1 1
Step 3 : Cross-multiply symbols of radicals with their respective valency.
H → 1
Cl → 1
Step 4 : Write down the chemical formula of the compound.
HCl
