1. Introduction
Change is the law of nature.
There are so many situations of daily life, where we can
observe various changes.
Like,
(i) Conversion of water into vapours from a cup of hot
tea.
(ii) Corrosion of iron articles (rusting) if exposed to
humid atmosphere.
(iii) Cooking of food.
(iv) Digestion of food in our body.
(v) Breaking of any article like glass.
(vi) Combustion of fuel in our vehicle.
Scientist classify these changes as
(1) Physical changes (2) Chemical changes
(1) Physical changes
A change in which physical properties of a substance changes but the chemical
composition do not change.
For example, Freezing, melting, boiling, condensation, etc.
[1]
, Characteristic features of physical changes
(1) The identity of the substance is maintained.
(2) The change is generally temporary.
(3) Heat change may or may not take place.
(4) Only the physical state or some of the physical properties of the substances are
changed.
(2) Chemical changes
A change in which one or more substances change into new substances with a different
chemical composition.
For example, burning of a candle, rusting of iron, combustion of fuel, etc.
Characteristic features of chemical changes
(1) The identity of original substance is completely lost.
(2) The change is generally permanent.
(3) The change is generally accompanied by energy change.
(4) The change cannot be reversed generally.
1. From the given examples, identify the chemical changes.
(a) Fermentation of grapes.
(b) Burning of a candle.
(c) Evaporation of alcohol.
(d) Freezing of water.
(e) Turning brown of a freshly cut apple in the air.
(f) Growth of a plant.
(g) Dissolution of sugar in water.
(h) Fading of coloured clothes in the sun.
★ In earlier standards, we have seen how compounds are formed by chemical
combination of element.
★ We have also learnt that the driving force behind formation of a chemical bond is to
attain an electronic configuration with a complete octet.
★ The atoms attain a complete octet by giving, taking or sharing of electrons with each
other.
2. Chemical reaction and its characteristics
[2]
,The process in which a substance or substances undergo a chemical change to produce
new substances, with entirely new properties are known as chemical reaction.
Aim
To study the reaction between magnesium and oxygen to
form magnesium oxide.
Caution
Perform this activity in the presence of a teacher. It would
be better to wear eye protection (as used by welders).
Materials Required
Burner, tong, magnesium ribbon, sand paper, watch glass.
Method
(i) Clean a magnesium ribbon about 2 cm long, by rubbing it with sand paper.
(ii) Hold it with a pair of tongs. Burn it using a spirit lamp or burner and collect the ash so
formed in a watch glass. Burn the magnesium ribbon keeping it as far as possible from
your eyes.
Burning of a magnesium ribbon in air and collection
of magnesium oxide in a watch glass.
Now answer
What do you observe?
Observation
It is observed that magnesium ribbon burns with a dazzling
white light and changes into a white powder. This powder is
magnesium oxide.
Conclusion
Magnesium burns in air to combine with oxygen to form
magnesium oxide.
[3]
, Aim
To study the reaction between lead nitrate solution and
potassium iodide solution.
Materials required
Test tube, lead nitrate solution, potassium iodide solution.
Method
(i) Take lead nitrate solution in a test tube.
(ii) Add potassium iodide solution to this.
Observation
It is observed that a yellow solid (precipitate) is formed.
Conclusion
Lead nitrate solution reacts with potassium iodide solution to form a yellow precipitate of
lead iodide.
Pb(NO3)2(aq) + 2KI(aq) ⎯→ PbI2(s)(↓) + 2KNO3(aq)
Lead nitrate Potassium iodide Lead (II) iodide Potassium nitrate
(Colourless) (Colourless) (Yellow ppt.) (Colourless)
Aim
To study the reaction between zinc and sulphuric acid or hydrochloric acid.
Materials required
Conical flask or test tube, zinc granules, dilute hydrochloric acid or sulphuric acid.
Method
(i) Take a few zinc granules in a conical flask or a test tube.
(ii) Add dilute hydrochloric acid or sulphuric acid to this.
[4]
Change is the law of nature.
There are so many situations of daily life, where we can
observe various changes.
Like,
(i) Conversion of water into vapours from a cup of hot
tea.
(ii) Corrosion of iron articles (rusting) if exposed to
humid atmosphere.
(iii) Cooking of food.
(iv) Digestion of food in our body.
(v) Breaking of any article like glass.
(vi) Combustion of fuel in our vehicle.
Scientist classify these changes as
(1) Physical changes (2) Chemical changes
(1) Physical changes
A change in which physical properties of a substance changes but the chemical
composition do not change.
For example, Freezing, melting, boiling, condensation, etc.
[1]
, Characteristic features of physical changes
(1) The identity of the substance is maintained.
(2) The change is generally temporary.
(3) Heat change may or may not take place.
(4) Only the physical state or some of the physical properties of the substances are
changed.
(2) Chemical changes
A change in which one or more substances change into new substances with a different
chemical composition.
For example, burning of a candle, rusting of iron, combustion of fuel, etc.
Characteristic features of chemical changes
(1) The identity of original substance is completely lost.
(2) The change is generally permanent.
(3) The change is generally accompanied by energy change.
(4) The change cannot be reversed generally.
1. From the given examples, identify the chemical changes.
(a) Fermentation of grapes.
(b) Burning of a candle.
(c) Evaporation of alcohol.
(d) Freezing of water.
(e) Turning brown of a freshly cut apple in the air.
(f) Growth of a plant.
(g) Dissolution of sugar in water.
(h) Fading of coloured clothes in the sun.
★ In earlier standards, we have seen how compounds are formed by chemical
combination of element.
★ We have also learnt that the driving force behind formation of a chemical bond is to
attain an electronic configuration with a complete octet.
★ The atoms attain a complete octet by giving, taking or sharing of electrons with each
other.
2. Chemical reaction and its characteristics
[2]
,The process in which a substance or substances undergo a chemical change to produce
new substances, with entirely new properties are known as chemical reaction.
Aim
To study the reaction between magnesium and oxygen to
form magnesium oxide.
Caution
Perform this activity in the presence of a teacher. It would
be better to wear eye protection (as used by welders).
Materials Required
Burner, tong, magnesium ribbon, sand paper, watch glass.
Method
(i) Clean a magnesium ribbon about 2 cm long, by rubbing it with sand paper.
(ii) Hold it with a pair of tongs. Burn it using a spirit lamp or burner and collect the ash so
formed in a watch glass. Burn the magnesium ribbon keeping it as far as possible from
your eyes.
Burning of a magnesium ribbon in air and collection
of magnesium oxide in a watch glass.
Now answer
What do you observe?
Observation
It is observed that magnesium ribbon burns with a dazzling
white light and changes into a white powder. This powder is
magnesium oxide.
Conclusion
Magnesium burns in air to combine with oxygen to form
magnesium oxide.
[3]
, Aim
To study the reaction between lead nitrate solution and
potassium iodide solution.
Materials required
Test tube, lead nitrate solution, potassium iodide solution.
Method
(i) Take lead nitrate solution in a test tube.
(ii) Add potassium iodide solution to this.
Observation
It is observed that a yellow solid (precipitate) is formed.
Conclusion
Lead nitrate solution reacts with potassium iodide solution to form a yellow precipitate of
lead iodide.
Pb(NO3)2(aq) + 2KI(aq) ⎯→ PbI2(s)(↓) + 2KNO3(aq)
Lead nitrate Potassium iodide Lead (II) iodide Potassium nitrate
(Colourless) (Colourless) (Yellow ppt.) (Colourless)
Aim
To study the reaction between zinc and sulphuric acid or hydrochloric acid.
Materials required
Conical flask or test tube, zinc granules, dilute hydrochloric acid or sulphuric acid.
Method
(i) Take a few zinc granules in a conical flask or a test tube.
(ii) Add dilute hydrochloric acid or sulphuric acid to this.
[4]
