STD 8 SCIENCE 13. Chemical Change and Chemical Bond

Question 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,  equality sign)

a. An................... is drawn in between the reactants and products while writing the equation for a chemical reation.

b. Rusting of iron is a............. chemical change.

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

d. A colourless solution of calcium hydroxide in a test tube turns..............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.....................change.

f. Oxygen is a....................... in respiration.

g. Sodium chloride  is....................compound while hydrogen chlorid is........................ compound.

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

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

Answer 1:

a. An arrow is drawn in between the reactants and products while writing the equation for a chemical reation.

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 achemical 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.

Question 2:

Explain by writing a word equation.

a. Respiration is a chemical change.

b. Hard water gets softened on mixing with a solutions of washing soda.

c. Lime stone powder disappears on adding to dilute hydrochloric acids.

d. Bubbles are seen on adding lemon juice to baking soda.

Answer 2:

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

• Respiration is irreversible process
• During respiration, oxygen is convert into carbon-dioxide( a new substance is formed).
• During respiration, there is change in their chemical composition.
• During respiration, absorption and evolution of energy takes place.

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

b.Sodium carbonate, Na2CO3, is also known as washing soda. It can remove temporary and permanent hardness from water. Sodium carbonate is soluble but calcium carbonate and magnesium carbonate are insoluble.

The carbonate ions from sodium carbonate reacts with calcium and magnesium ions in the water to produce insoluble precipitate.

For example:

$C{a}^{2+}\left(aq\right)+N{a}_{2}C{O}_3\to CaC{O}_3\left(s\right)+2N{a}^{+}\left(aq\right)$

The water is softened because it no longer contains dissolved calcium ions and magnesium ions. 

c.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 gas (brisk effervescence).

$2HCl+CaC{O}_3\to CaC{l}_2+C{O}_2+H_{2}0$

In this reaction, we add hydrogen ions (H+), which will react with the carbonate ion to form hydrogen carbonate HCO3- ions, which are soluble in water and the limestone will dissolve.

d.Bubbles are seen on adding lemon juice to baking soda because lemon juice is citric acid and baking soda is sodium bicarbonate. On adding lemon juice in baking soda, we add acid in base then neutralization reaction is taking place resulting in the formation of salt ,water and brisk effervesence of carbon-dioxide.This CO2 is released in the form of bubbles.

$H_3{C}_6{H}_5{O}_7\left(\mathrm{aq}\right)+3{\mathrm{NaHCO}}_3\left(\mathrm{aq}\right)\to {\mathrm{Na}}_3{C}_6{H}_5{O}_7\left(\mathrm{aq}\right)+3H_{2}O\left(l\right)+3{\mathrm{CO}}_2\left(g\right)citricacid+bakingsoda\to salt+water+carbondioxide$

Question 3:

Match the pairs.

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

Answer 3:

a. Photosynthesis	i.    Chemical change
b. Water	ii.   Covalent bond
c. Sodium chloride	iii.  Ionic bond
d. Dissolution of salt in water	iv.   Physical change
e. Carbon	v.    Reactant in combustion process
f.  Fluorine	vi.  Tendency to form anion
g. Magnesium	vii. Tendency to lose electrons

Question 4:

Show with the heip of diagram of electronic configuration how the following compound are formed from the constituent atoms.

a. Sodium chloride  

b. Potassium fluoride

c.Water  

d. Hydrogen chloride

Answer 4:

a. Sodium chloride :

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.

                         

Step 4 : Write down the chemical formula of the compound.

        

              NaCl

b. Potassium fluoride :

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.

                         

Step 4 : Write down the chemical formula of the compound.

        

              KF

c.Water :

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.

                         

Step 4 : Write down the chemical formula of the compound.

        

              H2O

d. Hydrogen chloride :

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.

                         

Step 4 : Write down the chemical formula of the compound.

        

              HCl