Class 10 Science: CBSE (Official) Marking Scheme with Solution (2022-23) | Science Class 10 PDF Download

 Page 1


SCIENCE (086) 
CLASS X 
MARKING SCHEME (2022-23) 
Q.  No Questions 
 
Marks 
SECTION – A 
1.  (c) Only iii 1 
2.  (b) MnO
2
 is reduced to MnCl
2 
& HCl is oxidized to Cl
2
 1 
3.  (c) Magnesium ribbon burns with brilliant white light 1 
4.  (b) CO
2
, Cl
2
, CO
2
, CO 1 
5.  (d) Ferrous sulphate solution remains green with no change in the copper coin. 1 
6.  (a) Only i  1 
7.  (c) Addition of hydrogen in presence of catalyst changes A to C 1 
8.  (b) II,III 1 
9.  (b) 1 
10.  (d) 1 
11.  (d) C only 1 
12.  (b) B and D 1 
13.  (c) increases 1 
14.  (b) 2 (Either North or South) 1 
15.  (b) diameter d of the wire 1 
16.  (d) The field consists of concentric circles centred around the wire. 1 
17.  (c)  A is true but R is false 1 
18.  (a) Both A and R are true and R is the correct explanation of A 1 
19.  (c)  A is true but R is false 1 
20.  (a) Both A and R are true and R is the correct explanation of A 1 
SECTION – B 
21.  
Calcium hydroxide reacts with Carbon dioxide present in the atmosphere to form 
Calcium carbonate which results in milkiness/white ppt / Formation of Calcium 
carbonate   (1mark) 
Ca(OH) 2 + CO 2 ?  CaCO 3 + H 2O   (1mark) 
OR 
Fe + HCl ? FeCl 2/ FeCl 3 + H 2    (1mark) (No deduction for balancing/ states) 
Zn + HCl ? ZnCl 2 + H 2 - 1M 
2 
Page 2


SCIENCE (086) 
CLASS X 
MARKING SCHEME (2022-23) 
Q.  No Questions 
 
Marks 
SECTION – A 
1.  (c) Only iii 1 
2.  (b) MnO
2
 is reduced to MnCl
2 
& HCl is oxidized to Cl
2
 1 
3.  (c) Magnesium ribbon burns with brilliant white light 1 
4.  (b) CO
2
, Cl
2
, CO
2
, CO 1 
5.  (d) Ferrous sulphate solution remains green with no change in the copper coin. 1 
6.  (a) Only i  1 
7.  (c) Addition of hydrogen in presence of catalyst changes A to C 1 
8.  (b) II,III 1 
9.  (b) 1 
10.  (d) 1 
11.  (d) C only 1 
12.  (b) B and D 1 
13.  (c) increases 1 
14.  (b) 2 (Either North or South) 1 
15.  (b) diameter d of the wire 1 
16.  (d) The field consists of concentric circles centred around the wire. 1 
17.  (c)  A is true but R is false 1 
18.  (a) Both A and R are true and R is the correct explanation of A 1 
19.  (c)  A is true but R is false 1 
20.  (a) Both A and R are true and R is the correct explanation of A 1 
SECTION – B 
21.  
Calcium hydroxide reacts with Carbon dioxide present in the atmosphere to form 
Calcium carbonate which results in milkiness/white ppt / Formation of Calcium 
carbonate   (1mark) 
Ca(OH) 2 + CO 2 ?  CaCO 3 + H 2O   (1mark) 
OR 
Fe + HCl ? FeCl 2/ FeCl 3 + H 2    (1mark) (No deduction for balancing/ states) 
Zn + HCl ? ZnCl 2 + H 2 - 1M 
2 
22.  
 
 
Any four points (½ x4=2 marks) 
 
2 
23.  Gallbladder stores bile which helps in emulsification of lipids   (1mark). In the 
absence of stored bile, emulsification of fats will be negligible/ affected/ less 
(1mark) and thus fat digestion will be slow. Hence there are such diet restrictions. 
 
2 
24.  Glucose, amino acids, salts (any 2, 1 mark each) and a major amount of water are 
selectively re-absorbed as the urine flows along the tube.   
The amount of water reabsorbed depends on how much excess water there is in 
the body (0.5 marks), and on how much of dissolved waste there is to be excreted 
(0.5marks)  
 
2 
25.  Dispersion- The splitting of white light into seven colours on passing through a 
prism.                                                                                                           (1 mark)               
Velocity is directly proportional to wavelength given constant frequency. So 
yellow will have greater wavelength than blue as the velocity of yellow light is 
greater than blue.                                                                             (0.5 + 0.5 mark) 
OR 
Angle of deflections of the two prisms need to be equal and opposite. While the 
first prism splits the light in the seven colours due to different angles of deflection, 
the second prism combines the spectrum along a single ray and the colours again 
combine to give white light as the emergent light.  (1mark)                                                       
 
 
 
 
 
 
(1mark) 
 
 
2 
26.  
Excess generation of biodegradable wastes can be harmful as - 
Its decomposition is a slow process leading to production of foul smell and gases. 
(1mark) 
It can be the breeding ground for germs that create unhygienic conditions.(1 mark) 
2 
Page 3


SCIENCE (086) 
CLASS X 
MARKING SCHEME (2022-23) 
Q.  No Questions 
 
Marks 
SECTION – A 
1.  (c) Only iii 1 
2.  (b) MnO
2
 is reduced to MnCl
2 
& HCl is oxidized to Cl
2
 1 
3.  (c) Magnesium ribbon burns with brilliant white light 1 
4.  (b) CO
2
, Cl
2
, CO
2
, CO 1 
5.  (d) Ferrous sulphate solution remains green with no change in the copper coin. 1 
6.  (a) Only i  1 
7.  (c) Addition of hydrogen in presence of catalyst changes A to C 1 
8.  (b) II,III 1 
9.  (b) 1 
10.  (d) 1 
11.  (d) C only 1 
12.  (b) B and D 1 
13.  (c) increases 1 
14.  (b) 2 (Either North or South) 1 
15.  (b) diameter d of the wire 1 
16.  (d) The field consists of concentric circles centred around the wire. 1 
17.  (c)  A is true but R is false 1 
18.  (a) Both A and R are true and R is the correct explanation of A 1 
19.  (c)  A is true but R is false 1 
20.  (a) Both A and R are true and R is the correct explanation of A 1 
SECTION – B 
21.  
Calcium hydroxide reacts with Carbon dioxide present in the atmosphere to form 
Calcium carbonate which results in milkiness/white ppt / Formation of Calcium 
carbonate   (1mark) 
Ca(OH) 2 + CO 2 ?  CaCO 3 + H 2O   (1mark) 
OR 
Fe + HCl ? FeCl 2/ FeCl 3 + H 2    (1mark) (No deduction for balancing/ states) 
Zn + HCl ? ZnCl 2 + H 2 - 1M 
2 
22.  
 
 
Any four points (½ x4=2 marks) 
 
2 
23.  Gallbladder stores bile which helps in emulsification of lipids   (1mark). In the 
absence of stored bile, emulsification of fats will be negligible/ affected/ less 
(1mark) and thus fat digestion will be slow. Hence there are such diet restrictions. 
 
2 
24.  Glucose, amino acids, salts (any 2, 1 mark each) and a major amount of water are 
selectively re-absorbed as the urine flows along the tube.   
The amount of water reabsorbed depends on how much excess water there is in 
the body (0.5 marks), and on how much of dissolved waste there is to be excreted 
(0.5marks)  
 
2 
25.  Dispersion- The splitting of white light into seven colours on passing through a 
prism.                                                                                                           (1 mark)               
Velocity is directly proportional to wavelength given constant frequency. So 
yellow will have greater wavelength than blue as the velocity of yellow light is 
greater than blue.                                                                             (0.5 + 0.5 mark) 
OR 
Angle of deflections of the two prisms need to be equal and opposite. While the 
first prism splits the light in the seven colours due to different angles of deflection, 
the second prism combines the spectrum along a single ray and the colours again 
combine to give white light as the emergent light.  (1mark)                                                       
 
 
 
 
 
 
(1mark) 
 
 
2 
26.  
Excess generation of biodegradable wastes can be harmful as - 
Its decomposition is a slow process leading to production of foul smell and gases. 
(1mark) 
It can be the breeding ground for germs that create unhygienic conditions.(1 mark) 
2 
SECTION - C 
Q.no. 27 to 33 are short answer questions. 
27.  
i) Displacement - ½ M 
? Fe(s) + CuSO
4
(aq) ? FeSO
4
(aq) + Cu(s)          (1 mark) 
? Zn(s) + CuSO
4
(aq)? ZnSO
4
(aq) + Cu(s) 
? Pb(s) + CuCl
2
(aq) ? PbCl
2
(aq) + Cu(s) 
(Any one of the reaction or other displacement reaction.) 
ii) Double displacement  (½ mark) 
Na
2
SO
4
 (aq) + BaCl
2
(aq) ? BaSO
4 
(s) + 2NaCl(aq)   (1 mark) 
(Any one of the reaction or other double displacement reaction.) 
3 
28.  
(a) Anode: Chlorine; Cathode: Hydrogen 
(b) Chlor alkali process as the products obtained are alkali, chlorine gas and 
hydrogen gas               Electric current 
(c) 2NaCl(aq) + 2H
2
O(l)                         2NaOH(aq) + Cl
2
 (g) + H
2
 (g) 
3 
29.  
No photosynthesis will occur so no glucose will be made. Also no respiration will 
take place as no Oxygen will be taken in. (1) 
No transpiration will occur so there would be no upward movement of water or 
minerals from the soil as there will be no transpirational pull.(1) 
Temperature regulation of leaf surface will be affected. (1) 
OR 
Lymph carries digested and absorbed fat from the intestine (1) and drains excess 
fluid from extracellular space back into the blood (1). Blockage of lymphatic 
system will lead to water retention and poor fat absorption in the body  
(1- any one) 
3 
30.  
(a) The object has to be placed at a distance between 0 - 40 cm. This is because 
image is virtual, erect and magnified when the object is placed between F and P. 
(1mark) 
 
(b) 
 
 
 
 
 
 
                                                                                                                                   
 
 
 
 
(1mark) 
 
(c) Used as shaving mirror or used by dentists to get enlarged image of teeth  
(any one use ) (1mark) 
3 
Page 4


SCIENCE (086) 
CLASS X 
MARKING SCHEME (2022-23) 
Q.  No Questions 
 
Marks 
SECTION – A 
1.  (c) Only iii 1 
2.  (b) MnO
2
 is reduced to MnCl
2 
& HCl is oxidized to Cl
2
 1 
3.  (c) Magnesium ribbon burns with brilliant white light 1 
4.  (b) CO
2
, Cl
2
, CO
2
, CO 1 
5.  (d) Ferrous sulphate solution remains green with no change in the copper coin. 1 
6.  (a) Only i  1 
7.  (c) Addition of hydrogen in presence of catalyst changes A to C 1 
8.  (b) II,III 1 
9.  (b) 1 
10.  (d) 1 
11.  (d) C only 1 
12.  (b) B and D 1 
13.  (c) increases 1 
14.  (b) 2 (Either North or South) 1 
15.  (b) diameter d of the wire 1 
16.  (d) The field consists of concentric circles centred around the wire. 1 
17.  (c)  A is true but R is false 1 
18.  (a) Both A and R are true and R is the correct explanation of A 1 
19.  (c)  A is true but R is false 1 
20.  (a) Both A and R are true and R is the correct explanation of A 1 
SECTION – B 
21.  
Calcium hydroxide reacts with Carbon dioxide present in the atmosphere to form 
Calcium carbonate which results in milkiness/white ppt / Formation of Calcium 
carbonate   (1mark) 
Ca(OH) 2 + CO 2 ?  CaCO 3 + H 2O   (1mark) 
OR 
Fe + HCl ? FeCl 2/ FeCl 3 + H 2    (1mark) (No deduction for balancing/ states) 
Zn + HCl ? ZnCl 2 + H 2 - 1M 
2 
22.  
 
 
Any four points (½ x4=2 marks) 
 
2 
23.  Gallbladder stores bile which helps in emulsification of lipids   (1mark). In the 
absence of stored bile, emulsification of fats will be negligible/ affected/ less 
(1mark) and thus fat digestion will be slow. Hence there are such diet restrictions. 
 
2 
24.  Glucose, amino acids, salts (any 2, 1 mark each) and a major amount of water are 
selectively re-absorbed as the urine flows along the tube.   
The amount of water reabsorbed depends on how much excess water there is in 
the body (0.5 marks), and on how much of dissolved waste there is to be excreted 
(0.5marks)  
 
2 
25.  Dispersion- The splitting of white light into seven colours on passing through a 
prism.                                                                                                           (1 mark)               
Velocity is directly proportional to wavelength given constant frequency. So 
yellow will have greater wavelength than blue as the velocity of yellow light is 
greater than blue.                                                                             (0.5 + 0.5 mark) 
OR 
Angle of deflections of the two prisms need to be equal and opposite. While the 
first prism splits the light in the seven colours due to different angles of deflection, 
the second prism combines the spectrum along a single ray and the colours again 
combine to give white light as the emergent light.  (1mark)                                                       
 
 
 
 
 
 
(1mark) 
 
 
2 
26.  
Excess generation of biodegradable wastes can be harmful as - 
Its decomposition is a slow process leading to production of foul smell and gases. 
(1mark) 
It can be the breeding ground for germs that create unhygienic conditions.(1 mark) 
2 
SECTION - C 
Q.no. 27 to 33 are short answer questions. 
27.  
i) Displacement - ½ M 
? Fe(s) + CuSO
4
(aq) ? FeSO
4
(aq) + Cu(s)          (1 mark) 
? Zn(s) + CuSO
4
(aq)? ZnSO
4
(aq) + Cu(s) 
? Pb(s) + CuCl
2
(aq) ? PbCl
2
(aq) + Cu(s) 
(Any one of the reaction or other displacement reaction.) 
ii) Double displacement  (½ mark) 
Na
2
SO
4
 (aq) + BaCl
2
(aq) ? BaSO
4 
(s) + 2NaCl(aq)   (1 mark) 
(Any one of the reaction or other double displacement reaction.) 
3 
28.  
(a) Anode: Chlorine; Cathode: Hydrogen 
(b) Chlor alkali process as the products obtained are alkali, chlorine gas and 
hydrogen gas               Electric current 
(c) 2NaCl(aq) + 2H
2
O(l)                         2NaOH(aq) + Cl
2
 (g) + H
2
 (g) 
3 
29.  
No photosynthesis will occur so no glucose will be made. Also no respiration will 
take place as no Oxygen will be taken in. (1) 
No transpiration will occur so there would be no upward movement of water or 
minerals from the soil as there will be no transpirational pull.(1) 
Temperature regulation of leaf surface will be affected. (1) 
OR 
Lymph carries digested and absorbed fat from the intestine (1) and drains excess 
fluid from extracellular space back into the blood (1). Blockage of lymphatic 
system will lead to water retention and poor fat absorption in the body  
(1- any one) 
3 
30.  
(a) The object has to be placed at a distance between 0 - 40 cm. This is because 
image is virtual, erect and magnified when the object is placed between F and P. 
(1mark) 
 
(b) 
 
 
 
 
 
 
                                                                                                                                   
 
 
 
 
(1mark) 
 
(c) Used as shaving mirror or used by dentists to get enlarged image of teeth  
(any one use ) (1mark) 
3 
31.  (a)  
  
                                                                                                                                      
(2 marks) 
(b)  This is because the least distance of distinct vision is 25 cm.   (1 mark) 
 
3 
32.  (a) When iron filings are placed in a magnetic field around a bar magnet, they 
behave like tiny magnets. The magnetic force experienced by these tiny magnets 
make them rotate and align themselves along the direction of field lines.  (1 mark) 
(b)The physical property indicated by this arrangement is the magnetic field 
produced by the bar magnet. (1 mark) 
(c) Magnetic field lines never intersect, magnetic field lines are closed curves.   
(1mark) 
OR 
(a) The deflection in the compass needle increases as Magnetic field of the current 
carrying conductor is directly proportional to current flowing through it.   
(1.5marks) 
(b) The deflection in the needle decreases as the magnetic field is inversely 
proportional to the perpendicular distance from the wire.   (1.5marks) 
3 
33.  
Damage to the ozone layer is a cause for concern because the ozone layer shields 
the surface of earth from harmful UV radiations from the sun which cause skin 
cancer in human beings. 
Synthetic chemicals like chlorofluorocarbons (CFCs) which are used as 
refrigerants and in the fire  - extinguishers are the main reason for the depletion of 
the ozone layer. 
Steps taken to limit this damage - Many developing and developed countries have 
signed and are obeying the directions of UNEP (United Nations Environment 
Programme) to freeze or limit the production and usage of CFCs at 1986 levels. 
(1 x 3 = 3 marks) 
3 
SECTION - D 
34.  (a) A – Ethanoic acid/ Or any other carboxylic acid , C- Sodium salt of ethanoic 
acid/ any other carboxylic acid/ sodium ethanoate  ( ½ + ½ mark) 
(b) Use of  A- dil solution used as vinegar in cooking/ preservative in pickles  
(1mark) 
Use of  B – making perfumes, flavoring agent  (1 mark) 
                                   Conc H
2
SO
4
 
(c) CH
3
COOH + C
2
H
5
OH ----------------> CH
3
COOC
2
H
5
 +H
2
O  (1mark) 
CH
3
COOC
2
H
5
 + NaOH----------------> CH
3
COONa + C
2
H
5
OH  (1mark) 
  OR 
5 
Page 5


SCIENCE (086) 
CLASS X 
MARKING SCHEME (2022-23) 
Q.  No Questions 
 
Marks 
SECTION – A 
1.  (c) Only iii 1 
2.  (b) MnO
2
 is reduced to MnCl
2 
& HCl is oxidized to Cl
2
 1 
3.  (c) Magnesium ribbon burns with brilliant white light 1 
4.  (b) CO
2
, Cl
2
, CO
2
, CO 1 
5.  (d) Ferrous sulphate solution remains green with no change in the copper coin. 1 
6.  (a) Only i  1 
7.  (c) Addition of hydrogen in presence of catalyst changes A to C 1 
8.  (b) II,III 1 
9.  (b) 1 
10.  (d) 1 
11.  (d) C only 1 
12.  (b) B and D 1 
13.  (c) increases 1 
14.  (b) 2 (Either North or South) 1 
15.  (b) diameter d of the wire 1 
16.  (d) The field consists of concentric circles centred around the wire. 1 
17.  (c)  A is true but R is false 1 
18.  (a) Both A and R are true and R is the correct explanation of A 1 
19.  (c)  A is true but R is false 1 
20.  (a) Both A and R are true and R is the correct explanation of A 1 
SECTION – B 
21.  
Calcium hydroxide reacts with Carbon dioxide present in the atmosphere to form 
Calcium carbonate which results in milkiness/white ppt / Formation of Calcium 
carbonate   (1mark) 
Ca(OH) 2 + CO 2 ?  CaCO 3 + H 2O   (1mark) 
OR 
Fe + HCl ? FeCl 2/ FeCl 3 + H 2    (1mark) (No deduction for balancing/ states) 
Zn + HCl ? ZnCl 2 + H 2 - 1M 
2 
22.  
 
 
Any four points (½ x4=2 marks) 
 
2 
23.  Gallbladder stores bile which helps in emulsification of lipids   (1mark). In the 
absence of stored bile, emulsification of fats will be negligible/ affected/ less 
(1mark) and thus fat digestion will be slow. Hence there are such diet restrictions. 
 
2 
24.  Glucose, amino acids, salts (any 2, 1 mark each) and a major amount of water are 
selectively re-absorbed as the urine flows along the tube.   
The amount of water reabsorbed depends on how much excess water there is in 
the body (0.5 marks), and on how much of dissolved waste there is to be excreted 
(0.5marks)  
 
2 
25.  Dispersion- The splitting of white light into seven colours on passing through a 
prism.                                                                                                           (1 mark)               
Velocity is directly proportional to wavelength given constant frequency. So 
yellow will have greater wavelength than blue as the velocity of yellow light is 
greater than blue.                                                                             (0.5 + 0.5 mark) 
OR 
Angle of deflections of the two prisms need to be equal and opposite. While the 
first prism splits the light in the seven colours due to different angles of deflection, 
the second prism combines the spectrum along a single ray and the colours again 
combine to give white light as the emergent light.  (1mark)                                                       
 
 
 
 
 
 
(1mark) 
 
 
2 
26.  
Excess generation of biodegradable wastes can be harmful as - 
Its decomposition is a slow process leading to production of foul smell and gases. 
(1mark) 
It can be the breeding ground for germs that create unhygienic conditions.(1 mark) 
2 
SECTION - C 
Q.no. 27 to 33 are short answer questions. 
27.  
i) Displacement - ½ M 
? Fe(s) + CuSO
4
(aq) ? FeSO
4
(aq) + Cu(s)          (1 mark) 
? Zn(s) + CuSO
4
(aq)? ZnSO
4
(aq) + Cu(s) 
? Pb(s) + CuCl
2
(aq) ? PbCl
2
(aq) + Cu(s) 
(Any one of the reaction or other displacement reaction.) 
ii) Double displacement  (½ mark) 
Na
2
SO
4
 (aq) + BaCl
2
(aq) ? BaSO
4 
(s) + 2NaCl(aq)   (1 mark) 
(Any one of the reaction or other double displacement reaction.) 
3 
28.  
(a) Anode: Chlorine; Cathode: Hydrogen 
(b) Chlor alkali process as the products obtained are alkali, chlorine gas and 
hydrogen gas               Electric current 
(c) 2NaCl(aq) + 2H
2
O(l)                         2NaOH(aq) + Cl
2
 (g) + H
2
 (g) 
3 
29.  
No photosynthesis will occur so no glucose will be made. Also no respiration will 
take place as no Oxygen will be taken in. (1) 
No transpiration will occur so there would be no upward movement of water or 
minerals from the soil as there will be no transpirational pull.(1) 
Temperature regulation of leaf surface will be affected. (1) 
OR 
Lymph carries digested and absorbed fat from the intestine (1) and drains excess 
fluid from extracellular space back into the blood (1). Blockage of lymphatic 
system will lead to water retention and poor fat absorption in the body  
(1- any one) 
3 
30.  
(a) The object has to be placed at a distance between 0 - 40 cm. This is because 
image is virtual, erect and magnified when the object is placed between F and P. 
(1mark) 
 
(b) 
 
 
 
 
 
 
                                                                                                                                   
 
 
 
 
(1mark) 
 
(c) Used as shaving mirror or used by dentists to get enlarged image of teeth  
(any one use ) (1mark) 
3 
31.  (a)  
  
                                                                                                                                      
(2 marks) 
(b)  This is because the least distance of distinct vision is 25 cm.   (1 mark) 
 
3 
32.  (a) When iron filings are placed in a magnetic field around a bar magnet, they 
behave like tiny magnets. The magnetic force experienced by these tiny magnets 
make them rotate and align themselves along the direction of field lines.  (1 mark) 
(b)The physical property indicated by this arrangement is the magnetic field 
produced by the bar magnet. (1 mark) 
(c) Magnetic field lines never intersect, magnetic field lines are closed curves.   
(1mark) 
OR 
(a) The deflection in the compass needle increases as Magnetic field of the current 
carrying conductor is directly proportional to current flowing through it.   
(1.5marks) 
(b) The deflection in the needle decreases as the magnetic field is inversely 
proportional to the perpendicular distance from the wire.   (1.5marks) 
3 
33.  
Damage to the ozone layer is a cause for concern because the ozone layer shields 
the surface of earth from harmful UV radiations from the sun which cause skin 
cancer in human beings. 
Synthetic chemicals like chlorofluorocarbons (CFCs) which are used as 
refrigerants and in the fire  - extinguishers are the main reason for the depletion of 
the ozone layer. 
Steps taken to limit this damage - Many developing and developed countries have 
signed and are obeying the directions of UNEP (United Nations Environment 
Programme) to freeze or limit the production and usage of CFCs at 1986 levels. 
(1 x 3 = 3 marks) 
3 
SECTION - D 
34.  (a) A – Ethanoic acid/ Or any other carboxylic acid , C- Sodium salt of ethanoic 
acid/ any other carboxylic acid/ sodium ethanoate  ( ½ + ½ mark) 
(b) Use of  A- dil solution used as vinegar in cooking/ preservative in pickles  
(1mark) 
Use of  B – making perfumes, flavoring agent  (1 mark) 
                                   Conc H
2
SO
4
 
(c) CH
3
COOH + C
2
H
5
OH ----------------> CH
3
COOC
2
H
5
 +H
2
O  (1mark) 
CH
3
COOC
2
H
5
 + NaOH----------------> CH
3
COONa + C
2
H
5
OH  (1mark) 
  OR 
5 
(a) Sulphuric acid acts as dehydrating agent   (1mark) 
              Conc H
2
SO
4
, 443K 
C
2
H
5
OH —--------------------> C
2
H
4
 + H
2
O  (1mark) 
(b) By reaction with  sodium carbonate/ bi carbonate 1M with the samples, 
ethanol will not react whereas ethanoic acid gives brisk effervescence  
(1mark) 
      2CH
3
COOH + Na
2
 CO
3 
? 2CH
3
COONa + H
2
O+ CO
2
 
                        OR 
             CH
3
COOH + NaHCO 3 ? CH 3COONa + H
2
O + CO
2
 (1 mark) 
35.  
(a) The reason is that many multi-cellular organisms are not simply a random 
collection of cells. Specialised cells are organised as tissues, and tissues are 
organised into organs, which then have to be placed at definite positions in the 
body. Therefore, cell-by-cell division would be impractical.                (2 marks) 
(b) Sexual maturation of reproductive tissues is a necessary link for reproduction 
because of the need for specialised cell called germ-cells to participate in 
sexual reproduction. The body of the individual organism has to grow to its 
adult size, the rate of general body growth begins to slow down, reproductive 
tissues begin to mature.                    (1½ marks) 
A whole new set of changes in the appearance of the body takes place like 
change in body proportions, new features appear. This period during 
adolescence is called puberty. 
There are also changes taking place that are different between boys and girls. In 
girls, breast size begins to increase, with darkening of the skin of the nipples at 
the tips of the breasts. Also, girls begin to menstruate at around this time. Boys 
begin to have new thick hair growth on the face and their voices begin to crack.                      
(1½ marks) 
OR 
(a) If the niche were drastically altered, the population could be wiped out. 
However, if some variations were to be present in a few individuals in these 
populations, there would be some chance for them to survive. Variation is thus 
useful for the survival of species over time.                                        (2 marks) 
(b) 
? The lining of the uterus thickens and is richly supplied with blood to 
nourish the growing embryo.                                                                                   
(½ mark) 
? The embryo gets nutrition from the mother’s blood with the help of 
placenta. It is embedded in the uterine wall.                                                            
( ½ mark) 
?  It contains villi on the embryo’s side of the tissue. On the mother’s side 
are blood spaces, which surround the villi.                                                    
( ½ mark) 
?  This provides a large surface area for glucose and oxygen to pass from 
the mother to the embryo. The developing embryo will also generate 
waste substances which can be removed by transferring them into the 
mother’s blood through the placenta.                                              (1 mark) 
? The child is born as a result of rhythmic contractions of the muscles in the 
uterus.                                                                                          ( ½ mark) 
5