CBSE Class 10 Science Practicals (2)

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SECOND TERM

1. To study the following properties of acetic acid (ethanoic acid):
    i) odour
    ii) solubility in water
    iii) effect on litmus
    iv) reaction with sodium bicarbonate

Answer:
1. Odour: Acetic acid has a pungent, characteristic vinegar-like smell.
2. Solubility: It is completely miscible in water in all proportions, forming a homogeneous solution.
3. Effect on litmus: It turns blue litmus paper red, indicating its acidic nature.
4. Reaction with sodium bicarbonate: It reacts briskly with sodium bicarbonate (NaHCO₃) to evolve carbon dioxide (CO₂) gas, which turns lime water milky.

Teacher's Note:
a) Ethanoic acid is a weak organic acid belonging to the carboxylic acid group with formula CH₃COOH.
b) Ensure test tubes are clean and dry before adding reagents to avoid false observations.
c) The brisk effervescence due to carbon dioxide gas is a confirmatory test for carboxylic acids.
d) Always use dilute solutions and handle laboratory chemicals with standard safety precautions.

2. To study saponification reaction for preparation of soap.

Answer:
1. Saponification is the alkaline hydrolysis of an ester, specifically animal fat or vegetable oil (like castor oil, coconut oil) using sodium hydroxide (NaOH).
2. Oil or fat is heated with concentrated sodium hydroxide solution in a beaker with constant stirring.
3. Common salt (sodium chloride, NaCl) is added to the mixture to precipitate out (salting out) the soap.
4. The soap separates as a solid layer floating on the surface of the mixture and can be filtered or collected.

Teacher's Note:
a) Soap is chemically the sodium salt of long-chain fatty acids.
b) Common salt is added to decrease the solubility of soap and separate it completely from the aqueous solution.
c) Glycerol is obtained as a byproduct of this reaction.
d) Care must be taken while handling hot concentrated sodium hydroxide solution to prevent skin burns.

3. To study the comparative cleaning capacity of a sample of soap in soft and hard water.

Answer:
1. Take two test tubes, label them A and B, and add soft water to A and hard water to B.
2. Add equal drops of soap solution to both test tubes and shake them vigorously.
3. Abundant lather (foam) is formed easily in test tube A containing soft water.
4. Curdy white precipitate or scum is formed in test tube B containing hard water, and very little or no lather is produced until all calcium and magnesium ions are precipitated.

Teacher's Note:
a) Hard water contains dissolved salts of calcium and magnesium (chlorides, sulphates, or hydrogen carbonates).
b) Soap reacts with calcium and magnesium ions to form insoluble salts called scum, wasting soap.
c) Synthetic detergents work effectively even in hard water because they do not form insoluble calcium or magnesium salts.
d) Shake both test tubes uniformly for the same duration to ensure accurate comparative results.

4. To determine the focal length of:
    i. Concave mirror,
    ii. Convex lens,
by obtaining the image of a distant object.

Answer:
1. Mount the concave mirror or convex lens on a stand and place it facing a distant object (like a tree or a distant window).
2. Place a white screen or a measuring screen behind the mirror or lens and adjust its position until a sharp, inverted, diminished image of the distant object is obtained on the screen.
3. Measure the distance between the pole of the concave mirror (or optical center of the convex lens) and the screen using a meter scale.
4. This measured distance corresponds approximately to the focal length (f) of the given mirror or lens.

Teacher's Note:
a) Rays coming from a distant object are considered parallel to the principal axis.
b) For a parallel beam of light incident on a mirror or lens, the image is formed at the principal focus.
c) Ensure the distant object is well-lit for a sharp and clear image on the screen.
d) Take multiple observations to minimize experimental errors.

5. To trace the path of a ray of light passing through a rectangular glass slab for different angles of incidence. Measure the angle of incidence, angle of refraction, angle of emergence and interpret the result.

Answer:
1. Fix a white sheet of paper on a drawing board and place a rectangular glass slab centrally on it, marking its boundary.
2. Draw a normal line and incident ray at various angles of incidence (like 30°, 45°, 60°) and fix pins along the incident ray.
3. View the pins through the opposite face of the slab and fix two more pins to trace the emergent ray.
4. Remove the slab, join the points to complete the path, and measure the angle of incidence (i), angle of refraction (r), and angle of emergence (e).
5. Result interpretation: The angle of incidence is equal to the angle of emergence (\(i = e\)), and the emergent ray is parallel to the incident ray, showing lateral displacement.

Teacher's Note:
a) Light bends towards the normal when entering from a rarer medium (air) to a denser medium (glass).
b) Light bends away from the normal when exiting from a denser medium (glass) to a rarer medium (air).
c) Lateral displacement depends on the thickness of the glass slab and the angle of incidence.
d) Ensure the pins are fixed in a straight vertical line to avoid parallax errors.

6. To study (a) binary fission in Amoeba, and (b) budding in yeast with the help of prepared slides.

Answer:
1. Place the prepared slide of binary fission in Amoeba under the microscope and focus under low and high power.
2. Observe that the elongated nucleus of Amoeba divides first, followed by the constriction and division of the cytoplasm into two equal-sized daughter amoebae.
3. Place the prepared slide of budding in yeast under the microscope and focus.
4. Observe small outgrowths or bulges known as buds forming on the parent yeast cell, which eventually detach to form new individual yeast cells.

Teacher's Note:
a) Binary fission is a type of asexual reproduction where a single parent cell divides into two equal halves.
b) Budding is an asymmetric division where a new organism develops from an outgrowth or bud due to cell division at one specific site.
c) Amoeba reproduces by binary fission under favorable conditions.
d) Handle microscope slides carefully by the edges to avoid scratching or contamination.

7. To trace the path of the rays of light through a glass prism.

Answer:
1. Fix a white sheet of paper on a drawing board and place a glass prism on it, drawing its triangular boundary.
2. Draw a normal line on one refracting surface and draw an incident ray making an angle of incidence with the normal, placing two pins on it.
3. Look through the opposite refracting surface, locate the images of the pins, and fix two more pins to represent the emergent ray.
4. Remove the prism, trace the complete path of the ray, and measure the angle of incidence (i), angle of refraction (r), angle of prism (A), and angle of deviation (D).

Teacher's Note:
a) The angle between the incident ray and the emergent ray is called the angle of deviation.
b) Light undergoes refraction twice - first when entering the prism from air and second when leaving the prism into air.
c) The relation between prism angle, angle of deviation, angle of incidence, and angle of emergence is given by \(A + D = i + e\).
d) Ensure the pins are placed at a sufficient distance apart (at least 5 cm) for accurate alignment.

8. To find the image distance for varying object distances in case of a convex lens and draw corresponding ray diagrams to show the nature of image formed.

Answer:
1. Mount a convex lens on an optical bench and determine its approximate focal length.
2. Place a lighted candle (object) at various distances (greater than 2f, at 2f, between f and 2f, at f, and between optical center and f) in front of the lens.
3. Adjust the position of the white screen on the other side of the lens to obtain a sharp image and measure the object distance (u) and image distance (v).
4. Record observations in a tabular column and draw the corresponding ray diagrams for each case, noting the nature, position, and size of the image formed.

Teacher's Note:
a) The lens formula connecting focal length, object distance, and image distance is \(\frac{1}{f} = \frac{1}{v} - \frac{1}{u}\).
b) Real images formed by a convex lens are always inverted, whereas virtual images are erect.
c) When the object is placed at the focus (f), the image is formed at infinity.
d) Sign conventions must be strictly followed during calculations with object and image distances.

9. To study homology and analogy with the help of models/charts of animals and models/ charts/ specimens of plants.

Answer:
1. Homologous organs: Examine structures such as the forelimbs of a frog, bird, lizard, and human, which have similar basic anatomical designs and embryonic origins but perform different functions.
2. Interpret homology: Homologous organs indicate common ancestry and divergent evolution.
3. Analogous organs: Examine structures such as the wings of an insect and the wings of a bird or bat, which have different basic structures and embryonic origins but perform similar functions (flying).
4. Interpret analogy: Analogous organs indicate convergent evolution and adaptation to similar environmental demands without common ancestry.

Teacher's Note:
a) Homology refers to similarity in origin and structure, indicating common evolutionary descent.
b) Analogy refers to similarity in function despite differences in fundamental structure and origin.
c) Plant examples of homology include thorn of Bougainvillea and tendril of Cucurbita (both modified stems).
d) Plant examples of analogy include sweet potato (modified root) and potato (modified stem), both serving storage functions.

10. To identify the different parts of an embryo of a dicot seed (Pea, gram or red kidney bean).

Answer:
1. Soak dicot seeds (such as gram, pea, or red kidney bean) in water overnight to soften the seed coat and make internal structures easily accessible.
2. Carefully peel off the outer seed coat (testa and tegmen) using forceps to expose the two thick cotyledons.
3. Gently separate the two cotyledons without breaking them to observe the embryonic axis.
4. Identify the distinct parts of the embryo: the plumule (future shoot), the radicle (future root), and the hypocotyl/epicotyl region.

Teacher's Note:
a) Dicot seeds possess two cotyledons which store food reserves for the developing embryo.
b) The radicle develops into the root system of the new plant, while the plumule develops into the shoot system.
c) Seeds must be soaked adequately so the delicate embryonic parts do not get damaged during dissection.
d) Use a magnifying lens or dissecting microscope for clear visualization of the micropyle and embryonic axis.