CBSE Class 10 Science Reproduction, Carbon Compounds, Light and Heredity Assignment Set 11

Answer: The seminal vesicles and prostate gland add their secretions to the sperms, forming semen; these secretions provide nourishment to the sperms and make their transport easier. The testes lie in the scrotum because sperm formation needs a temperature about 2–3 °C lower than the normal body temperature.

Teacher's Note:
1. Both glands add fluids to the sperms.
2. The scrotum acts as a natural cooling arrangement.

Answer: Contraception is the use of methods to prevent pregnancy. It is needed to control population growth, to space children, to protect the health of the mother and to prevent sexually transmitted infections (in the case of barrier methods). Surgical methods are vasectomy in males and tubectomy in females.

Teacher's Note:
1. Contraceptive methods can be barrier, chemical, surgical or based on devices.
2. Surgical methods block the gamete passage permanently.

Answer: 1. They work well in hard water without forming scum.
2. They can be used in acidic water as well.
3. They have a stronger cleansing action on oily dirt.
4. They can be made with specific properties, e.g. for washing machines or dishes.
5. They lather easily with small quantities and rinse out cleanly.

Teacher's Note:
1. Soaps react with calcium and magnesium salts in hard water to form insoluble scum.
2. Detergents do not, which is their main advantage.

Answer: (i) Ethanoic acid: CH₃–COOH, i.e. a methyl group attached to a carboxyl group.
(ii) Butanone: CH₃–CO–CH₂–CH₃, a four-carbon chain with the carbonyl group on carbon 2.
(iii) Pentane: CH₃–CH₂–CH₂–CH₂–CH₃.
(iv) Hexanal: CH₃–CH₂–CH₂–CH₂–CH₂–CHO, a six-carbon chain whose first carbon is the aldehyde group.

Teacher's Note:
1. The name ending tells the functional group: -oic acid, -one and -al.
2. The prefix gives the number of carbons: but-, pent-, hex-.

Answer: f = +20 cm, v = +40 cm. From 1/v − 1/u = 1/f: 1/u = 1/40 − 1/20 = −1/40, so u = −40 cm. Magnification m = v/u = 40/(−40) = −1. Image height = m × 2 cm = −2 cm. The object is 40 cm (at 2F) from the lens, and the image is real, inverted and of the same size (2 cm).

Teacher's Note:
1. An object at 2F gives a real, inverted image of the same size at 2F on the other side.
2. Use the sign convention with u negative for the object.

Answer: Yes, the lens will still form a complete image. Every part of the lens refracts rays from every point of the object, so the uncovered half is enough to form the full image, but the image is dimmer because fewer rays pass through.

Teacher's Note:
1. Image completeness does not depend on the size of the aperture.
2. The covered part only reduces the brightness.

Answer: Acquired traits are changes in the body (somatic) cells caused by the environment or by use and disuse. They do not alter the DNA in the germ cells, and only DNA in the germ cells is passed to the offspring, so acquired traits are not inherited.

Teacher's Note:
1. Give an example like a weight loss due to starvation, which does not affect the offspring.
2. The DNA of germ cells remains unchanged.

Answer: Bring each mirror close to the face and look at the image. The plane mirror gives an erect image of the same size. The concave mirror gives an erect and enlarged image. The convex mirror gives an erect and diminished image.

Teacher's Note:
1. The size of the image seen for a nearby face separates the mirrors.
2. Only the concave mirror magnifies a close object.

Answer: f = +300 cm, u = −50 cm. 1/v = 1/f − 1/u = 1/300 + 1/50 = 7/300, so v = +300/7 ≈ +42.9 cm. m = −v/u = 42.9/50 ≈ +0.86. The image is formed about 42.9 cm behind the mirror; it is virtual, erect and diminished (magnification about 0.86).

Teacher's Note:
1. Positive v for a mirror means the image is behind it.
2. A positive m less than 1 shows an erect, smaller image.

Answer: A small piece of tissue (explant) is taken from the parent plant and placed on a sterile nutrient medium containing growth hormones. It grows into a mass of undifferentiated cells called callus. The callus is transferred to a medium with suitable hormones to form roots and shoots, and the tiny plantlets are shifted to soil.
Advantages: many identical plants are produced quickly from a small tissue, disease-free plants are obtained, and it helps to grow plants that do not produce seeds easily.

Teacher's Note:
1. Callus is the key stage between the explant and the plantlet.
2. All the plants obtained are clones of the parent.

Answer: (a) 2CH₃COOH + 2Na → 2CH₃COONa + H₂↑
(b) 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑
(c) CH₃COOH + C₂H₅OH → (conc. H₂SO₄, heat) CH₃COOC₂H₅ + H₂O

Teacher's Note:
1. Reaction (c) is esterification and gives a sweet-smelling ester.
2. Balance the sodium and carbon atoms in (a) and (b).

Answer: (a) Sexually transmitted diseases are infections that spread from an infected person to a healthy person through sexual contact.
(b) Gonorrhoea is caused by a bacterium (Neisseria gonorrhoeae) and herpes by a virus (herpes simplex virus).
(c) They can be prevented by using condoms, avoiding multiple partners, using sterilised needles and getting early treatment.

Teacher's Note:
1. Some STDs are bacterial and some are viral.
2. Condoms give protection against many of them.

Answer: The lens must form a virtual image at 150 cm of an object kept at 25 cm: u = −25 cm, v = −150 cm. 1/f = 1/v − 1/u = −1/150 + 1/25 = 5/150, so f = +30 cm. A convex lens of focal length 30 cm (power about +3.3 D) is needed. The defect is hypermetropia (far-sightedness).

Teacher's Note:
1. For near-point correction the image is formed at the defective near point.
2. A convex lens is always used for hypermetropia.

Answer: Presbyopia is the difficulty in seeing nearby objects clearly that occurs with old age. It is caused by weakening of the ciliary muscles and loss of flexibility of the eye lens, so the near point recedes. It is corrected by using bifocal lenses having a concave upper part for distant vision and a convex lower part for reading.

Teacher's Note:
1. This defect develops with age and may occur along with myopia or hypermetropia.
2. Bifocal lenses fix both.

Answer: Burning ethyne with pure oxygen produces a very hot flame, about 3000 °C, which is hot enough to melt and join metals. Air contains about 80 percent nitrogen that absorbs heat and prevents complete combustion, so the flame would be cooler and sooty.

Teacher's Note:
1. Pure oxygen allows complete combustion of ethyne.
2. The nitrogen in air lowers the flame temperature.

Answer: In binary fission the parent cell divides into two nearly equal daughter cells: the nucleus divides first, followed by the cytoplasm, as in Amoeba. Other methods of asexual reproduction are multiple fission (Plasmodium), budding (Hydra, yeast), fragmentation (Spirogyra), regeneration (Planaria), spore formation (Rhizopus) and vegetative propagation in plants.

Teacher's Note:
1. Binary fission is common in unicellular organisms.
2. Cite one example with each method.

Answer: Vegetative reproduction is the formation of new plants from vegetative parts such as roots, stems or leaves, e.g. by cutting or grafting, or through potato eyes. It takes place in natural conditions or simple field methods. Tissue culture uses a small piece of tissue grown in sterile laboratory conditions on an artificial medium with hormones, and can yield many plants in a short time.

Teacher's Note:
1. Both give offspring identical to the parent.
2. Tissue culture needs controlled laboratory conditions, whereas vegetative propagation does not.

Answer: He is suffering from myopia (short-sightedness): the image of a distant object forms in front of the retina, because the eyeball is too long or the lens is too curved. It is corrected with a concave (diverging) lens of suitable focal length, which makes the rays diverge so the image forms on the retina.

Teacher's Note:
1. A myopic eye sees near objects clearly but not distant ones.
2. Concave lenses reduce the converging power of the eye.

Answer: Women have two X chromosomes (XX) and men have one X and one Y (XY). All eggs carry an X chromosome, while half of the sperms carry X and half carry Y. Fertilisation by an X sperm gives XX (girl) and by a Y sperm gives XY (boy). The two events are equally likely, so the chance is 50:50.

Teacher's Note:
1. The sex of the child is decided by the sperm, not by the egg.
2. Equal numbers of X and Y sperms give equal chances.

Answer: Covalent compounds have no free ions or free electrons, since atoms share electrons, so they cannot conduct electricity. Carbon dioxide is evolved: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂.

Teacher's Note:
1. Conduction needs mobile charged particles.
2. Carbon dioxide turns lime water milky.

Answer: (a) Air molecules scatter shorter wavelengths (blue) more strongly than red.
(b) Atmospheric refraction bends the light from the Sun over the horizon before it actually rises.
(c) Red light has the longest wavelength, so it is scattered the least and can be seen from a distance.
(d) Starlight is refracted repeatedly by the atmosphere with changing density, so the apparent position and brightness of the star keep changing.
(e) Planets are much closer and act as extended sources, so the variations from many point sources average out and they do not twinkle.

Teacher's Note:
1. Scattering and atmospheric refraction explain all five.
2. Stars are point sources, while planets are extended sources.

Answer: (a) Catenation, the ability of carbon to bond with other carbon atoms to form long chains, branches and rings; and tetravalency, which lets carbon form four strong covalent bonds with many elements.
(b) Saturated hydrocarbons have only single bonds between carbon atoms (ethane, C₂H₆); unsaturated ones contain at least one double or triple bond (ethene, C₂H₄).
(c) Silicon forms such chains, but its compounds are far less stable and reactive because Si–Si bonds are weaker than C–C bonds.

Teacher's Note:
1. Catenation and tetravalency together explain the large number of carbon compounds.
2. Silicon chains break more easily than carbon chains.

Answer: (a) Isomers are compounds having the same molecular formula but different structures. For example, butane C₄H₁₀ exists as straight-chain n-butane (CH₃CH₂CH₂CH₃) and branched isobutane (CH₃CH(CH₃)CH₃).
(b) (i) Alkenes. (ii) Alkynes.

Teacher's Note:
1. Isomerism begins at four carbon atoms in alkanes.
2. Alkenes end in -ene and alkynes in -yne.

Answer: Pollination is the transfer of pollen from the anther to the stigma; fertilisation is the fusion of the male and female gametes to form a zygote. Pollination can occur without fertilisation when the pollen fails to germinate or grow a pollen tube. Fertilisation needs pollination because the male gametes reach the ovule only through the pollen tube that grows from a pollen grain on the stigma.

Teacher's Note:
1. Pollination is the first step and fertilisation is the final result.
2. The pollen tube is the link between them.