CBSE Class 10 Science Light, Chemistry, Life Processes and Electricity Assignment Set 07

Answer: The change in the speed of light when it goes from one medium to another.

Teacher's Note:
1. The passage states this in its first sentence.
2. Copy the idea in your own words.

Answer: Its speed increases, so the ray bends at the glass-air interface (away from the normal).

Teacher's Note:
1. The passage says the speed increases and refraction occurs.
2. Adding 'away from the normal' shows understanding.

Answer: It depends on the difference between the speeds of light in the two media; a larger difference gives a larger angle of bending.

Teacher's Note:
1. The passage links angle and speed difference directly.
2. Quote both ideas.

Answer: The speed of light drops from 3 × 10⁸ m/s to 2 × 10⁸ m/s in glass, but only to 2.25 × 10⁸ m/s in water. The speed difference is larger for glass, so the bending is greater.

Teacher's Note:
1. Compare the two speed differences using the values given.
2. Glass has the higher refractive index.

Answer: (c) Real. Rays actually converge to form a real image, which can be caught on a screen.

Teacher's Note:
1. A screen can show only an image formed by actually meeting rays.
2. Virtual images cannot be caught on a screen.

Answer: (a) Red. Red has the longest wavelength and the smallest deviation.

Teacher's Note:
1. Deviation depends on wavelength.
2. The colour with the longest wavelength bends least.

Answer: (d) 25 W. R = V²/P = 220²/100 = 484 Ω, so P = 110²/484 = 25 W.

Teacher's Note:
1. Resistance stays the same, so power varies with the square of voltage.
2. Halving the voltage cuts the power to one-fourth.

Answer: (d) VR. Power can be written as I²R, V²/R or VI, but VR is not a power expression.

Teacher's Note:
1. Power equals VI, I²R and V²/R.
2. VR has no such meaning.

Answer: (d) Weak acid and strong base. It is formed from NaOH (strong base) and carbonic acid (weak acid), so it hydrolyses to give a basic solution.

Teacher's Note:
1. The nature of a salt depends on the acid and base that form it.
2. Sodium carbonate comes from NaOH and H₂CO₃.

Answer: Silver reacts with hydrogen sulphide present in air to form a black coating of silver sulphide. This is corrosion of silver.

Teacher's Note:
1. Tarnishing of silver is a type of corrosion.
2. The black substance is silver sulphide.

Answer: Tomato (also spinach and wood sorrel) contains oxalic acid.

Teacher's Note:
1. Many plant foods contain organic acids.
2. Tomatoes are the textbook example for oxalic acid.

Answer: Anodising is the process of forming a thick protective layer of aluminium oxide on aluminium by making it the anode in electrolysis of dilute sulphuric acid. The thick oxide layer resists corrosion and can be dyed.

Teacher's Note:
1. Aluminium already has a thin oxide coat.
2. Anodising makes it thicker.

Answer: (a) (i). CaO + H₂O → Ca(OH)₂ + heat; the heat release shows the reaction is exothermic.

Teacher's Note:
1. The release of heat is exactly what exothermic means.
2. So R explains A.

Answer: They come from burning fossil fuels in vehicles, power plants and industries, and from some industrial processes.

Teacher's Note:
1. These gases form when fuels containing sulphur and nitrogen burn.
2. They dissolve in rain to form acids.

Answer: (a) Less than 2.0 g. Acetic acid reacts with calcium carbonate, releasing carbon dioxide and dissolving part of the chip.

Teacher's Note:
1. Marble is calcium carbonate and vinegar contains acetic acid.
2. The acid reacts with it and some marble dissolves.

Answer: The cerebellum.

Teacher's Note:
1. It is part of the hind-brain.
2. It also coordinates precise voluntary movements.

Answer: (a) Scattering is the spreading of light in different directions when it strikes fine particles in the atmosphere. The colour of scattered light depends on the size of the scattering particles and on the wavelength of light: very fine particles scatter blue light, while larger particles scatter longer wavelengths and look whitish.
(b) The blue colour of the sky and the red colour of the sun at sunrise and sunset (also the Tyndall effect).

Teacher's Note:
1. Remember that shorter wavelengths scatter more strongly.
2. Link particle size to colour.

Answer: u = −24 cm, f = +30 cm. 1/v = 1/f − 1/u = 1/30 + 1/24 = 9/120, so v = +13.3 cm, behind the mirror. m = −v/u = 13.3/24 ≈ +0.56, so the image is erect, virtual and about 2.5 cm tall.

Teacher's Note:
1. Substitute with proper signs in the mirror formula.
2. A positive magnification below 1 means an erect, diminished image.

Answer: (a) For light passing from one medium to another, the ratio sin i / sin r is constant for a given pair of media (and colour of light).
(b) The speed of light in diamond is 1/2.42 of its speed in vacuum, that is, light travels 2.42 times slower in diamond than in vacuum.

Teacher's Note:
1. Refractive index is the ratio of the speed of light in vacuum to that in the medium.
2. Diamond has one of the highest refractive indices among common materials.

Answer: Electrolytic reduction is the reduction of a metal ion to the metal at the cathode during electrolysis of its molten compound; it is used for highly reactive metals. On electrolysing molten NaCl, sodium ions gain electrons at the cathode (Na⁺ + e⁻ → Na) and chlorine is released at the anode.

Teacher's Note:
1. Highly reactive metals cannot be reduced by carbon.
2. Name the cathode and the anode product.

Answer: (a) Excess acid in the stomach causes pain; an antacid is a mild base that neutralises it.
(b) Distilled water has no ions, but rain water contains dissolved acids such as carbonic acid that give ions.
(c) Sodium hydroxide reacts with aluminium to form sodium aluminate and hydrogen gas, so the container is corroded.

Teacher's Note:
1. Each part needs one clear reason.
2. Use the words neutralisation, ions and amphoteric reaction.

Answer: Transpiration is the loss of water as vapour from the aerial parts of a plant, mainly through the stomata of leaves. It creates a suction pull that helps in the upward movement of water and minerals, and it helps in cooling the plant.

Teacher's Note:
1. Transpiration pull works in tall trees where root pressure is not enough.
2. Cooling is the second function.

Answer: The carpel is the female reproductive part of a flower, also called the pistil. The stigma receives the pollen, the style is the tube through which the pollen tube grows, and the ovary contains ovules, each with an egg cell. After fertilisation the ovary becomes the fruit and the ovules become seeds.

Teacher's Note:
1. The carpel has three parts: stigma, style and ovary.
2. Link each part to its function.

Answer: f = −20 cm, v = −15 cm. From 1/v − 1/u = 1/f: 1/u = 1/v − 1/f = −1/15 + 1/20 = −1/60, so u = −60 cm. Magnification m = v/u = (−15)/(−60) = 0.25, so image height = 0.25 × 6 = 1.5 cm (virtual, erect).

Teacher's Note:
1. A concave lens always gives a virtual image on the same side as the object.
2. Check that the image is nearer than the object.

Answer: (a) Gypsum is CaSO₄·2H₂O and plaster of Paris is CaSO₄·½H₂O. Heating gypsum at 373 K gives plaster of Paris, and adding water to plaster of Paris gives gypsum again. It is used for plaster for fractured bones and for making toys and smooth surfaces.
(b) Dissolving an acid or base in water is highly exothermic, so the beaker feels warm. Adding acid to water spreads the heat safely, while adding water to concentrated acid can cause spurting.

Teacher's Note:
1. Write the formulae with the water of crystallisation.
2. Include one safety reason in part (b).

Answer: (a) Manganese dioxide is heated with aluminium powder in a displacement reaction: 3MnO₂ + 4Al → 3Mn + 2Al₂O₃ + heat.
(b) Sodium atom transfers one electron to chlorine to form Na⁺ and Cl⁻, which attract each other to give NaCl. Ionic compounds are solids with high melting and boiling points, are soluble in water, and conduct electricity in molten state or in solution.

Teacher's Note:
1. Reactive aluminium reduces the oxide.
2. Ionic bonds form by transfer of electrons.