CBSE Class 10 Science Sample Paper 1: marking scheme and solutions

Answer: (b) The eye lens forms an inverted, real image of the object on the retina.

Answer: (b) A rheostat changes the resistance in a circuit without changing the voltage source, so it changes the current through the lamp.

Answer: (c) In yeast pyruvate gives ethanol and carbon dioxide; in mitochondria it gives carbon dioxide and water. Only carbon dioxide is common. Lactic acid forms in muscle.

Answer: (a) Drawing a metal into thin wires uses ductility. Malleability is beating into sheets; sonorousness and lustre are sound and shine.

Answer: (d) Baking soda is sodium hydrogencarbonate, NaHCO₃. Sodium carbonate is formed from it on heating, and sodium chloride is one of its raw materials.

Answer: (b) Cyclohexane is C₆H₁₂ with single bonds only; benzene, C₆H₆, is a cyclic unsaturated hydrocarbon.

Answer: (a) Both are true. Because red is scattered least by fog or smoke, it reaches a distant observer in the same colour, which is exactly why danger signals are red.

Answer: (c) Conventional current is taken opposite to the flow of electrons, so in the external circuit it is shown from the positive terminal to the negative terminal.

Answer: (b) Both statements are true. R explains why the filament survives the heat without melting; it does not explain why most of the power becomes heat.

Answer: (d) A convex mirror always forms an erect, diminished image, so a small one can show a full-length image of a large object such as a tall building; a plane or concave mirror of the same size cannot.

Answer: (b) Both are true. Hibiscus is bisexual because it contains both stamens and pistil; that the stamen makes pollen does not explain why the flower has both parts.

Answer: (a) Paddy is the producer, the rat the primary consumer, the snake the secondary consumer and the eagle, the larger carnivore at the fourth level, the tertiary consumer.

Answer: (c) Tissue culture lets many plants be grown from one parent in disease-free conditions, and it is commonly used for ornamental plants.

Answer: (a) Milk of magnesia is magnesium hydroxide, Mg(OH)₂, a mild base that neutralises excess stomach acid.

Answer: (d) Information picked up at the dendritic tip creates an impulse that travels from the dendrite to the cell body and then along the axon to its end.

Answer: (b) Both are true. Baking soda works as an antacid because it is a mild base that neutralises excess acid; the raw material used to make it does not explain that.

Answer: (d) Oxygen atoms on the right: 3 × 2 + 4 × 1 = 10, so 10/2 = 5 molecules of O₂. Option (b) gives the number of oxygen atoms, not O₂ molecules.

Answer: (d) The end that points towards the north is the north-seeking or north pole; the other end is the south pole.

Answer: (a) Buds produced in the notches along the leaf margin of Bryophyllum fall on the soil and develop into new plants.

Answer: (b) Every child gets an X from the mother, boy or girl. A boy is XY, so the deciding chromosome is the Y he received from his father.

The ester is converted back to the alcohol and the sodium salt of the acid, neither of which has the ester's sweet smell: CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa. This is saponification.

• equation: CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa, the ester being used up so the smell goes (1)
• the reaction is saponification (1)

An ester heated with sodium hydroxide gives the alcohol and the sodium salt of the carboxylic acid: CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa. This saponification reaction is used to prepare soap.

• products: alcohol and sodium salt of the acid, with the balanced equation for ethyl ethanoate (1)
• saponification, used to prepare soap (1)

(i) 10: each germ-cell takes one chromosome from each of the 10 pairs. (ii) 20: the zygote forms by fusion of two germ-cells of 10 each, which restores the normal number.

• germ-cell 10, one from each pair (1)
• zygote 20, fusion restores the number (1)

Any two of: (i) a ray parallel to the principal axis, which after reflection appears to diverge from the principal focus behind the mirror; (ii) a ray directed towards the centre of curvature, which is reflected back along the same path; (iii) a ray directed towards the principal focus, which emerges parallel to the principal axis. The image lies where the reflected rays, produced backwards, meet.

• first ray and its path after reflection (1)
• second ray and its path after reflection (1)

(a) The table fan and the toaster: they have metallic bodies, and earthing matters especially for appliances with a metallic body. (b) The green earth wire is connected to a metal plate buried deep in the earth near the house.

• table fan and toaster, because their bodies are metallic (1)
• a metal plate buried deep in the earth near the house (1)

(a) A small amount of carbon, about 0.05%, which turns it into hard, strong steel. (b) Stainless steel, made by mixing iron with nickel and chromium; it is hard and does not rust.

• carbon, about 0.05% (1)
• stainless steel with nickel and chromium (1)

A sphygmomanometer. Hypertension is caused by the constriction of arterioles, which increases resistance to blood flow.

• sphygmomanometer (1)
• constriction of arterioles increasing resistance to blood flow (1)

P: the cerebellum of the hind-brain, which gives precision of voluntary actions and maintains posture and balance. Q: the medulla of the hind-brain, which controls involuntary actions such as salivation and vomiting. R: the fore-brain, which has areas specialised for smell.

• P: cerebellum, for balance and precision of voluntary actions (1)
• Q: medulla, for involuntary actions such as salivation and vomiting (1)
• R: fore-brain, with an area specialised for smell (1)

L: the cerebellum, responsible for the precision of voluntary actions and for balance. M: the medulla, which controls blood pressure among involuntary actions. N: the fore-brain, which has areas specialised for hearing.

• L: cerebellum, precision of voluntary actions and balance (1)
• M: medulla, controls blood pressure (1)
• N: fore-brain, area specialised for hearing (1)

(i) R(A) = 3.0/0.25 = 12 Ω, R(B) = 6.0/1.5 = 4 Ω. Each graph is a straight line through the origin, of slope V/I = R; A's line is steeper. (ii) Same material and length, so R is inversely proportional to the area. B has a third of A's resistance, so B is thicker, with three times the area of cross-section of A. (iii) I = V/R: A 9/12 = 0.75 A, B 9/4 = 2.25 A.

• straight lines through origin; A steeper (R = 12 Ω against 4 Ω) (1)
• B thicker, three times the area of A (1)
• 0.75 A in A, 2.25 A in B (1)

(a) Rancidity, caused by oxidation of the fats and oils in the food. (b) Any two: adding antioxidants; packing in airtight containers; flushing the packet with an unreactive gas such as nitrogen.

• rancidity (1)
• oxidation of fats and oils (1)
• two methods of prevention (1)

In a fish the heart pumps blood to the gills, where it is oxygenated, and the blood passes directly to the rest of the body, so the heart only receives blood returning from the body. Blood passes through a fish's heart once in each cycle and through a human heart twice (double circulation).

• heart pumps to the gills; oxygenated there (1)
• blood goes straight to the body, so the heart handles only blood from the body (1)
• fish once per cycle; human twice (double circulation) (1)

f = 1/P = 1/3.2 m = 0.3125 m = +31.25 cm. The lens forms the image of the cloth at 25 cm at his near point: u = −25 cm, 1/v = 1/f + 1/u = 1/31.25 − 1/25 = 8/250 − 10/250 = −2/250, so v = −125 cm. Without spectacles he sees clearly only from 125 cm (1.25 m). The power is positive, so the lens is convex; he has hypermetropia.

• f = 1/P = +31.25 cm (1)
• uses u = −25 cm in the lens formula to get v = −125 cm, near point 1.25 m (1)
• convex lens, because the power is positive (1)

(a) Ethane H₃C–CH₃ (single bond), ethene H₂C=CH₂ (double bond), ethyne HC≡CH (triple bond). (b) Ethane: it is saturated, and saturated hydrocarbons generally give a clean flame, while unsaturated ones give a yellow, sooty flame. (c) HC≡CH + 2H₂ → H₃C–CH₃, ethane.

• (a) the three structures with single, double and triple carbon-carbon bonds (1)
• (b) ethane, because it is saturated (1)
• (c) ethane, H₃C–CH₃ (1)

(a) Chlorofluorocarbons (CFCs); also used in fire extinguishers. (b) The sharp fall in the amount of ozone in the atmosphere since the 1980s. (c) Ozone is a deadly poison to breathe; it is only at the higher levels of the atmosphere that it does its useful work of shielding the earth from the Sun's ultraviolet radiation.

• CFCs, also in fire extinguishers (1)
• fall in atmospheric ozone (1)
• ozone is a poison at ground level; its shielding is at the higher levels of the atmosphere (1)

(a) Gustatory receptors detect taste; receptors are the specialised tips of some nerve cells, located in sense organs such as the tongue. (b) The information acquired at the dendritic tip sets off a chemical reaction that creates an electrical impulse. The impulse travels from the dendrite to the cell body and then along the axon to its end. (c) Once a cell has generated and transmitted an impulse it needs time to reset before it can send another, so it cannot fire continually. The body therefore also uses chemical communication: a released compound diffuses to surrounding cells, which recognise it with special molecules on their surfaces; it is slower but reaches all cells, steadily and persistently.

• (a) gustatory receptors, the specialised tips of nerve cells in the tongue (1)
• (b) chemical reaction at the dendritic tip creates an electrical impulse (1)
• (b) impulse runs dendrite → cell body → axon to its end (1)
• (c) a cell needs time to reset after each impulse, so cannot fire continually (1)
• (c) chemical communication: compounds reach all cells, slower but persistent (1)

(a) In the inner ear; receptors are the specialised tips of some nerve cells, located in sense organs; olfactory receptors in the nose detect the smell. (b) The information acquired at the dendritic tip sets off a chemical reaction that creates an electrical impulse, which travels from the dendrite to the cell body and then along the axon to its end. (c) After generating and transmitting an impulse a cell needs time to reset, so it cannot fire continually. The body therefore also uses chemical communication: a released compound diffuses to surrounding cells, which recognise it with special surface molecules; it is slower but reaches all cells, steadily and persistently.

• (a) inner ear; specialised tips of nerve cells; olfactory receptors for the smell (1)
• (b) chemical reaction at the dendritic tip creates an electrical impulse (1)
• (b) impulse runs dendrite → cell body → axon to its end (1)
• (c) a cell needs time to reset after each impulse, so cannot fire continually (1)
• (c) chemical communication: compounds reach all cells, slower but persistent (1)

(a) Concentric circles with their centre on the rod, lying in the horizontal plane of the sheet, perpendicular to the rod. (b) Right-hand thumb rule: thumb pointing down along the current, the fingers curl clockwise as seen from above. (c) The field is smaller at 6 cm, because the field decreases as the distance from the conductor increases; the circles there are farther apart. (d) Increasing the current increases the field at the point; reversing the current reverses the direction of the field, so the lines go anticlockwise.

• concentric circles centred on the rod (1)
• in a plane perpendicular to the rod (1)
• clockwise from above, by the right-hand thumb rule (1)
• smaller at 6 cm, field falls with distance (1)
• more current, stronger field; reversed current, reversed field (1)

(a) Concentric circles in the plane of the card, which is perpendicular to the wire, with their centre on the wire. (b) Right-hand thumb rule: thumb pointing towards Sunita along the current, the fingers curl anticlockwise as she sees them. (c) The field becomes smaller, since it decreases as the distance from the conductor increases. (d) Halving the current makes the field at the point smaller; reversing the current reverses its direction, so the lines go clockwise.

• concentric circles centred on the wire (1)
• in the plane of the card, perpendicular to the wire (1)
• anticlockwise, by the right-hand thumb rule (1)
• smaller at 8 cm, field falls with distance (1)
• less current, smaller field; reversed current, reversed field (1)

(a) 2H₂O(l) → 2H₂(g) + O₂(g), with 'electricity' written over the arrow (H 4, O 2 on each side). Tube X: hydrogen is evolved at the cathode, the negative electrode, and X is joined to the negative terminal. (b) Pure water is a poor conductor; the acid provides ions so the current can pass. (c) Hydrogen : oxygen by mass is 1 : 8, so 0.5 × 8 = 4 g of oxygen. (d) Endothermic: the decomposition goes on only while electrical energy is supplied.

• 2H₂O → 2H₂ + O₂ with electricity over the arrow (1)
• tube X, hydrogen at the negative electrode (cathode) (1)
• acid provides ions for conduction (1)
• 4 g of oxygen (1)
• endothermic, energy supplied as electricity (1)

(a) Oxygen, which is evolved at the anode, the positive electrode. 2H₂O(l) → 2H₂(g) + O₂(g), with 'electricity' over the arrow (H 4, O 2 on each side). (b) Pure water is a poor conductor; without the acid there are too few ions to carry the current. (c) By mass hydrogen and oxygen form in the ratio 1 : 8; 2 : 1 is the ratio by volume. (d) Electrical energy; the reaction absorbs energy, so it is endothermic.

• oxygen over the positive electrode (1)
• 2H₂O(l) → 2H₂(g) + O₂(g) with electricity over the arrow (1)
• pure water a poor conductor; acid gives ions (1)
• mass ratio 1 : 8, not 2 : 1 (1)
• electricity; absorbs energy (endothermic) (1)