CBSE EXAMINATION PAPER-2023

Answer: (i) When dilute hydrochloric acid is added to copper oxide, copper(II) chloride is formed. The colour of this compound in solution is blue-green.
(ii) The balanced chemical equation for the reaction is:
CuO + 2HCl → CuCl₂ + H₂O
Here, copper oxide (CuO) reacts with hydrochloric acid (HCl) to form copper(II) chloride (CuCl₂) and water (H₂O). The blue-green colour indicates the presence of CuCl₂ in the solution.

Answer: In the chlor-alkali process, an aqueous solution of sodium chloride (brine) is electrolyzed. Here, compound X is sodium chloride solution. Gas Y, hydrogen, is liberated at the cathode. Gas Z, chlorine, is liberated at the anode. When chlorine gas is treated with dry slaked lime, it forms bleaching powder, which is compound B. Thus, X is sodium chloride solution, Y is hydrogen gas, Z is chlorine gas, and B is bleaching powder.

Answer:

(i) The cerebellum is responsible for maintaining posture and balance. It controls precise voluntary movements and helps in coordination.

(ii) The medulla, part of the hind-brain, controls the beating of the heart by regulating heartbeat automatically.

(iii) Thinking is a function of the cerebrum, the largest part of the brain involved in thinking, reasoning, memory, and voluntary actions.

(iv) Blood pressure is regulated by the medulla, which controls various involuntary functions including blood pressure.

Answer: Auxins are primarily synthesized in the tips of the shoots and young leaves of a plant. (i) Positive phototropism is shown by the shoot of the plant, as it grows towards the light source. (ii) Negative geotropism is also shown by the shoot, as it grows upwards, away from the direction of gravity. (iii) Positive hydrotropism is exhibited by roots, which grow towards moisture or water sources.

Answer:

(i) Renal Artery: It carries oxygen-rich blood containing waste products to the kidneys for filtration.

(ii) Urethra: It is the tube through which urine is expelled out of the body from the urinary bladder.

(iii) Glomerulus: It filters blood in the kidneys, allowing waste and excess substances to pass into the nephron.

(iv) Tubular part of nephron: It reabsorbs useful substances like glucose, salts, and water back into the blood and allows waste to remain to form urine.

Answer: The plant kept in the dark could not survive longer because it was unable to perform photosynthesis. Photosynthesis requires sunlight to produce food for the plant. Without sunlight, the plant cannot make food and hence it will eventually die. The plant in sunlight can carry out photosynthesis, producing food and oxygen, which supports its survival.

Answer: The excessive use of pesticides leads to their accumulation in rivers and ponds, causing water pollution. These chemicals harm aquatic life by poisoning fish and other organisms. Pesticides can also enter the food chain through such water bodies, resulting in biological magnification, which means the concentration of these toxic substances increases as they move up the food chain. This makes water unsafe for drinking and affects human health. Moreover, pesticides damage the soil’s natural organisms, reducing fertility. Therefore, their excessive use is harmful to the environment and living beings.

Answer:

(i) The defect of vision shown is Myopia or Near-sightedness.

(ii) Its two causes are: (1) The focal length of the eye lens is too short, and (2) The eyeball is too long.

(iii) The type of lens used for the correction of this defect is a concave lens.

Answer: The sky appears blue from Earth because the atmosphere contains air molecules and tiny particles that scatter sunlight. These particles scatter blue light, which has a shorter wavelength, more effectively than other colours. This scattered blue light reaches our eyes, making the sky look blue. From space, there is no atmosphere to scatter sunlight, so the sky appears black against the backdrop of stars and space.

Answer:

(i) Before electrolysis of water, a few drops of acid (like dilute sulfuric acid) are added to increase the water's conductivity because pure water is a poor conductor of electricity. During electrolysis, hydrogen gas is liberated at the cathode (negative electrode) due to the reduction of water molecules, and oxygen gas is liberated at the anode (positive electrode) due to the oxidation of water molecules. The volume of hydrogen gas collected at the cathode is twice the volume of oxygen gas collected at the anode. This is because water (H2O) decomposes into H2 and O2 in a 2:1 molar ratio.

(ii) When silver chloride (AgCl) is exposed to sunlight, it decomposes into silver metal (Ag) and chlorine gas (Cl2). The silver chloride turns grey due to the formation of silver particles. This type of reaction is called a photochemical reaction or photodecomposition reaction because it occurs in the presence of light.

Answer:

(i) Safe procedure for diluting a strong concentrated acid: Always add acid to water, not water to acid. This is because concentrated acid is highly exothermic when mixed with water and adding water to acid may cause splashing of the acid. First, take some water in a beaker and then slowly add the acid to it while stirring continuously to dissipate the heat. This ensures safety during dilution.

(ii) Salt formed and its pH: When sulphuric acid (H₂SO₄) is added to sodium hydroxide (NaOH), the salt formed is sodium sulphate (Na₂SO₄) and water. The reaction is a neutralization reaction. The pH of the resulting solution is neutral, approximately 7.

(iii) Why dry HCl gas does not change the colour of dry blue litmus paper: Dry HCl gas does not change the colour of dry blue litmus paper because the acidic character of HCl is due to the presence of hydronium ions (H₃O⁺) formed when HCl dissolves in water. Since dry HCl gas lacks moisture, it cannot produce these ions and hence does not turn blue litmus paper red. However, wet blue litmus paper changes colour because water is present to ionize HCl into H₃O⁺ ions.

Answer: (i) How does Paramecium obtain its food?
Paramecium obtains its food by using its cilia, which are tiny hair-like structures covering its surface. The cilia beat rhythmically to move the paramecium through water and also help sweep food particles into a mouth-like opening called the oral groove. The food particles are then enclosed in food vacuoles where digestion takes place.

(ii) Roles of the following in our digestive system:
(a) Hydrochloric acid: It is present in the stomach and helps to create an acidic environment, which activates the enzyme pepsin to digest proteins and also kills harmful bacteria in food.
(b) Trypsin: This is an enzyme produced by the pancreas that breaks down proteins into smaller peptides in the small intestine.
(c) Muscular walls of stomach: They help in churning and mixing the food with digestive juices, turning it into a semi-liquid substance called chyme, aiding digestion.
(d) Salivary amylase: This enzyme is present in saliva and begins the digestion of carbohydrates by breaking down starch into simpler sugars in the mouth.

Answer:

(i) Double circulation means that the blood passes through the heart twice during one complete cycle in the body. In double circulation, blood first goes from the heart to the lungs to get oxygenated (pulmonary circulation) and then returns to the heart. From there, the heart pumps the oxygen-rich blood to the rest of the body (systemic circulation). This system is different from fish, where blood passes through the heart only once per cycle.

(ii) The separation of the right and left sides of the heart is useful because it prevents the mixing of oxygen-poor blood (deoxygenated) from the body and oxygen-rich blood from the lungs. The partition called the septum divides the heart into two halves, ensuring clean and separate blood flows.

This separation is especially important for birds and mammals as they have high energy requirements to maintain their constant body temperature. By keeping oxygen-rich and oxygen-poor blood separate, the heart efficiently supplies oxygen and nutrients to the body cells, supporting their active lifestyles and energy needs.

Answer: Given, object height (h) = 10 cm, object distance (u) = -25 cm (since the object is in front of the lens), and focal length (f) = +15 cm (for a converging lens). Using the lens formula: 1/f = 1/v - 1/u, we get 1/v = 1/f + 1/u = 1/15 + 1/(-25) = (5 - 3)/75 = 2/75, so v = 75/2 = 37.5 cm. The positive value of v indicates that the image is formed on the opposite side of the lens. To find the height of the image (h'), use magnification m = v/u = 37.5 / -25 = -1.5. Therefore, h' = m × h = -1.5 × 10 = -15 cm. The negative sign shows the image is inverted. Hence, the image is real, inverted, magnified, formed at 37.5 cm from the lens, and its height is 15 cm.

Answer: Power (P) of the lens is +4D. The focal length (f) in meters is given by the formula f = 1/P. So, f = 1/4 = 0.25 meters or 25 cm. Since the power is positive, the lens is convex. Given, object distance (u) = -50 cm (object distance is taken negative as per sign convention, assuming light travels from left to right). Using lens formula, 1/f = 1/v - 1/u, we get 1/v = 1/f + 1/u = 1/25 + 1/(-50) = (2 - 1)/50 = 1/50. So, image distance v = +50 cm. The positive image distance indicates that the image is real and formed on the opposite side of the lens. Magnification (m) = v/u = +50 / (-50) = -1. The negative magnification means the image is inverted and magnification value 1 means the image is of the same size as the object. Thus, the image is real, inverted and of the same size, formed at 50 cm on the opposite side of the lens. A ray diagram for a convex lens shows rays from the top of the object converging at the image point on the opposite side, confirming this result.

Answer:

(i) Alternating current (A.C.) is preferred for long-distance transmission of electric power because it can be easily transformed to higher or lower voltages using transformers. High voltage transmission reduces the current flowing through the wires, which in turn reduces energy loss due to the resistance of the wires. Direct current (D.C.) cannot be easily transformed, so transmitting electricity over long distances using D.C. causes greater energy loss.

(ii) The current supplied to households is alternating current (A.C.), which periodically reverses its direction. In contrast, the current given by a battery of dry cells is direct current (D.C.), which flows in only one direction. The A.C. supply in homes typically has a constant frequency (e.g., 50 Hz) and voltage, unlike the steady, unidirectional flow from a battery.

(iii) An electric fuse protects the circuit and appliances from damage caused by short-circuiting or overloading by breaking the circuit when the current exceeds a safe limit. It contains a thin wire that melts when excess current flows, stopping the flow of electricity and preventing potential damage or fire hazards.

Answer: The magnetic field lines around a current-carrying solenoid are similar to those of a bar magnet. Inside the solenoid, the magnetic field lines are nearly parallel and very close to each other, indicating a strong and uniform magnetic field. Outside the solenoid, the field lines spread out and loop from one end of the solenoid to the other, showing a weaker magnetic field. Among the three points A, B, and C, point C is inside the solenoid where the field lines are dense and parallel, so the magnetic field strength is maximum there. Points A and B are near the ends of the solenoid where magnetic field lines start and return, so the field strength is less compared to inside. Hence, the magnetic field strength is minimum at points A and B, and maximum at point C inside the solenoid.

Answer: Biodegradable wastes are substances that can be broken down by natural processes and microorganisms, whereas non-biodegradable wastes cannot be decomposed easily and persist in the environment for a long time. If biodegradable waste is not disposed properly, it promotes the growth of harmful bacteria and creates foul odor, leading to unhygienic conditions. On the other hand, accumulated non-biodegradable waste can harm the ecosystem by polluting soil and water and can take hundreds of years to degrade, causing long-lasting environmental damage.

Answer: (i) (a) Structure of Butanoic acid (C₃H₇COOH): CH₃-CH₂-CH₂-COOH. It consists of a four carbon chain with a carboxyl group (-COOH) attached at the end.
(b) Structure of Chloropentane: CH₃-CHCl-CH₂-CH₂-CH₃. It is a five carbon chain (pentane) where a chlorine atom (Cl) replaces one hydrogen atom, usually shown on the second carbon.
(ii) The two structures are isomers of each other. They have the same molecular formula but different structural arrangements. Butanoic acid has a carboxyl group (-COOH) making it a carboxylic acid, whereas chloropentane has a chlorine atom substituted in the hydrocarbon chain. This difference in functional groups changes their properties.
(iii) Difference between saturated and unsaturated carbon compounds based on general formula: Saturated compounds contain only single bonds between carbon atoms and have the general formula C_nH_2n+2. Unsaturated compounds contain one or more double or triple bonds and have fewer hydrogen atoms than saturated compounds; their general formula is C_nH_2n (for alkenes) or C_nH_2n-2 (for alkynes). Saturated compounds are usually less reactive, whereas unsaturated compounds are more reactive due to the presence of multiple bonds.

Answer: (a) When a small piece of sodium is dropped in ethanol, it reacts vigorously to form sodium ethoxide and hydrogen gas is released. This is a typical reaction of alcohols with active metals like sodium.
Reaction: 2Na + 2C₂H₅OH → 2C₂H₅ONa + H₂

(b) Glacial acetic acid is called "glacial" because it solidifies just below room temperature at about 16.7 deg C, forming ice-like crystals. It is a pure and water-free form of acetic acid.

(c) When ethanol is heated at 443 K in the presence of concentrated sulfuric acid, it undergoes dehydration to form ethene (an alkene) and water. Concentrated H₂SO₄ acts as a dehydrating agent and catalyst here.
Reaction: C₂H₅OH → C₂H₄ + H₂O (with hot conc. H₂SO₄)

(d) Saponification is the hydrolysis of an ester in the presence of a base to form alcohol and soap (salt of fatty acid).
Example reaction:
CH₃COOC₃H₇ + NaOH → CH₃COONa + C₃H₇OH
(Propyl acetate + sodium hydroxide → sodium acetate + propanol)

Answer: (i) The two modes of asexual reproduction observed in hydra are budding and regeneration.
Budding: In budding, a new organism develops as an outgrowth or bud from the parent hydra. This bud grows while attached to the parent and after maturing, it detaches to live independently as a new hydra. It is a common method of reproduction in hydra.
Regeneration: Although not a mode of reproduction in the true sense, hydra can regenerate its lost parts. If a hydra is cut into pieces, each piece can grow into a new individual hydra.

(ii) Vegetative propagation is a type of asexual reproduction in plants where new plants are produced from the vegetative parts like roots, stems, or leaves without the formation of seeds.
Advantages of vegetative propagation:
1. It produces plants faster compared to growing from seeds.
2. It helps in producing plants that are genetically identical to the parent, preserving desirable traits.

Answer:

(i) Electric current (I) through a conductor is directly proportional to the potential difference (V) across its terminals, provided the temperature and other physical conditions remain constant. This relationship is known as Ohm's Law and can be written as V = I × R, where R is the resistance of the conductor. To verify this, a simple circuit is set up with a battery, a resistor, an ammeter connected in series to measure the current, and a voltmeter connected in parallel to measure the voltage across the resistor.

The circuit diagram includes:

• A battery providing the potential difference.
• A resistor connected in series.
• An ammeter connected in series to measure current.
• A voltmeter connected in parallel across the resistor to measure voltage.
• Connecting wires with negligible resistance.

By varying the voltage and measuring current, the proportionality is confirmed as a straight line when V is plotted against I.

(ii) An ammeter should have very low resistance because it is connected in series in a circuit. If it had high resistance, it would reduce the overall current flowing through the circuit, thus affecting the circuit's normal operation and giving incorrect current readings. Low resistance ensures that the ammeter does not alter the current it is intended to measure.

(iii) In the V-I graphs given for series and parallel combinations of resistors:

• (a) Series Combination: In series, the equivalent resistance is higher, so for the same voltage, the current is less making the slope of the V-I graph steeper. The graph with a steeper slope (lower current for same voltage) corresponds to the series combination.
• (b) Parallel Combination: In parallel, the equivalent resistance is lower, so for the same voltage, the current is higher making the slope of the V-I graph less steep. The graph with a gentler slope (higher current for the same voltage) corresponds to the parallel combination.