CBSE EXAMINATION PAPER-2023

Answer: (a) When aqueous solutions of potassium iodide and lead nitrate are mixed, a yellow precipitate is formed. This precipitate is lead iodide (PbI₂).
(b) The chemical equation for the reaction is:
Pb(NO₃)₂(aq) + 2KI(aq) -> PbI₂(s) + 2KNO₃(aq).
(c) Yes, this reaction is a double displacement reaction because the positive and negative ions of the two reactants exchange partners to form new products, one of which is the insoluble lead iodide precipitate.

Answer:

(i) The black substance is formed because when copper powder is heated, it reacts with oxygen in the air. This reaction forms copper oxide, which is black in color. The name of this black substance is copper(II) oxide.

(ii) The black copper oxide can be reversed to copper by passing hydrogen gas over the heated copper oxide. This causes copper oxide to get reduced back to copper, restoring its original form.

Answer:

(a) In birds and mammals, it is important to prevent mixing of oxygenated and deoxygenated blood because these animals have high energy requirements and need a highly efficient supply of oxygen to their body tissues. The complete separation of the heart into two halves (left and right) through a septum keeps oxygen-rich blood and oxygen-poor blood separate, ensuring efficient oxygen delivery for energy production and maintaining their constant body temperature.

(b) Animals like amphibians and many reptiles can tolerate some mixing of oxygenated and deoxygenated blood since they have three-chambered hearts. Their body temperature depends on the environmental temperature, meaning they do not regulate it internally like birds and mammals.

Answer: A rainbow appears in the sky when sunlight passes through raindrops, usually after a rain shower or near waterfalls. It is seen as a colorful arc opposite the Sun's position. The formation of a rainbow involves the refraction, reflection, and dispersion of sunlight inside water droplets. Sunlight bends when entering the droplet, reflects off the inner surface, and bends again while exiting. This process splits light into seven colors: red, orange, yellow, green, blue, indigo, and violet, forming a rainbow.

Answer: Scattering of light is the phenomenon where light rays are deflected in different directions when they encounter tiny particles in the air. These particles are smaller than the wavelength of visible light and scatter shorter wavelengths more effectively. The clear sky appears blue because blue light has a shorter wavelength and is scattered more by the air molecules than other colors. This scattered blue light reaches our eyes from all directions, making the sky look blue.

Answer:

(a) Initially, silver chloride is white in colour when kept in the china dish.

(b) The type of chemical reaction taking place is called photochemical decomposition or photolysis, where silver chloride decomposes on exposure to sunlight. The reaction can be written as:
2AgCl (white) —sunlight→ 2Ag (grey) + Cl₂ (chlorine gas)

(c) One use of this reaction is in photographic processes where silver halides like silver chloride break down upon exposure to light to form the image. Another chemical used for the same purpose is silver bromide (AgBr), which is also light sensitive and used in photographic films.

Answer:

Roasting and calcination are two important processes used to convert ores into oxides. The two main differences between them are:

• Roasting involves heating the ore in the presence of excess oxygen or air. It is used mainly for sulphide ores. For example, when zinc sulphide is roasted, it reacts with oxygen to form zinc oxide and sulfur dioxide gas:
  2ZnS + 3O₂ → 2ZnO + 2SO₂
• Calcination involves heating the carbonate ores in limited air or absence of oxygen. It is mainly used for carbonate ores. For example, zinc carbonate on calcination decomposes to give zinc oxide and carbon dioxide gas:
  ZnCO₃ → ZnO + CO₂

After roasting or calcination, the metal oxide produced is reduced to extract the pure metal. Reduction involves removing oxygen from the metal oxide, usually by heating with a reducing agent such as carbon. For example, zinc oxide is reduced by carbon to produce zinc metal and carbon monoxide:

ZnO + C → Zn + CO

This step is called reduction and it helps to obtain the metal from its oxide.

Answer:

(i) The first step in the breakdown of glucose during both aerobic and anaerobic respiration is glycolysis. In this process, one molecule of glucose (a six-carbon molecule) is broken down into two molecules of pyruvate (each containing three carbons). This step takes place in the cytoplasm of the cell. Glycolysis releases a small amount of energy and does not require oxygen.

(ii) ATP, or adenosine triphosphate, is called the energy currency of the cell because it stores and transports chemical energy within cells. When ATP is broken down into ADP (adenosine diphosphate) and a phosphate group, energy is released, which the cell uses to perform various functions such as muscle contraction, nerve impulses, and synthesis of molecules. Thus, ATP acts as a usable form of energy for cellular activities.

(iii) Residual volume of air is the amount of air that remains in the lungs even after the strongest possible exhalation during a breathing cycle. This air cannot be voluntarily expelled from the lungs and helps keep the lungs inflated, allowing continuous gas exchange between breaths.

Answer: To experimentally verify that sunlight is essential for photosynthesis, follow these steps: 1. Take two healthy potted green plants of the same species and size. 2. Place one plant in sunlight and the other in a dark room or cover it with a black cloth to block sunlight. 3. After a few days, test the presence of starch in the leaves of both plants using iodine solution. 4. To do this, first boil the leaves in water to kill them, then in alcohol to remove chlorophyll, and finally add iodine solution. 5. If the leaf turns blue-black, it indicates the presence of starch produced by photosynthesis. 6. The plant kept in sunlight will show a blue-black color indicating photosynthesis occurred, while the plant kept in darkness will show no starch. 7. This experiment proves that sunlight is essential for photosynthesis to take place in plants.

Answer: During scary or stressful situations, the hormone adrenaline is released in human beings. This hormone is secreted by the adrenal glands, which are located above the kidneys. When a person faces fear, the adrenal glands secrete adrenaline into the bloodstream. Adrenaline prepares the body for a 'fight or flight' response. It increases the heart rate and blood flow to muscles, providing more oxygen and energy to the body. It also causes rapid breathing and heightens alertness, enabling a person to react quickly. The body becomes ready to either fight the danger or run away from it efficiently. These changes help the body to cope with and respond actively to scary situations.

Answer: Given: Height of object (h) = 3 cm, Focal length (f) = 12 cm, Object distance (u) = -18 cm (since object is in front of the mirror, distance is negative). Using the mirror formula: 1/f = 1/v + 1/u, where v is the image distance. Calculating 1/v = 1/f - 1/u = 1/12 - 1/(-18) = 1/12 + 1/18 = (3 + 2)/36 = 5/36. Therefore, v = 36/5 = 7.2 cm. Image distance is +7.2 cm, meaning image is formed on the same side as the reflected rays (real image). Magnification (m) = -v/u = -7.2 / -18 = 0.4. Height of image (h') = m × h = 0.4 × 3 = 1.2 cm. Hence, the image is real, inverted, and smaller than the object.

Answer: Power of a lens is defined as the ability of the lens to converge or diverge light rays and is given by the formula P = 100/f, where P is the power in diopters (D) and f is the focal length in centimeters. For a lens with focal length of 10 cm, the power will be P = 100/10 = +10 D. Since the focal length is positive, the lens is convex in nature. When an object is placed at 20 cm from the optical centre of this convex lens, and considering the New Cartesian Sign Convention where distances in the direction of incident light are positive, magnification will be negative as the image formed by a convex lens beyond focal length is real and inverted. Hence, the sign of magnification will be negative.

Answer:

Overloading of an electrical circuit occurs when more electrical devices are connected to the circuit than it can safely handle, causing excessive current flow. This can damage appliances and wiring, and may even cause fires. Two common causes of overloading in household circuits are (1) connecting too many appliances to a single socket, which draws more current than the socket is designed for, and (2) a sudden increase in supply voltage which causes a higher current flow than normal. To prevent overloading, one should avoid using multiple appliances from the same socket and ensure that wiring and circuit components are properly rated. Installing a fuse or circuit breaker helps protect the circuit by breaking the circuit if the current exceeds safe limits.

Answer: Harmful chemicals, such as pesticides, enter our bodies through the food chain mainly because these chemicals are used in agriculture and get washed into the soil and water bodies. Plants absorb these chemicals from contaminated soil and water. Animals then consume these plants, and the chemicals accumulate in their bodies. As humans consume plants and animals at higher trophic levels, these chemicals accumulate in our bodies in larger amounts. This process, where the concentration of harmful chemicals increases at each trophic level, is called biological magnification. Since humans are at the top of the food chain, the concentration of these chemicals is maximum in our bodies, leading to potential health risks.

Answer: (a) The acid 'X' is Ethanoic acid (CH₃COOH), and the alcohol 'Y' is Ethanol (CH₃CH₂OH). The sweet smelling substance 'Z' formed is Ethyl ethanoate (CH₃COOCH₂CH₃), which is an ester.
Reaction: Ethanoic acid reacts with ethanol in presence of acid catalyst (usually concentrated H₂SO₄) to form ethyl ethanoate and water. This reaction is called an esterification reaction.

CH₃COOH + CH₃CH₂OH --H₂SO₄ (acid catalyst) --> CH₃COOCH₂CH₃ + H₂O

(b) When ester 'Z' (ethyl ethanoate) is treated with sodium hydroxide (NaOH), it produces ethanol and sodium ethanoate. This reaction is called saponification or alkaline hydrolysis.

CH₃COOCH₂CH₃ + NaOH → CH₃COONa + CH₃CH₂OH

This reaction is called alkaline hydrolysis because the ester is hydrolysed by a base (NaOH) to give an alcohol and a salt of the acid (sodium ethanoate).

Answer:

(i) The simplest saturated hydrocarbon is methane. Its molecular formula is CH4. The electron dot structure of methane shows one carbon atom forming four single covalent bonds with four hydrogen atoms, each sharing one electron with carbon. The bonds present in methane are single covalent bonds.

Electron dot structure of methane (CH4):
H
|
H - C - H
|
H
Each line indicates a single covalent bond, where electrons are shared.

(ii) Two mixtures of carbon compounds used as fuel in daily life, which contain methane as an important component, are natural gas and LPG (liquefied petroleum gas). Natural gas is widely used for cooking and heating, while LPG is commonly used as cooking gas in households.

(iii) Methane belongs to the homologous series called alkanes. Alkanes are saturated hydrocarbons where carbon atoms are connected by single bonds only. The general formula of alkanes is CnH2n+2, where 'n' is the number of carbon atoms.

(iv) When methane is burnt in sufficient oxygen, it produces a non-luminous, blue, and hot flame. This clean flame is ideal for cooking and heating because it burns completely producing carbon dioxide and water without soot.

Answer:

(i) Differences between Nervous and Hormonal Mechanisms for Control and Coordination in Animals:

1. Speed of Response: Nervous mechanisms provide rapid responses that are immediate and short-lived, while hormonal mechanisms produce slower responses that last for a longer duration.

2. Mode of Transmission: Nervous system transmits information through nerve impulses traveling along neurons, whereas hormones are chemical messengers secreted into the bloodstream to reach target organs.

3. Area of Effect: Nervous coordination acts on specific target cells or organs directly connected by nerves, while hormones can affect many different cells and organs throughout the body.

(ii) Role of Auxins in the Bending of Plant Shoot towards Unidirectional Light (Phototropism):

Phototropism is the growth movement of a plant towards light. When light comes from one direction (unidirectional light), auxin, a plant hormone, redistributes itself unevenly within the shoot. Auxin concentration increases on the side of the shoot that is away from the light (the shaded side). This higher concentration of auxin stimulates the cells on the shaded side to elongate more than those on the illuminated side. As a result, the shoot bends towards the light source. This bending allows the plant to maximize light absorption for photosynthesis, which is essential for its growth and survival.

Answer: Part (a): When an object is placed at a distance of 2f (twice the focal length) from a convex lens, the image formed is real, inverted, and of the same size as the object. The image is formed on the opposite side of the lens at a distance 2f from the lens.

Ray Diagram Explanation:

• A ray parallel to the principal axis passes through the focal point on the other side.
• A ray passing through the optical center goes straight without deviation.
• A ray passing through the focal point on the object side becomes parallel to the principal axis after refraction.

These rays intersect at the image position at 2f on the other side of the lens, confirming the image location and size. The magnification (m) is given by image distance divided by object distance, which is 2f/2f = 1. Hence, the image size equals the object size.

Part (b): Given: Length of flame = 2 cm, focal length (f) = 12 cm, object distance (u) = 16 cm.

(i) To find the distance of the image (v), we use the lens formula:
1/f = 1/v - 1/u
=> 1/v = 1/f + 1/u = 1/12 + 1/16 = (4/48) + (3/48) = 7/48
=> v = 48/7 ≈ 6.86 cm.
The image is formed at approximately 6.86 cm from the lens on the opposite side.

(ii) Magnification (m) = v/u = 6.86/16 ≈ 0.429
Length of image = magnification × length of object = 0.429 × 2 cm ≈ 0.86 cm.
Therefore, the image formed is smaller than the object and inverted perpendicular to the principal axis.