CBSE EXAMINATION PAPER-2023
SCIENCE
(Solved)
General Instructions :
Read the following instructions carefully and follow them :
- This question paper contains 45 questions. All questions are compulsory.
- This question paper is divided into 5 sections.
- Section A – questions number 1 to 3 are case based questions
- Section B – questions number 4 to 23 are multiple choice questions
- Section C – questions number 24 to 31 are very short answer
- Section D – questions number 32 to 40 are short answer
- Section E – questions number 41 to 45 are long answer
- There is no overall choice given in the question paper. However, an internal choice has been provided in few questions.
- Use of calculator is NOT allowed.
Section A
The teacher while conducting practicals in the laboratory divided the students into three groups and gave them various solutions to find out their pH and classify them into acidic, basic and neutral solutions.
Group A: Lemon juice, vinegar, colourless aerated drink.
Group B: Tomato juice, coffee, ginger juice.
Group C: Sodium hydroxide, sodium chloride, lime water.
(1) List two ways of determining pH of a solution.
[1 Mark](2) Explain, why the sour substances such as lemon juice are effective in cleaning the tarnished copper vessels.
[2 Marks](3) For the solutions provided, which group is/are likely to have pH value (i) less than 7, and (ii) greater than 7?
[1 Mark](4) "pH has great importance in our daily life." Justify this statement by giving two examples.
All the reproductive methods of living organisms are broadly categorized into two types: 1. Asexual reproduction, and 2. Sexual reproduction.
Asexual reproduction involves the participation of a single parent without the formation of gametes, fertilisation and transfer of genetic material. This method is a common means of rapidly increasing offsprings under favourable conditions.
(1) Write one advantage of sexual mode of reproduction over asexual reproduction.
[1 Mark](2) Name the type of fission that occurs in Leishmania and Plasmodium.
[1 Mark](3) Give reasons why: (i) Colonies of yeast fail to multiply in water but multiply in sugar solution. (ii) Rhizopus individuals do not grow on a dry slice of bread.
(4) Name the filamentous structures a student could identify when he collected water from a pond that appeared dark green. How do these organisms multiply ? Explain.
(1) What is the relationship between the readings of A₁ and A₃? Give reasons for your answer.
(2) What is the relationship between the readings of A₂ and A₃?
(3) Determine the reading of the voltmeter V₁.
(4) Find the total resistance of the circuit.
Section B
In the given experimental set-up, if the experiment is carried out separately with each of the following solutions, the cases in which the bulb will glow is/are:
(i) Dilute hydrochloric acid
(ii) Dilute sulphuric acid
(iii) Glucose solution
(iv) Alcohol
In order to balance the following chemical equation, the values of the coefficients x and y respectively are:
The correct coefficients x = 2 and y = 2 balance the chemical equation because they ensure that the number of atoms of each element on the reactants side is equal to the number on the products side. Balancing chemical equations involves adjusting these coefficients to maintain the conservation of mass, meaning atoms are neither created nor destroyed. According to the context, coefficients represent the number of molecules involved, and balancing requires matching the atom counts on both sides.
The total number of electrons shared in the formation of an ethyne molecule is:
In an ethyne (C2H2) molecule, there is a triple bond between the two carbon atoms. Each bond consists of a pair of shared electrons, so a triple bond means 3 pairs, or 6 electrons, are shared between the two carbons. Additionally, each carbon atom forms a single bond with one hydrogen atom, sharing 2 electrons (1 pair) with hydrogen. However, the question asks about the total number of electrons shared in the molecule formation, which primarily concerns the carbon-carbon bonds. The triple bond between the two carbon atoms involves sharing 6 electrons in total. Therefore, the correct answer is 6.
In the following diagram, identify the cells through which massive amounts of gaseous exchange takes place for photosynthesis:
When an 'X' bearing sperm fertilises the egg, the resulting zygote has the following combination of chromosomes:
Each human gamete (sperm or egg) carries 22 autosomes and 1 sex chromosome. The egg always carries an X chromosome, and when the sperm carrying an X chromosome fertilizes it, the resulting zygote will have XX as the sex chromosomes. Thus, the zygote has 22 pairs of autosomes plus XX sex chromosomes, making the correct answer 22 + XX.
An object is placed in front of a convex mirror at infinity. According to the New Cartesian Sign Convention, the sign of the focal length and the sign of the image distance in this case are respectively:
For a convex mirror, the focal length (f) is positive because it is a diverging mirror. According to the New Cartesian Sign Convention, object distance (u) is negative when the object is in front of the mirror. When the object is at infinity (very far away), the image formed by the convex mirror will be virtual, diminished, and located behind the mirror, which means the image distance (v) is negative. Therefore, the focal length is positive and the image distance is negative.
For verifying Ohm's law, we design an electric circuit diagram in which we show the arrangement of different circuit components. We find that with respect to the resistor, the:
In a resistive circuit, if the current is increased to two times, the percentage change in the amount of heat dissipated in the circuit would be:
A positron enters a uniform magnetic field at right angles to it as shown. The direction of force experienced by the positron will be:
A positron is the antiparticle of the electron and has the same magnitude of charge but positive instead of negative. According to the Lorentz force law, the direction of force on a charged particle moving in a magnetic field is given by the right-hand rule for positive charges. In the provided context, an electron moving downward in a magnetic field directed to the right experiences a force directed out of the page. Since a positron has opposite charge, the force direction will be opposite to that of the electron. Therefore, the force on the positron will be directed into the page.
Assertion (A): Sodium oxide is an amphoteric oxide.
Reason (R): Metal oxides which react with both acids as well as bases are known as amphoteric oxides.
Both A and R are true and R is the correct explanation of A. Sodium oxide (Na2O), being a metal oxide, reacts with acids to form salt and water and also reacts with bases. Amphoteric oxides are defined as metal oxides which react with both acids and bases. Hence, since sodium oxide shows such behavior, it is amphoteric, and the reason correctly explains the assertion.
Assertion (A): When a bacterium divides into two, and the resultant two bacteria divide again, the four bacteria produced would be almost similar.
Reason (R): DNA copying involves small inaccuracies in the reproduction process.
Assertion (A): A person suffering from myopia cannot see distant objects clearly.
Reason (R): A converging lens is used for the correction of myopic eye as it can form real as well as virtual images of the objects placed in front of it.
Assertion (A): Magnetic field lines do not intersect each other.
Reason (R): Magnetic field lines are imaginary lines, the tangent to which at any point gives the direction of the field at that point.
Section C
(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.
When copper powder is heated in a watch glass, a black substance is formed.
(i) Why is this black substance formed? Name it.
(ii) How can this black substance be reversed to its original form?
(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.
(a) Why is it important to prevent oxygenated and deoxygenated blood from mixing in birds and mammals?
(b) Which animals can tolerate some mixing of the oxygenated and deoxygenated blood streams ? On what factor does the body temperature of these animals depend ?
(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.
Mendel crossed a pure tall pea plant (TT) with a pure short pea plant (tt) and obtained all tall plants in F generation.
(a) What is the gene combination present in the plants of F₁ generation ?
(b) Give reason why only tall plants are observed in F₁ progeny.
(c) What will be the ratio of the plants obtained in the F₂ generation when F₁ plants are self-pollinated ?
(b) Only tall plants are observed because the allele for tallness (T) is dominant over the allele for shortness (t). The F₁ plants receive one dominant allele T from the tall parent and one recessive allele t from the short parent, so all show the tall trait.
(c) When F₁ plants (Tt) are self-pollinated, the F₂ generation shows a phenotypic ratio of 3 tall plants : 1 short plant.
When and where does a rainbow appear in the sky ? Draw a labelled ray diagram to show its formation.
What is scattering of light ? Why does the clear sky appear blue ?
V - I graph for two conducting wires A and B are as shown. If both wires are of the same length and same diameter, which of the two is made of a material of high resistivity? Give reasons to justify your answer.
This conclusion is drawn from the V-I graphs where the slope (V/I) represents the resistance of the wire. Since both wires have the same length and diameter, the difference in resistance is due to their material's resistivity. Wire B shows a higher slope on the V-I graph, indicating greater resistance, and therefore higher resistivity.
Hence, wire B has higher resistivity because resistivity is proportional to resistance when length and area are constant.
We do not clean ponds or lakes but an aquarium needs to be cleaned regularly. Why ?
Section D
Silver chloride kept in a china dish turns grey in sunlight.
(a) Write the colour of silver chloride when it was kept in the china dish.
(b) Name the type of chemical reaction taking place and write the chemical equation for the reaction.
(c) State one use of the reaction. Name one more chemical which can be used for the same purpose.
(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.
With the help of suitable chemical equations, list the two main differences between roasting and calcination. How is metal reduced from the product obtained after roasting/calcination of the ore ? Write the chemical equation for the reaction involved.
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.
(i) What is the first step in the breakdown of glucose during aerobic and anaerobic respiration ? Where does it take place ?
(ii) ATP is called the energy currency of the cell. Why ?
(iii) What is meant by residual volume of air in a breathing cycle ?
(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.
Write in sequence the steps for experimental verification of the fact that sunlight is essential for photosynthesis .
Name the hormone released and the gland which secretes it in human beings during scary situations. How does the body respond to enable it to deal with the situation ?
A student has focussed the image of an object of height 3 cm on a white screen using a concave mirror of focal length 12 cm. If the distance of the object from the mirror is 18 cm, find the values of the following :
(i) Distance of the image from the mirror
(ii) Height of the image
Define power of a lens. The focal length of a lens is 10 cm. Write the nature of the lens and find its power. If an object is placed at a distance of 20 cm from the optical centre of this lens, according to the New Cartesian Sign Convention, what will be the sign of magnification in this case ?
Explain the meaning of overloading of an electrical circuit. List two possible causes due to which overloading may occur in household circuits. Write one preventive measure that should be taken to avoid overloading of domestic circuits.
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.
Explain how some harmful chemicals enter our bodies through the food chain. Why is the concentration of these harmful chemicals found to be maximum in human beings ?
Section E
An acid 'X' and an alcohol 'Y' react with each other in the presence of an acid catalyst to form a sweet smelling substance 'Z'. Identify 'X', 'Y' and 'Z'. Write the chemical equation for the reaction involved and name it. The substance 'Z' on treatment with sodium hydroxide produces back the alcohol 'Y' and sodium ethanoate. Write the chemical equation for the reaction involved and name it, giving justification for the name.
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).
(i) Name the simplest saturated hydrocarbon. Draw its electron dot structure. Which type of bonds exist in this compound ?
(ii) Name any two mixtures of the carbon compound used as a fuel in daily life, of which the above mentioned compound is an important component.
(iii) In which homologous series of carbon compounds can this compound be placed ? Write the general formula of the series.
(iv) Which type of flame is produced on burning it ?
(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.
(i) List three points of difference between nervous and hormonal mechanisms for control and coordination in animals.
(ii) How are auxins related with the bending of plant shoot towards unidirectional light ? Explain.
(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.
(i) Name the disorder which a person is likely to suffer from due to the following :
(I) Over-secretion of growth hormone
(II) Deficiency of oestrogen in females
(III) Less secretion of thyroxine
Also name the gland that secretes each of the hormones mentioned above.
(ii) How is the timing and amount of hormone released regulated ? Explain with the help of an example.
(i)
(I) Over-secretion of growth hormone leads to a disorder called gigantism (in children) and acromegaly (in adults). The hormone is secreted by the pituitary gland.
(II) Deficiency of oestrogen in females causes menopause symptoms such as osteoporosis, irregular periods, and other health issues. Oestrogen is secreted by the ovaries.
(III) Less secretion of thyroxine causes hypothyroidism, which results in symptoms like fatigue, weight gain, and slow metabolism. Thyroxine is secreted by the thyroid gland.
(ii)
The timing and amount of hormone released are regulated by feedback mechanisms that maintain body balance. For example, growth hormone secretion is controlled by the hypothalamus-pituitary axis. When growth hormone levels are low, the hypothalamus releases growth hormone releasing factor (GHRF), which stimulates the pituitary gland to secrete growth hormone. When levels are sufficient, the hypothalamus releases somatostatin, which inhibits growth hormone secretion. This feedback mechanism ensures hormone levels stay balanced according to the body's needs.
(a) An object is placed in front of a convex lens of focal length f. If the distance of the object from the lens is 2f, draw a ray diagram to show the formation of the image. Write the value of magnification in this case.
(b) A student has focussed the image of a candle flame on a white screen using a convex lens. The situation is as given below : Length of the flame = 2 cm Focal length of the lens = 12 cm Distance of the flame from the lens = 16 cm If the flame is perpendicular to the principal axis of the lens, calculate the values of the following :
(i) Distance of the image from the lens
(ii) Length of the image formed
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.
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.
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