Important Questions — CBSE Class class-10 Science

C) Uterus

C

D

(B) DDT

$44 + XY$

Double circulation means that blood passes through the heart twice in one complete circuit of the body — once through the pulmonary circuit (heart to lungs and back) and once through the systemic circuit (heart to body and back) — keeping oxygenated and deoxygenated blood separate.

(a) Harmful effect of plastic bags and alternatives:

Plastic is non-biodegradable — it is not broken down by the enzymes of decomposer microorganisms (bacteria, fungi) and so persists in the environment for a very long time. One major harmful effect: plastic bags choke drains and soil, and when ingested by grazing animals (cattle, stray animals often eat plastic mixed with food waste), they block the digestive tract, causing illness or death of the animal. (Other valid effects: they clog soil pores and waterlogged drains, preventing water percolation and causing water-logging/flooding; they persist for hundreds of years, accumulating as permanent litter and polluting land and water bodies.)

Better alternatives: cloth bags, jute bags, paper bags — all made of natural, biodegradable materials that decomposers can break down, unlike plastic.

(b) How pesticides/fertilizers reach our bodies:

Paddy fields require large amounts of water, and pesticides and chemical fertilizers applied to the field are washed down by this water into the soil or into water bodies (rivers, ponds). From the soil, these chemicals are absorbed by plant roots along with water and minerals; from water bodies, they are taken up by aquatic plants and animals. In this way the chemicals enter the food chain at the producer level.

Since these chemicals are non-degradable, they are not excreted and instead get accumulated progressively at each successive trophic level as one organism eats another — herbivores accumulate more than plants, carnivores accumulate more than herbivores, and so on. Because human beings occupy the topmost level of most food chains, the maximum concentration of these harmful chemicals gets accumulated in the human body, through eating contaminated grains (wheat, rice), vegetables, fruits, and meat.

This phenomenon — the increasing concentration of a harmful, non-degradable chemical at successive trophic levels of a food chain — is known as biological magnification.

Part (A):

(i) Sugarcane reproduces through vegetative propagation, a mode of asexual reproduction in which parts of the plant such as the stem develop into new plants, used through methods such as layering or grafting.

(ii) Two advantages of vegetative propagation:

1. Plants raised by vegetative propagation bear flowers and fruits earlier than plants produced from seeds.
2. It makes possible the propagation of plants (such as banana, orange, rose and jasmine) that have lost the capacity to produce seeds; also, all plants produced are genetically similar enough to the parent plant to have all its characteristics.

(iii) Two more examples of plants that reproduce by vegetative propagation: Rose and grapes (other acceptable examples from the chapter: banana, orange, jasmine, potato, Bryophyllum, money-plant).

(iv) Regeneration is not possible in all types of animals because it is carried out by specialised cells that proliferate and form a mass of cells, which then undergo an organised sequence of changes (development) to form various cell types and tissues. Only simple animals with relatively simple body organisation, such as Hydra and Planaria, possess enough of these specialised regenerative cells throughout their bodies to allow any cut piece to regrow into a complete organism. More complex animals have highly specialised tissues and organs arranged in a carefully organised body plan, and they lack such widespread specialised regenerative cells, so regeneration (as opposed to normal sexual reproduction) is not a viable strategy for them.

Part (B):

(i) Changes taking place after fertilisation:

(a) Zygote – The zygote, formed by fusion of the male and female gametes, divides several times to form an embryo within the ovule.

(b) Ovule – The ovule develops a tough coat and is gradually converted into a seed, containing the future plant (embryo) within it.

(c) Ovary – The ovary grows rapidly and ripens to form a fruit.

(d) Sepals – The sepals (along with the petals, stamens, style and stigma) shrivel and fall off after fertilisation.

(ii) Germination: Germination is the process by which the embryo present inside a seed develops into a seedling under appropriate conditions (such as adequate moisture, air and warmth).

D

$Al_2O_3$ (option D) is an amphoteric oxide.

C

(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).

(A)(a) Chemical name: calcium sulphate hemihydrate; formula: $CaSO_4.\frac{1}{2}H_2O$.

(b) $CaSO_4.2H_2O \xrightarrow{373K} CaSO_4.\frac{1}{2}H_2O + 1\frac{1}{2}H_2O$

(c) Used by doctors as plaster for supporting fractured bones in the right position; used for making toys, materials for decoration, and for making surfaces smooth.

OR

(B)(a) Methanoic acid (formic acid).

(b)(i) Dissolving an acid in water is a highly exothermic process. Adding acid to a large volume of water lets the heat generated spread through the water and dissipate safely, keeping the mixture cool; if water is instead added to a concentrated acid, the small amount of water added heats up very rapidly and violently, and the heat generated may cause the mixture to splash out, causing burns, and may even crack the glass container due to excessive local heating.

(ii) Baking soda ($NaHCO_3$) is a mild, non-corrosive basic salt. When the stomach produces excess hydrochloric acid causing indigestion, baking soda neutralises this excess acid, relieving the pain and irritation, which is why it is used as an antacid.

A converging (convex) lens.

Assertion (A) is false, but Reason (R) is true.

Esterification is a chemical reaction in which an acid reacts with an alcohol to form an ester and water. The general equation for esterification is: R-COOH + R'-OH $\rightarrow$ R-COO-R' + H2O. Saponification is a chemical reaction in which an ester reacts with a base to form a carboxylic acid and an alcohol. The general equation for saponification is: R-COO-R' + NaOH $\rightarrow$ R-COO-Na+ + R'-OH. One use of esters is in the manufacture of perfumes and fragrances. One use of the saponification process is in the manufacture of soap.

a) The valency of an element is the number of valence electrons present in the outermost shell of an atom of that element. The valency of Na is 1, Mg is 2 and Al is 3. Hence, Al has the highest valency among the given elements.

Three techniques that have been developed to prevent pregnancy are:

1. Barrier method: This method prevents the sperm from meeting the egg by using barriers such as condoms, diaphragms, and cervical caps.
2. Oral contraceptives: These are pills that contain hormones that prevent ovulation.
3. Intrauterine devices (IUDs): These are devices that are inserted into the uterus to prevent implantation of the fertilized egg.

The technique that is not meant for males is oral contraceptives.

The use of these techniques has a direct impact on the health and prosperity of a family by allowing them to plan their family size and spacing, which can lead to improved health and economic outcomes.

Mendel explained that it is possible that a trait is inherited but not expressed in an organism through his experiments on pea plants. He proposed that each trait is controlled by two factors (now called genes), one inherited from each parent. These two factors may be identical or different, depending on the parentage. If the two factors are identical, the trait will be expressed. However, if the two factors are different, only one of the traits will be expressed, and the other will be hidden. This hidden trait is not lost but is inherited by the offspring.

Evolution and classification of organisms are interlinked because the classification of organisms is based on their evolutionary relationships. The characteristics of an organism are a result of its evolutionary history, and the classification of organisms reflects their shared characteristics and evolutionary relationships. The classification of organisms helps us to understand their evolutionary history and the relationships between different groups of organisms.

(a) Two Ways to Bring Awareness in the Neighbourhood

1. Street Plays and Community Workshops (Nukkad Nataks): Organizing interactive street plays in local parks or community centers to demonstrate simple, daily water-saving habits (e.g., closing the tap while brushing, using a bucket instead of a hose to wash cars, and reusing kitchen wastewater for watering plants).
2. Door-to-Door Campaigns and Pamphlet Distribution: Distributing informative pamphlets that highlight the local water crisis, provide a checklist of water-saving tips, and encourage residents to install aerators on taps to reduce water flow.

(b) Method to Prevent the Underground Water Table from Depleting

• Name of the Method: Rainwater Harvesting (or Rooftop Rainwater Harvesting)
• Explanation:

Rainwater harvesting is a sustainable technique used to collect, filter, and store rainwater for future use or to recharge groundwater aquifers. In urban areas, rainwater falling on rooftops is redirected through PVC pipes into a filtration tank (containing sand, gravel, and charcoal). From the filtration tank, the clean water is directed into a recharge pit, borewell, or dug well. This allows the water to percolate deep into the soil layers, directly replenishing the underground aquifers and preventing the water table from falling further.

Why Compounds are Called Hydrocarbons

Compounds that are composed entirely of only carbon and hydrogen atoms are called hydrocarbons.

General Formulas and First Members of Homologous Series

1. Alkanes (Saturated Hydrocarbons):

• General Formula: $C_nH_{2n+2}$
• First Member: Methane ($CH_4$)
• Structure:

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(Note: Methene and methyne do not exist because a carbon-carbon double or triple bond requires at least two carbon atoms.)

Conversion of Alkenes into Alkanes

• Name of the Reaction: Hydrogenation Reaction (or Addition Reaction).
• Chemical Equation and Conditions:

Alkenes react with hydrogen gas ($H_2$) in the presence of catalysts like finely divided Nickel ($Ni$), Palladium ($Pd$), or Platinum ($Pt$) at an elevated temperature to form saturated alkanes.

$$\text{CH}_2=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni catalyst, } \Delta} \text{CH}_3-\text{CH}_3$$

(a) (i) Ovary: The ovary is the female reproductive organ that produces eggs (ovum) and hormones.

(ii) Uterus: The uterus is a hollow, muscular organ that supports the growth of the fertilized egg during pregnancy.

(iii) Fallopian tube: The fallopian tube is a muscular tube that connects the ovary to the uterus and provides a pathway for the egg to travel from the ovary to the uterus.

(b) Structure and functions of placenta:

The placenta is a flat, disk-shaped organ that develops in the uterus during pregnancy. It is made up of both fetal and maternal tissues and plays a crucial role in providing oxygen and nutrients to the fetus and removing waste products. The placenta also produces hormones that help to maintain pregnancy.

Acquired traits are characteristics that an individual develops during its lifetime, such as scars or knowledge, whereas inherited traits are characteristics that are passed down from parents to offspring through genes. For example, a person may acquire a scar on their skin due to an injury, but this scar is not inherited by their offspring. On the other hand, eye color is an inherited trait that is determined by genes passed down from parents. The reason why acquired traits are not inherited is that they are not encoded in the genes of the individual. Genes are the units of heredity, and they determine the characteristics of an individual. Acquired traits are not encoded in genes and therefore cannot be passed down to offspring.

This question is not related to the provided chapter.

(a) The mirror is a convex mirror. The ray diagram for the observation is shown below:

The convex mirror is used as a rear-view mirror in vehicles because it always gives an erect, though diminished, image. Also, it has a wider field of view as it is curved outwards.

(b) The radius of curvature of a spherical mirror is the radius of the sphere of which the reflecting surface of the mirror forms a part. The focal length of a spherical mirror is half of its radius of curvature. Therefore, the focal length of the given spherical mirror is +12 cm. Since the focal length is positive, the mirror is a convex mirror.

(a) Two possible reasons for a student suffering from myopia are: (i) excessive curvature of the eye lens, or (ii) elongation of the eyeball. (i) The student is unable to see distinctly the objects placed beyond 5 m from his eyes because in a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself. (ii) The defect is corrected by the use of a concave lens of suitable power. This lens will bring the image back on to the retina and thus the defect is corrected.

When acetic acid is added to sodium bicarbonate powder, carbon dioxide gas is released with brisk effervescence. The reaction is as follows: CH3COOH + NaHCO3 $\rightarrow$ CH3COONa + H2O + CO2

Option C is correct because saponification is an exothermic reaction, which releases heat energy, causing the temperature of the reaction mixture to rise and making the outer surface of the beaker feel hot.

Option D is correct because hard water contains dissolved hydrogen carbonates, chlorides, or sulphates of calcium and magnesium. Dissolving calcium sulphate ($CaSO_4$) and calcium chloride ($CaCl_2$) in distilled water introduces calcium ions ($Ca^{2+}$), thereby converting it into hard water.

Option B is correct because Gram, Pea, and Ground-nut are all dicotyledonous (dicot) seeds, meaning their embryos have two cotyledons.

The correct option is C (Radish and Carrot).

Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 20 cm and 25 cm respectively.

The correct option is D (either towards or away from the screen depending upon the position of the object).

B

The angle of incidence is the angle at which a light ray enters a medium. In this diagram, it is represented by x. The angle of emergence is the angle at which a light ray leaves a medium. In this diagram, it is represented by q. The angle of deviation is the angle between the incident ray and the emergent ray. In this diagram, it is represented by z.

1. Essential materials (chemicals) required to prepare soap in the laboratory:

• Vegetable oil (such as castor oil, coconut oil, or cottonseed oil)
• Sodium hydroxide ($NaOH$) (caustic soda)
• Sodium chloride ($NaCl$) (common salt, used to precipitate the soap out of the solution)

1. Test to determine the nature of the reaction mixture:

• Take a small amount of the reaction mixture and dissolve it in distilled water to make a solution.
• Dip a strip of red litmus paper into this solution. The red litmus paper turns blue.
• Dip a strip of blue litmus paper into the solution. The blue litmus paper remains blue.
• Conclusion: Since the reaction mixture turns red litmus blue, it is alkaline (basic) in nature. This is because soap is a salt of a strong base ($NaOH$) and a weak organic acid (fatty acid), which undergoes hydrolysis in water to release hydroxyl ions ($OH^-$).

For the molecular formula $\text{C}_3 ext{H}_6 ext{O}$, the two functional group representations are:

1. Aldehyde:

• Name: Propanal
• Structure:

$$\text{CH}_3-\text{CH}_2-\text{CHO}$$

(Or structurally represented as:)

$$\text{H}-\overset{\begin{matrix}\text{H}\\\|\end{matrix}}{\underset{\begin{matrix}|\\\text{H}\end{matrix}}{\text{C}}}-\overset{\begin{matrix}\text{H}\\\|\end{matrix}}{\underset{\begin{matrix}|\\\text{H}\end{matrix}}{\text{C}}}-\overset{\begin{matrix}\text{O}\\\|\!\|\end{matrix}}{\text{C}}-\text{H}$$

1. Ketone:

• Name: Propanone (commonly known as acetone)
• Structure:

$$\text{CH}_3-\text{CO}-\text{CH}_3$$

(Or structurally represented as:)

$$\text{H}-\overset{\begin{matrix}\text{H}\\\|\end{matrix}}{\underset{\begin{matrix}|\\\text{H}\end{matrix}}{\text{C}}}-\overset{\begin{matrix}\text{O}\\\|\!\|\end{matrix}}{\text{C}}-\overset{\begin{matrix}\text{H}\\\|\end{matrix}}{\underset{\begin{matrix}|\\\text{H}\end{matrix}}{\text{C}}}-\text{H}$$

Relation in the language of science:

These two compounds are structural isomers (specifically functional isomers). They share the same molecular formula ($\text{C}_3 ext{H}_6 ext{O}$) but possess different functional groups (an aldehyde group $-\text{CHO}$ in propanal, and a ketone group $-\text{CO}-$ in propanone), which gives them distinct chemical and physical properties.

Definition of Reproduction

Reproduction is the biological process by which living organisms produce new individuals (offspring) that look very much like themselves, ensuring the continuation of their species.

How Reproduction Provides Stability to Populations

1. Niche Occupation: Populations of organisms use their reproductive ability to fill well-defined places or niches in the ecosystem.
2. Consistency of Body Design: The consistency of DNA copying during reproduction is crucial for maintaining the specific body design features that allow a species to survive and function in its particular niche.
3. Population Maintenance: By continuously replacing dying individuals with new ones of similar body designs, reproduction maintains the stability and continuity of the population of a species over time.

Regeneration

Regeneration is the ability of a fully differentiated organism to give rise to a complete new individual from its body parts. If the individual is somehow cut or broken up into many pieces, many of these pieces can grow into separate, complete individuals.

How Regeneration is Carried Out in Multicellular Organisms (like Hydra)

1. Specialised Cells: Regeneration is carried out by highly specialised cells present in the organism.
2. Proliferation: When the organism is cut, these specialised cells proliferate rapidly, making a large mass of cells.
3. Differentiation and Development: From this mass of cells, different cells undergo changes to become various cell types and tissues. These changes occur in an organised sequence referred to as development, ultimately forming a complete new organism.

The two areas of study, "evolution" and "classification", are deeply interlinked and mutually support each other. This can be justified through the following points:

1. Basis of Classification:

• Classification is the systematic grouping of organisms based on their similarities and differences in characteristics (features of form or function).
• Organisms that share a large number of characteristics are grouped together, while those with fewer similarities are placed in different groups.

1. Reflection of Common Ancestry:

• The presence of shared characteristics among different species indicates that they have inherited these traits from a common ancestor.
• For example, the presence of a vertebral column in mammals, birds, reptiles, and amphibians indicates they all evolved from a common vertebrate ancestor.

1. Tracing Evolutionary Relationships (Phylogeny):

• The more characteristics two species share, the more closely related they are.
• The closer the relationship, the more recently they shared a common ancestor.
• Therefore, as we build a classification hierarchy from species to kingdoms, we are actually tracing the evolutionary relationships and history of life on Earth. Thus, classification is a direct reflection of evolutionary relationships.

The correct answer is A.

(a) Necessity of Conserving Our Environment

It is necessary to conserve our environment for the following reasons:

1. Interdependence and Ecological Balance: All biotic components (plants, animals, microorganisms) and abiotic components (soil, water, temperature) interact to maintain a balance in nature. Disrupting one part (e.g., through pollution or deforestation) destabilizes the entire ecosystem.
2. Prevention of Waste Accumulation and Toxicity: Human activities generate non-biodegradable wastes (like plastics) and toxic chemicals (like pesticides) that persist in nature. These chemicals undergo biological magnification, accumulating at maximum concentrations in human bodies at the top of the food chain, causing severe health hazards.
3. Protection of Vital Shields: Synthetic chemicals like chlorofluorocarbons ($CFCs$) deplete the ozone ($O_3$) layer in the stratosphere, which shields us from harmful ultraviolet ($UV$) radiation that causes skin cancer and damages other organisms.

(b) Importance of Green and Blue Dust-bins

• Green Dust-bins (for Biodegradable Waste): Used to collect organic kitchen wastes, spoilt food, vegetable peels, and paper. These substances can be broken down by decomposers (bacteria and fungi) into simple inorganic substances. This allows for natural replenishment of the soil through composting.
• Blue Dust-bins (for Non-Biodegradable Waste): Used to collect materials like plastics, metals, glass, and synthetic materials. Since these materials cannot be broken down by biological enzymes and persist in the environment for a long time, separating them ensures they can be sent for recycling or safe industrial disposal, preventing them from polluting ecosystems or harming wildlife.

(c) Values Exhibited by the Classmate

1. Environmental Consciousness and Responsibility: Demonstrates a deep awareness of ecological issues and a proactive attitude towards protecting the biosphere.
2. Leadership and Social Citizenship: Shows the initiative to educate others, spread awareness, and work collectively for the betterment of society and the local environment.

Based on the chemical reactions and properties described:

• Compound P: Ethanol ($\text{CH}_3 ext{CH}_2 ext{OH}$ or $ext{C}_2 ext{H}_5 ext{OH}$)
• Compound Q: Ethene ($\text{CH}_2=\text{CH}_2$ or $ext{C}_2 ext{H}_4$)
• Compound R: Ethane ($\text{CH}_3-\text{CH}_3$ or $ext{C}_2 ext{H}_6$)

Chemical Equations Involved:

1. Dehydration of P to form Q:

When ethanol ('P') is heated with excess concentrated sulfuric acid at $443\text{ K}$, it undergoes dehydration to form ethene ('Q').

$$\text{CH}_3 ext{CH}_2 ext{OH} \xrightarrow{\text{Excess conc. } \text{H}_2\text{SO}_4, 443\text{ K}} \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O}$$

1. Hydrogenation of Q to form R:

Ethene ('Q') undergoes an addition reaction with hydrogen in the presence of a nickel catalyst to form ethane ('R').

$$\text{CH}_2=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni catalyst, } \Delta} \text{CH}_3-\text{CH}_3$$

1. Combustion of R:

Ethane ('R') burns in the presence of oxygen to produce carbon dioxide and water. One molecule of ethane yields exactly two molecules of carbon dioxide and three molecules of water.

$$\text{C}_2\text{H}_6 + \frac{7}{2}\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$$

(Or expressed with whole-number coefficients:)

$$2\text{C}_2\text{H}_6 + 7\text{O}_2 \rightarrow 4\text{CO}_2 + 6\text{H}_2\text{O}$$

Placenta

The placenta is a specialized, temporary physiological organ (a disc-like tissue) that connects the developing embryo to the uterine wall of the mother. It serves as the structural and functional interface between the maternal body and the fetus during pregnancy.

Structure of the Placenta

1. Disc-like Shape: It is a flat, disc-shaped structure embedded deeply within the uterine wall of the mother.
2. Embryonic Side (Villi): On the side facing the embryo, the placenta contains numerous finger-like projections called villi.
3. Maternal Side (Blood Spaces): On the mother's side, there are large blood spaces that surround and bathe these villi.
4. Surface Area: This specific arrangement provides a massive surface area for the exchange of nutrients, gases, and metabolic waste products between the mother and the embryo.

Functions of the Placenta in a Pregnant Female

1. Nutrition: It facilitates the transfer of essential nutrients (such as glucose, amino acids, vitamins, and minerals) from the mother's blood to the developing embryo to support its growth.
2. Respiration: It allows oxygen ($O_2$) to diffuse from the maternal blood into the fetal blood, and carbon dioxide ($CO_2$) produced by the embryo to diffuse back into the mother's blood for elimination.
3. Excretion: The metabolic waste products (like urea) generated by the developing embryo are transferred across the placenta into the mother's blood, which are then excreted by the mother's kidneys.
4. Endocrine Function: It secretes crucial hormones (such as progesterone and estrogen) necessary for maintaining the pregnancy and supporting fetal development.

Atmospheric refraction is the bending of light as it passes through the Earth's atmosphere due to the gradual change in air density.

1. Definition of Evolution

Evolution is the gradual, orderly, and continuous process of change in the heritable characteristics of biological populations over successive generations, leading to the formation of new species from pre-existing ones.

2. How Evolution Occurs

Evolution occurs primarily through the following mechanisms:

• Accumulation of Variations: During reproduction (especially sexual reproduction), inaccuracies in DNA copying and genetic recombination generate variations among individuals.
• Natural Selection: Environmental factors act as selecting agents. Variants that possess traits advantageous for survival in a given environment (e.g., heat-resistant bacteria during a heatwave) survive better, reproduce, and pass on these favorable traits to their offspring.
• Genetic Drift: Random changes in gene frequencies in small populations over generations can lead to significant changes in characteristics without necessarily providing a survival advantage.

3. Fossils as Evidence for Evolution

Fossils are the preserved remains, impressions, or traces of ancient organisms that lived millions of years ago. They provide direct, physical evidence of evolution in the following ways:

• Chronological Record (Stratigraphy): Fossils found in deeper, older sedimentary rock layers are simpler in structure (e.g., invertebrates), while those found in upper, newer layers are more complex (e.g., vertebrates). This shows a clear progression from simple to complex life forms over geological time.
• Connecting Links: Fossils often act as transitional forms or "connecting links" between two different groups of organisms, showing how one group evolved from another. For example, the fossil of Archaeopteryx exhibits features of both reptiles (teeth, long bony tail) and birds (feathers, wings), proving that birds evolved from reptilian ancestors.
• Estimation of Evolutionary Time: By dating fossils (using methods like carbon dating or relative depth analysis), scientists can establish a timeline of when specific traits and species emerged, helping reconstruct the evolutionary tree.

Castor oil and sodium hydroxide

Calcium chloride, magnesium chloride

The correct option is (d). The three correct parts of a pea seed embryo are the Radicle, Cotyledon, and Plumule.

The correct conclusion is that the angle of incidence ($\angle i$) is more than the angle of refraction ($\angle r$), but nearly equal to the angle of emergence ($\angle e$). Hence, option (a) is correct.

The correct option is (a). Carrot and radish form a group of vegetables having homologous structures.

The correct option is (b).

When a distant object (at infinity) is focused using a concave mirror, its image is formed at the principal focus ($F$). According to the properties of image formation by a concave mirror, the image of an object at infinity is real, inverted, and highly diminished (point-sized). Since it is obtained on a screen, it must be a real image, which is always inverted.

The correct option is (c).

There is no change in the blue litmus paper and the red litmus paper turns blue.

The correct option is (a).

Initially, the candle flame is at a finite distance (at the farthest end of the table), so its image is formed at a distance greater than the focal length ($v > f$). When focusing parallel rays from the sun (which is at infinity), the image is formed exactly at the principal focus ($v = f$). Since the focus is closer to the lens than the previous image position, the distance between the lens and the screen must be decreased. Therefore, the lens must be moved slightly towards the screen.

The correct answer is (c) $\angle i$, $\angle e$ and $\angle D$.

The two main observations that the student must note are:

1. Brisk effervescence is observed due to the rapid evolution of a colorless and odorless gas.
2. The evolved gas, when passed through lime water, turns the lime water milky (confirming the gas is carbon dioxide, $CO_2$).

Chemical Equation:

$$CH_3COOH + NaHCO_3 \rightarrow CH_3COONa + H_2O + CO_2$$

Acetic acid has a characteristic smell of vinegar.