CBSE EXAMINATION PAPER-2024

Answer: Saturated hydrocarbons produce a clean, blue flame during the flame test, indicating complete combustion with less soot. Unsaturated hydrocarbons give a yellow, sooty flame due to the presence of double or triple bonds, causing incomplete combustion and black smoke. The products of the combustion of a saturated hydrocarbon are carbon dioxide and water along with the release of heat and light energy.

Answer: Water is absorbed by the roots from the soil. It travels upward through the plant's xylem vessels towards the leaves. Tiny pores on the leaf surface called stomata allow water to evaporate into the air as water vapor, in a process called transpiration. This evaporation creates a suction force that pulls more water from the roots to the leaves. Thus, water moves from the soil to the roots, then through the plant to the leaves.

Answer: Variations within a species help individuals adapt to changing environments, promoting survival. For example, in bacteria, some may gain a variation that allows them to withstand heat. During a heatwave, these heat-resistant bacteria survive while others perish. This natural selection ensures that the species continues to exist over time with traits best suited for the environment. Such variations arise due to sexual reproduction and genetic changes, leading to greater diversity and better chances of survival.

Answer: In bifocal lenses used for presbyopia, the lens for distant vision is concave and the lens for near vision is convex. Here, the power 4.0 dioptre is likely negative (concave) for distant vision, and +2.0 dioptre (convex) is for near vision. The focal length (f) of a lens is calculated as 100 divided by power in dioptres. So, for the distant vision lens, focal length f = 100 / 4 = 25 cm or 0.25 m. Therefore, the lens of power 4.0 dioptre corrects distant vision, and its focal length is 0.25 meters.

Answer: Wire A, having a diameter of 0.2 mm, offers more resistance compared to wire B which has a diameter of 0.3 mm. This is because resistance in a wire depends inversely on its cross-sectional area. A wire with a smaller diameter has a smaller cross-sectional area, leading to higher resistance. Since both wires are of the same material and length, the difference in resistance is due to their differing diameters. Hence, wire A with the thinner diameter has greater resistance to current flow.

Answer: When metals like zinc react with dilute acids such as hydrochloric acid, they produce a salt and hydrogen gas. For example, zinc reacts with hydrochloric acid to form zinc chloride (a salt) and hydrogen gas. To test for hydrogen gas, collect the gas and bring a burning matchstick near it. If the gas is hydrogen, it will produce a 'pop' sound, confirming its presence.

Answer: Amoeba exhibits holozoic nutrition. It takes in food using its pseudopodia by surrounding and engulfing the food particle. The engulfed food gets enclosed in a food vacuole inside the cytoplasm. Inside the food vacuole, enzymes digest the food into simpler substances. The nutrients are absorbed into the cytoplasm, and undigested waste is expelled from the cell.

Answer: (a) The compound CaSO₄ · 1/2 H₂O is commonly known as Plaster of Paris. Its chemical name is Calcium sulfate hemihydrate.
(b) Preparation: Plaster of Paris is prepared by heating gypsum (Calcium sulfate dihydrate, CaSO₄ · 2H₂O) at about 150 deg C. On heating, loses water and gives CaSO₄ · 1/2 H₂O.
Chemical equation: CaSO₄ · 2H₂O (gypsum) --heat--> CaSO₄ · 1/2 H₂O (Plaster of Paris) + 1 1/2 H₂O
(c) When water reacts with Plaster of Paris, it absorbs water and forms gypsum (Calcium sulfate dihydrate).
Chemical equation: CaSO₄ · 1/2 H₂O + 1/2 H₂O --> CaSO₄ · 2H₂O
This reaction is called hydration and the product sets hard, which is why Plaster of Paris is used for making casts for broken bones.

Answer: (a) The conversion of ethanol to ethanoic acid is an oxidation reaction because ethanol loses hydrogen atoms and gains oxygen atoms during the process, which is the key characteristic of oxidation in organic chemistry.
(b) The common oxidising agents used for this conversion are acidified potassium dichromate (K₂Cr₂O₇) and alkaline potassium permanganate (KMnO₄).
(c) The chemical equation for this oxidation is:
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
Here, [O] represents oxygen provided by the oxidising agent.
(d) This reaction is different from the burning of ethanol in presence of oxygen because burning is a combustion reaction where ethanol reacts with excess oxygen to produce carbon dioxide and water with the release of heat energy. The equation for burning is:
CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O + energy
In oxidation, ethanol forms ethanoic acid by adding oxygen, whereas in burning, it completely oxidises to carbon dioxide and water releasing energy.

Answer: Budding in Hydra is a type of asexual reproduction where a new individual develops as an outgrowth or bud from the parent organism. First, certain cells called regenerative cells divide repeatedly at one specific site on the Hydra’s body, forming a bud. This bud grows gradually while attached to the parent, developing tentacles and other body parts. When the bud matures fully, it detaches from the parent Hydra and becomes an independent organism. The process involves only one parent and does not require gamete formation. Regenerative cells are responsible for the repeated cell division during budding. The labelled diagram would show the parent Hydra with a small outgrowth on its body, which grows bigger and eventually separates to form a new Hydra.

Answer:

Differences between dominant and recessive traits:
1. Dominant traits are expressed in the organism even if only one dominant allele is present, whereas recessive traits are expressed only when two recessive alleles are present.
2. Dominant traits mask the presence of recessive traits in heterozygous individuals, whereas recessive traits remain hidden if a dominant allele is present.

Percentage of yellow and green seed pea plants in F₂ generation:
In pea plants, yellow seed color (Y) is dominant over green seed color (y). The F₂ generation results from crossing heterozygous plants (Yy × Yy). According to Mendel’s inheritance, the genotype ratio for seed color in F₂ is 1 YY : 2 Yy : 1 yy.
This means 3 out of 4 plants (75%) have yellow seeds (YY or Yy) and 1 out of 4 plants (25%) have green seeds (yy). Specifically, 25% are homozygous yellow (YY), and 25% are homozygous green (yy).

Answer: The power of accommodation of the human eye is the ability of the eye lens to change its focal length by altering its curvature. This change is controlled by the ciliary muscles surrounding the lens. When looking at a nearby object, the ciliary muscles contract, causing the lens to become thicker and more curved, reducing the focal length and allowing the eye to focus on the object. When the object moves farther away, the ciliary muscles relax, making the lens thinner with a longer focal length. As the distance of an object from the eye increases, the image distance (the distance between the lens and the image formed on the retina) remains approximately constant because the image is always focused on the retina. Instead, the eye adjusts the focal length of the lens to form a clear image on the retina. The ciliary muscle is responsible for this adjustment. It changes the shape of the eye lens by contracting or relaxing. When it contracts, the lens thickens to focus on near objects; when it relaxes, the lens becomes thinner for distant vision. This process of adjusting the lens shape to focus on objects at different distances is known as accommodation.

Answer: In the human reproductive system, the seminal vesicles produce a nutrient-rich fluid that mixes with sperm to form semen. This fluid helps in the survival and movement of sperm. The prostate gland produces a fluid that also contributes to semen, which helps protect sperm and aids in their mobility. The oviduct, also called the Fallopian tube, carries the egg from the ovary to the uterus and is the usual site where fertilization occurs if sperm meet the egg. The testis is responsible for producing sperm, the male reproductive cells, and also produces testosterone, the hormone that controls the development of male secondary sexual characteristics and sperm production.

Answer:

A chemical reaction is a process in which one or more substances react to form new substances with different chemical properties. This change in composition is known as a chemical change, which may be recognised by changes such as change in colour, change in state, evolution of gas, or change in temperature.

To demonstrate chemical change due to change in colour, consider the reaction between blue copper sulfate solution and iron nails. When iron nails are dipped in copper sulfate solution, the blue color fades and a brown layer of copper forms on the nails. This change in color shows that a chemical reaction has taken place.

For change in temperature, take the example of mixing dilute hydrochloric acid with sodium hydroxide solution. When these two solutions are mixed, heat is released, and the mixture becomes warmer. This increase in temperature indicates a chemical reaction known as neutralization, confirming that a chemical change has occurred.

Thus, chemical reactions involve transformation of substances resulting in observable changes like color and temperature, proving that new substances are formed during the process.

Answer:

(i) Finding focal length without calculation:
For a convex lens, when the object is placed at infinity (very far away), the image is formed at the focal point. So, the image distance (v) is approximately equal to the focal length (f). By looking at the observation table, the image distance corresponding to a very large object distance will approximate the focal length. Hence, the focal length of the convex lens can be found directly from the image distance when the object distance is very large.

(ii) Incorrect observation and its reason:
In the observation table, if any image distance (v) is negative or inconsistent with lens behavior (for example, a virtual image formed on the same side as the object for a convex lens when not expected), that observation is incorrect. This usually happens when the object is placed within the focal length of the convex lens. The image formed in that case is virtual, erect, and magnified. The ray diagram for this situation shows the rays diverging on the image side and appears to come from behind the lens (same side as the object). One must identify the inconsistent observation from the table by checking if the image is real or virtual and if the image distance is positive or negative.

(iii) Magnification for u = 30 cm:
Magnification (m) is given by the ratio of image distance to object distance, m = v / u. Using the observation table, find the image distance v when the object distance u is 30 cm. Substitute values in the formula to calculate magnification. The magnification will indicate the size and orientation of the image formed.