CBSE EXAMINATION PAPER-2024

Answer: When magnesium ribbon burns in air, it reacts with oxygen forming a white ash called magnesium oxide. The chemical equation for the reaction is: Magnesium + Oxygen → Magnesium oxide or Mg + O₂ → MgO. This is a combination reaction, where two elements combine to form one compound. It is also an oxidation reaction because magnesium gains oxygen atoms during the process.

Answer: Auxins are synthesized mainly at the shoot tips of plants. They promote phototropism by diffusing towards the shaded side of the shoot when light comes from one direction. This causes a higher concentration of auxin on the side away from light, stimulating the cells there to elongate more. As a result, the shoot bends towards the light source, allowing the plant to maximize light absorption for photosynthesis.

Answer: Mendel used several pairs of contrasting characters in garden pea plants to study inheritance. Two such pairs are: 1. Seed Shape: Round seeds are dominant (represented as R), while wrinkled seeds are recessive (represented as r). 2. Plant Height: Tall plants are dominant (T), and short plants are recessive (t). These contrasting characters helped Mendel analyze how traits are passed from one generation to the next.

Answer: In humans, the mother has two X chromosomes (XX), so eggs always carry an X chromosome. The father has one X and one Y chromosome (XY), so sperm can carry either an X or a Y chromosome. When an X-carrying sperm fertilizes the egg (X), the child will be female (XX). When a Y-carrying sperm fertilizes the egg (X), the child will be male (XY). Since sperm carrying X or Y chromosomes are produced in equal amounts, the chance of having a male or female child is 50% each.

Answer: A person is said to be suffering from hypermetropia (or farsightedness) when they find it difficult to see nearby objects clearly but can see distant objects clearly. This happens because the image of nearby objects is formed behind the retina. Two main causes of this defect are: (1) the focal length of the eye lens is too long, and (2) the eyeball is smaller than normal. Hypermetropia can be corrected by using convex lenses, which help focus the image correctly onto the retina.

Answer: A straight conductor carrying current creates magnetic field lines that form concentric circles around it. The direction of the magnetic field lines can be determined by the right-hand thumb rule: if the right thumb points along the current direction, the curled fingers indicate the magnetic field direction. In the labelled diagram, the straight wire is drawn vertically with an arrow showing current flowing upwards. Around the wire, concentric circles are drawn with arrows indicating magnetic field direction as anti-clockwise when viewed from above. This illustrates the circular pattern of magnetic field lines around the straight current-carrying conductor.

Answer: The device used to magnetise a piece of magnetic material is called an electromagnet. It consists of a coil of insulated copper wire wrapped around a soft iron cylinder. When an electric current passes through the coil, it produces a magnetic field that magnetises the soft iron core. This method is commonly used to create temporary magnets. A labelled diagram would show a battery connected to the coil, the coil wrapped around the soft iron cylinder, and a switch to control the current.

Answer: Decomposers are organisms like fungi and bacteria that break down dead plants, animals, and waste products into simpler inorganic substances. This process is called decomposition. These nutrients are then released back into the soil and used by plants for growth. In the absence of decomposers, dead organisms and waste would accumulate, leading to a buildup of garbage in the ecosystem. Moreover, the natural replenishment of soil nutrients would not take place, affecting plant growth and disrupting the entire food chain.

Answer: When we water the soil, the water is first absorbed by the roots of the plant through a process called osmosis. From the roots, water moves upward through special tubes called xylem vessels that run through the stem and branches. This upward movement of water is helped by root pressure and the pull created by transpiration, which is the evaporation of water from tiny openings called stomata in the leaves. As water evaporates from the leaves, it pulls more water from the roots to the leaves, ensuring a continuous flow. This process not only transports water but also helps in the movement of minerals and keeps the plant cool. Thus, water travels from the soil through the roots, moves up through the xylem, and finally reaches the topmost leaves.

Answer: The phenomenon is called the Tyndall effect. It occurs when a beam of light passes through a colloidal solution or a medium containing fine particles, such as smoke or dust in the air. These tiny particles scatter the light in different directions, making the path of the beam visible to our eyes. For example, when sunlight enters a smoke-filled room through a small hole, the scattered light by the smoke particles makes the beam's path clearly visible. The colour of the light we receive depends on the size of the particles in the medium. Larger particles scatter longer wavelengths of light, like red or orange, while smaller particles mostly scatter shorter wavelengths like blue. This is why the sky appears blue, as the smaller particles in the atmosphere scatter blue light more effectively.

Answer: An electric fuse is a safety device used in a domestic circuit to protect electrical appliances and wiring from damage due to excessive current. It contains a thin wire that melts when the current exceeds a specific limit (called the rated current), thereby breaking the circuit and stopping the flow of electricity. This prevents overheating, short circuits, or electrical fires. For the electric heater rated at 3 kW and 220 V, the current drawn can be calculated as Current = Power / Voltage = 3000 W / 220 V = 13.64 A approximately. Since the circuit has a rating of only 5 A, the current drawn by the heater is much higher than the rated current of the circuit. When the heater is switched ON, the current will exceed 5 A, causing the fuse wire to heat up and melt quickly. This will break the circuit, preventing any damage to the wiring or appliances and avoiding hazards. Hence, the fuse acts as a protective device and the heater should not be used on a 5 A circuit.

Answer:

The process used to prepare sodium hydroxide from sodium chloride is called the chlor-alkali process. In this process, an aqueous solution of sodium chloride, known as brine, undergoes electrolysis. When electric current passes through the brine, it decomposes to form sodium hydroxide, chlorine gas, and hydrogen gas. The overall reaction is:
2NaCl (aqueous) + 2H₂O (liquid) → 2NaOH (aqueous) + Cl₂ (gas) + H₂ (gas).

In this electrolysis process, chlorine gas (Cl₂) is released at the anode (the positive electrode), and hydrogen gas (H₂) is released at the cathode (the negative electrode). Sodium hydroxide solution is formed near the cathode.

The two gases produced are X = Chlorine (Cl₂) at the anode and Y = Hydrogen (H₂) at the cathode.

When chlorine gas reacts with dry calcium hydroxide (Ca(OH)₂), it produces calcium oxychloride (called bleaching powder), represented as compound 'Z'. This compound is widely used for disinfecting water in water treatment plants and also in the textile industry.

The chemical reaction for the formation of bleaching powder is:
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.

Answer:

A convex mirror always diverges light rays, meaning the reflected rays spread out. When drawing ray diagrams for convex mirrors, we need to consider that the focal point is virtual and located behind the mirror.

Case 1: Ray parallel to the principal axis
When a ray of light is incident parallel to the principal axis on a convex mirror, it reflects off the mirror such that the reflected ray appears to come from the principal focus (F) located behind the mirror. To represent this in a diagram, draw the incident ray parallel to the principal axis, then the reflected ray diverging outwards. By extending the reflected ray backward behind the mirror, it should pass through the focal point (F).

Case 2: Ray directed towards the principal focus
If the incident ray is directed towards the principal focus (F) behind the mirror, it reflects from the convex surface and travels parallel to the principal axis. In the diagram, you will show the incident ray heading towards the focal point behind the mirror, and after reflection, the ray runs parallel to the principal axis.

Thus, in both cases, the law of reflection applies: angle of incidence equals angle of reflection, and the behavior of rays for a convex mirror shows that parallel rays appear to diverge from the virtual focus behind the mirror.