Secondary School Science Exam 2025

Iodised salt is recommended because it is a source of iodine, which is an essential nutrient for the thyroid gland to produce thyroid hormones. Lack of iodine can cause goitre, a condition where the thyroid gland swells.

A precipitation reaction is a chemical reaction that results in the formation of a solid precipitate from a solution. For example, when solutions of silver nitrate (AgNO3) and sodium chloride (NaCl) are mixed, a white precipitate of silver chloride (AgCl) forms.

Hydrochloric acid (HCl) secreted in the stomach helps in digestion by activating digestive enzymes, breaking down food, and killing harmful bacteria. It also helps in the absorption of nutrients.

Saturated hydrocarbons have only single bonds between carbon atoms, while unsaturated hydrocarbons have at least one double or triple bond between carbon atoms. For example, methane (CH4) is a saturated hydrocarbon, while ethene (C2H4) is an unsaturated hydrocarbon.

A combination reaction is a chemical reaction where two or more reactants combine to form a single product. An exothermic combination reaction is one that releases heat, for example, the reaction between sodium and chlorine to form sodium chloride (NaCl) produces heat.

Vinegar is a dilute solution of acetic acid in water (around 5%). It's commonly used as a condiment, cleaning agent, and preservative due to its acidic properties.

Esterification is a chemical reaction that involves the formation of an ester. It happens when an alcohol reacts with a carboxylic acid in the presence of a catalyst, e.g., sulfuric acid (H2SO4), with the elimination of water. The equation is: Alcohol + Carboxylic Acid --> Ester + Water

Drip irrigation is a water conservation technique where water is delivered directly to the roots of plants through a network of pipes and emitters. This method minimizes water loss through evaporation and runoff.

A chemical bond is a force of attraction that holds atoms together in a molecule. Bonds can be ionic, covalent, or metallic, depending on the type of atoms involved and how electrons are shared or transferred.

Carbon has many amazing properties, including the ability to form long chains and rings with itself and other elements. Carbon can form single, double, and triple bonds, creating a wide range of structures. This unique ability leads to an incredible number of possible compounds.

Digestive juices in the stomach, primarily pepsin and hydrochloric acid, break down protein into smaller peptides that can be absorbed by the body. They also kill harmful bacteria.

The main respiratory organs of fish are gills, earthworms use moist skin, mosquitos have trachea, and humans have lungs.

Double circulation means that blood passes through the heart twice during one complete cycle. Humans have a double circulatory system with a pulmonary circulation (heart to lungs) and systemic circulation (heart to body).

Dialysis is a process that filters the blood of a person with kidney failure. A machine performs the function of kidneys, removing waste products and excess fluids from the blood.

The nervous system is responsible for coordinating all body functions, receiving and transmitting information, controlling voluntary and involuntary actions, and maintaining homeostasis.

Common sexually transmitted diseases in humans include HIV/AIDS, syphilis, gonorrhea, chlamydia, and genital herpes. These diseases are spread through sexual contact.

Homologous organs have the same basic structure but perform different functions due to adaptation. For example, the forelimbs of humans, whales, and birds. Analogous organs perform the same function but have a different structure, like the wings of birds and insects.

Eliminating all organisms at a trophic level would have a cascading effect on the entire ecosystem. The absence of prey at a particular trophic level would lead to the decline of predator populations that depend on them. This can disrupt the balance of the food web and lead to an imbalance in the ecosystem.

Respiration in fish occurs through gills, specialized organs rich in blood capillaries. Water flows over the gills, and dissolved oxygen diffuses into the blood while carbon dioxide diffuses out.

Energy flows unidirectionally through an ecosystem, from producers (plants) to consumers (animals) and decomposers (microorganisms). Energy is lost as heat at each trophic level.

A dynamo is a device that converts mechanical energy into electrical energy using the principle of electromagnetic induction. It works on the principle that a changing magnetic field induces an electric current in a conductor. The turning of a coil within a magnetic field produces an electric current.

Convex lenses are thicker in the middle than at the edges. They converge parallel light rays to a single point called the focal point. Concave lenses are thinner in the middle than at the edges. They diverge parallel light rays as if they originated from a single point (focal point) behind the lens.

Convex lenses focus parallel light rays at a point called the focal point. This is caused by the refraction of light as it passes through the lens. In contrast, concave lenses cause parallel light rays to diverge as if they are coming from a focal point behind the lens. This diverging effect happens due to the curvature of the lens that bends light rays outwards.

Convex lenses form a real image when the object is placed beyond the focal point. The image is inverted and smaller than the object. Concave lenses always form virtual and upright images, smaller than the object. The image formed by a concave lens appears to be behind the lens.

Convex lenses converge parallel light rays at a point called the focal point. This is caused by the refraction of light as it passes through the lens, bending the light rays towards the center.

Concave lenses cause parallel light rays to diverge as if they are coming from a focal point behind the lens. This diverging effect happens due to the curvature of the lens, bending the light rays outwards.

Convex lenses can form a real image when the object is placed beyond the focal point. The image formed is inverted and smaller than the object. This happens because the refracted rays converge to form the image.

Concave lenses always form virtual and upright images, smaller than the object. This happens because the diverging rays create an image that appears behind the lens.