Haryana BSEH Secondary Science key terms

The number of hydrogen atoms or an equivalent combining capacity that one atom of an element can bond with. It explains why elements combine in fixed whole-number ratios to form compounds.

A short symbolic way of writing a compound using element symbols and small subscript numbers showing how many atoms of each element are present, such as H2O for water.

A chemical equation in which the number of atoms of each element is equal on the reactant and product sides, obeying conservation of mass. Coefficients are adjusted, never subscripts.

The principle that matter is neither created nor destroyed in a chemical reaction, so total mass of reactants equals total mass of products in a closed system.

An atom or group of atoms carrying a positive or negative electric charge because it has gained or lost electrons. Positive ions are cations; negative ions are anions.

A substance that releases hydrogen ions (H+) when dissolved in water, turns blue litmus red, and reacts with bases to form salt and water.

A substance that reacts with an acid to give salt and water, usually releasing hydroxide ions (OH-) in water. Soluble bases are called alkalis and turn red litmus blue.

A 0 to 14 scale measuring how acidic or basic a solution is. Values below 7 are acidic, 7 is neutral, and above 7 is basic; each unit means a tenfold change in hydrogen ion concentration.

The reaction between an acid and a base that produces salt and water, for example hydrochloric acid with sodium hydroxide giving sodium chloride and water.

A dye that changes colour according to whether a solution is acidic or basic. Litmus, methyl orange, phenolphthalein, and turmeric are common natural or laboratory examples.

A reaction in which a substance gains oxygen or loses electrons or hydrogen. Rusting of iron and burning of fuels are everyday examples of oxidation.

A reaction in which a substance loses oxygen or gains electrons or hydrogen. Oxidation and reduction always occur together in a redox reaction.

The gradual wearing away of a metal by chemical action with substances in its surroundings, such as moisture, oxygen, or acids. Rusting of iron is the commonest example.

Coating iron or steel with a thin layer of zinc to protect it from rusting, because zinc corrodes in preference to iron and seals the surface.

The process by which green plants use sunlight, water, and carbon dioxide to make glucose and release oxygen, with chlorophyll in chloroplasts trapping the light energy.

The green pigment inside chloroplasts that absorbs mainly red and blue light and converts light energy into chemical energy during photosynthesis.

Tiny pores mainly on leaf surfaces, guarded by two guard cells, that allow carbon dioxide to enter and oxygen and water vapour to leave the plant.

The loss of water vapour from aerial parts of a plant, mostly through stomata. It pulls water and dissolved minerals upward through the xylem.

A plant tissue made of hollow, dead vessel and tracheid cells that conducts water and dissolved minerals upward from roots to stems and leaves.

A living plant tissue of sieve tubes and companion cells that transports sugars made in leaves to growing and storage parts of the plant in both directions.

The breakdown of glucose inside cells, with or without oxygen, to release energy for life activities. Aerobic respiration uses oxygen; anaerobic respiration does not.

A fast, automatic, involuntary response to a stimulus, such as pulling the hand away from a hot object, completed through a reflex arc without waiting for the brain's decision.

The structural and functional unit of the nervous system, consisting of a cell body, dendrites, and an axon, which carries electrical impulses between parts of the body.

The tiny gap between the end of one neuron and the next, across which a chemical messenger diffuses to pass the nerve impulse onward.

A chemical messenger secreted by an endocrine gland directly into the blood, which travels to a target organ and controls its activity, such as insulin controlling blood sugar.

The bending of light as it passes from one transparent medium into another because its speed changes, for example when light enters water or a glass slab.

The ratio of the speed of light in vacuum to its speed in a given medium. It measures how strongly a medium bends light entering it.

A lens thicker at the centre than at the edges that converges a parallel beam of light to a real focus. It forms real or virtual images depending on object distance.

A lens thinner at the centre than at the edges that diverges a parallel beam of light. It always forms a virtual, erect, diminished image of a real object.

The distance between the optical centre of a lens or the pole of a mirror and its principal focus, measured along the principal axis.

At constant temperature, the current through a metallic conductor is directly proportional to the potential difference across its ends, giving V = IR.

The property of a conductor that opposes the flow of electric current, measured in ohms. It depends on length, area of cross-section, material, and temperature.

A component made of a material of known resistance, connected in a circuit to control current or to divide potential difference between parts of the circuit.

A circuit in which components are joined end to end so the same current flows through each and the total resistance is the sum of individual resistances.

A circuit in which components are connected across the same two points so each has the full supply voltage and the total current is the sum of branch currents.

The rate at which electrical energy is transferred by a circuit, equal to voltage multiplied by current and measured in watts.

The decomposition of a chemical compound in solution or molten state by passing an electric current through it, producing chemical change at the electrodes.

A safety device containing a thin wire of low melting point that melts and breaks the circuit when current exceeds a safe value, protecting appliances from damage.

A region of the upper atmosphere rich in ozone that absorbs most harmful ultraviolet radiation from the Sun, shielding living things on Earth's surface.

The trapping of heat near Earth's surface by gases such as carbon dioxide, methane, and water vapour, which let sunlight in but absorb outgoing infrared radiation.

The contamination of air by harmful gases, smoke, and particulate matter from vehicles, industries, and burning of fuels, which damages health and the environment.

Waste material that can be broken down into simpler harmless substances by the action of microorganisms, such as vegetable peels, paper, and cotton cloth.

A sequence of organisms in an ecosystem in which each is eaten by the next, showing how energy and matter pass from producers to consumers and decomposers.

A diet containing all nutrients in the right amounts and proportions, including carbohydrates, proteins, fats, vitamins, minerals, roughage, and water, for good health.

A disorder caused by lack of a particular nutrient in the diet over time, such as scurvy from vitamin C shortage or anaemia from iron shortage.

Poor health resulting from an unbalanced diet, which may involve lack of nutrients or excess of some foods, affecting growth and resistance to infection.

Giving a weakened or inactive form of a disease-causing agent so the body produces antibodies and develops immunity without suffering the full disease.

A microorganism or other agent that causes disease in a host, including certain bacteria, viruses, fungi, and protozoa that enter the body and multiply.

A standard setup in which the factor being tested is not changed, used for comparison so that the effect of the tested variable can be judged fairly.

Any factor in an experiment that can change. The independent variable is deliberately altered, the dependent variable is measured, and others are kept constant.

A testable, tentative explanation or prediction about a scientific question that is accepted or rejected on the basis of experimental evidence.

The curved upper surface of a liquid in a narrow container. It is read at eye level, with the lowest point of the curve taken for most laboratory liquids.

The digits in a measured value that carry meaningful information about its precision, used to report results without claiming false accuracy.

The internationally agreed base units of measurement, including metre, kilogram, second, ampere, kelvin, mole, and candela, used to express physical quantities consistently.

The ability to read, understand, and reason about scientific information and evidence well enough to make informed personal and public decisions.