Assam ASSEB HSLC General Science key terms

The direct change of a solid into its vapour on heating, and back to solid on cooling, without passing through the liquid state. Camphor, iodine and naphthalene show this behaviour.

The reverse of sublimation: a vapour changes directly into a solid on cooling, skipping the liquid stage. Frost forming on cold surfaces is a familiar natural example.

The three common physical forms of matter: solid, liquid and gas. They differ in the strength of attraction between particles, their spacing, and how freely the particles move.

Heat energy absorbed or released when a substance changes state at constant temperature, without any change in temperature being recorded. It is called latent because the heat appears hidden.

The escape of fast-moving molecules from the surface of a liquid below its boiling point. It causes cooling, increases with temperature and surface area, and is slowed by humidity.

In a solution, the solute is the substance that dissolves and is present in smaller amount; the solvent is the medium that dissolves it and is present in larger amount. Water is the commonest solvent.

A solution is a homogeneous mixture with particles too small to see or settle. A suspension is a heterogeneous mixture whose larger particles are visible and settle on standing.

The maximum mass of a solute that dissolves in a fixed mass of solvent at a given temperature to give a saturated solution. It usually rises with temperature for solids in water.

Separating an insoluble solid from a liquid by pouring the mixture through filter paper, which traps the solid as residue while the clear liquid passes through as filtrate.

Separating a liquid from dissolved solids or from other liquids by boiling it and condensing the vapour back to liquid. Fractional distillation separates liquids with different boiling points.

Purifying a solid by dissolving it in a hot solvent, then cooling so that pure crystals form while impurities stay in the mother liquor. Common for sugar and copper sulphate.

The combining capacity of an element or radical, equal to the number of electrons it gains, loses or shares to complete its outer shell. It fixes the formula of a compound.

A charged particle formed when an atom or group of atoms gains or loses electrons. Cations are positively charged because electrons are lost; anions are negatively charged because electrons are gained.

A symbolic representation of a compound showing the symbols of its elements and the number of atoms of each, using subscripts. It is built by crossing over valencies.

A chemical bond formed by the sharing of one or more pairs of electrons between atoms, usually non-metals. Shared pairs hold the atoms together and give molecules like water and methane.

The electrostatic attraction between oppositely charged ions formed when a metal transfers electrons to a non-metal. Sodium chloride, held by sodium and chloride ions, is a typical example.

A dye that changes colour over a particular pH range and so shows whether a solution is acidic or basic. Litmus, methyl orange and phenolphthalein are common examples.

A scale from 0 to 14 expressing the concentration of hydrogen ions in a solution. Values below 7 are acidic, 7 is neutral, and above 7 is basic or alkaline.

The reaction between an acid and a base that produces a salt and water, with the solution moving towards pH 7. Antacids and soil treatment use this reaction.

The compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion during neutralisation. Examples include sodium chloride, copper sulphate and calcium carbonate.

A list of metals arranged in order of decreasing chemical reactivity, from potassium and sodium down to gold. It predicts displacement reactions and the method used to extract a metal.

A reaction in which a more reactive element pushes a less reactive element out of its compound. Zinc displacing copper from copper sulphate solution is a standard example.

The slow attack on a metal by moisture, oxygen and other substances in the environment. Rusting of iron, which needs both air and water, is the most familiar case.

A homogeneous mixture of a metal with one or more other elements, made to improve hardness, strength or resistance to corrosion. Steel, brass and bronze are common alloys.

A family of carbon compounds with the same general formula and functional group, in which successive members differ by one CH2 unit and show gradually changing properties.

The atom or group of atoms in an organic molecule that determines its characteristic chemical properties. Examples are hydroxyl in alcohols and carboxyl in carboxylic acids.

A compound containing only carbon and hydrogen. Saturated hydrocarbons have single bonds only; unsaturated ones contain double or triple bonds and are more reactive.

A chemical reaction in which a substance burns in oxygen, releasing heat and often light. Complete combustion of hydrocarbons gives carbon dioxide and water.

A specialised structure inside a cell that performs a particular job, such as the nucleus, mitochondrion, chloroplast or ribosome. Some are bound by membranes, others are not.

Prokaryotic cells, such as bacteria, have no membrane-bound nucleus or organelles. Eukaryotic cells, found in plants, animals and fungi, have a true nucleus and membrane-bound organelles.

Cell division producing two genetically identical daughter cells with the same chromosome number as the parent. It is responsible for growth, repair and asexual reproduction.

A region of actively dividing undifferentiated plant cells found at root and shoot tips and in the cambium. Growth in length and thickness of the plant depends on it.

Stomata are tiny pores, mainly on leaves, that allow gas exchange. Transpiration is the loss of water vapour through them, which pulls water up the plant.

The two transport tissues of vascular plants. Xylem carries water and dissolved minerals upward from the roots; phloem carries prepared food in either direction.

A rapid, automatic response to a stimulus that does not involve conscious thought, such as blinking or withdrawing a hand from a hot object. It is mediated by a reflex arc.

A chemical messenger secreted by an endocrine gland into the blood, which acts on a specific target organ. Insulin, thyroxine and adrenaline are examples.

A dominant trait appears in an organism even when only one copy of its allele is present. A recessive trait appears only when both alleles are recessive.

The ability of the body to resist a particular disease. It may be innate, or acquired through previous infection or vaccination, which primes the immune system to respond quickly.

A self-regulating unit in which living organisms interact with one another and with their non-living surroundings, exchanging energy and matter. A pond or a forest is an example.

The variety of living organisms at genetic, species and ecosystem levels. High biodiversity supports ecosystem stability; it is threatened by habitat loss, pollution and overexploitation.

The product of an object's mass and its velocity, a vector quantity measured in kilogram metres per second. Total momentum of an isolated system is conserved in collisions.

The rate of change of velocity with time, a vector quantity measured in metres per second squared. A body accelerates when its speed, direction, or both change.

Three laws: a body stays at rest or in uniform motion unless a net force acts; force equals mass times acceleration; and every action has an equal and opposite reaction.

The upward force exerted by a fluid on a body immersed in it, equal to the weight of fluid displaced. It explains why objects float or appear lighter in water.

The force acting normally per unit area of a surface, measured in pascals. For a fluid at rest, pressure increases with depth and acts equally in all directions.

The statement that the net work done on a body equals the change in its kinetic energy. It links force, displacement and speed without needing the details of the path.

The bending of light as it passes from one transparent medium to another of different optical density, caused by a change in speed. It explains lenses and apparent depth.

The bouncing back of light from a surface. The angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal lie in one plane.

A lens is a transparent refracting medium with curved surfaces that converges or diverges light. Focal length is the distance from the lens to the point where parallel rays meet.

At constant temperature, the current through a conductor is directly proportional to the potential difference across it. The constant ratio is the resistance, measured in ohms.

The opposition offered by a conductor to the flow of electric current, measured in ohms. It increases with length and temperature and decreases with greater cross-sectional area.