chem all definitions

Anion

A negatively charged ion. Anion

Atom

The smallest part of an element that can exist. All substances are made up of atoms. Atoms contain a positively charged nucleus surrounded by negatively charged electrons. Atom

Atomic nucleus

Positively charged object composed of protons and neutrons at the centre of every atom with one or more electrons orbiting it. Atomic nucleus

Atomic number

The number of protons in the nucleus. Atomic number

Cation

A positively charged ion. Cation

Chromatography

A process used to separate substances in a mixture. It involves a mobile phase and a stationary phase. Chromatography

Compound

A substance made up of two or more types of atoms chemically combined together. Compound

Conductor

A material that contains charged particles which are free to move to carry electrical or thermal energy. Conductor

Covalent bond

A shared pair of electrons between two non-metals. Covalent bond

Crystallisation

A separation technique to obtain soluble solids from solutions. The process involves heating the solution until crystals start to form, leaving the solution to cool and then filtering the formed crystals from the solution. Crystallisation

Diamond

A giant covalent structure which is made up of carbon atoms, each of which form four covalent bonds with four other carbon atoms. Diamond

Electron

Negatively charged subatomic particle which orbits the nucleus at various energy levels. Very small relative mass (negligible). Electron

Electron shell

Different energy levels in atoms, occupied by electrons. Electrostatic forces

Empirical formula

The simplest whole number ratio of atoms of each element in a compound. Empirical formula

Filtration

A separation technique used to separate an insoluble solid from a solution. Filtration

Fractional distillation

A process used to separate a mixture of liquids. The liquids have different boiling points so can be separated into different fractions within a fractionating column. Fractional distillation

Fullerenes

Molecules of carbon atoms with hollow shapes. The structures are based on hexagonal rings of carbon atoms but they may also contain rings with five or seven carbon atoms. Examples include Graphene and C60. Fullerenes

Gas

The state of matter where the particles have the most energy. The particles in a gas are relatively spread out and move randomly in all directions. Gas

Giant covalent structure

A molecular structure containing many atoms covalently bonded together. The strong covalent bonds mean that giant covalent structures have high melting points. Giant covalent structure

Graphite

A giant covalent structure which is made up of carbon atoms each of which form three covalent bonds with three other carbon atoms, forming layers of hexagonal rings which have no covalent bonds between the layers. There is one delocalised electron per carbon atom which is free to move to carry charge. Graphite

Group (periodic table)

A column of the periodic table. Elements in the same group have similar chemical properties. Group (periodic table)

Inert

Unreactive. Inert electrodes are used in electrolysis to prevent the electrode material affecting the reactions. The noble gases are also described as being inert. Inert

Intermolecular forces

The forces which exist between molecules. The strength of the intermolecular forces impact physical properties like boiling/melting point. Intermolecular forces

Ion

An atom or molecule with an electric charge due to the loss or gain of electrons. Ion

Ionic bond

The bond formed between the oppositely charged ions when a metal atom loses electron(s) to form a positively charged ion and a non-metal gains these electron(s) to form a negatively charged ion. Ionic bond

Ionic compound

Chemical compound formed of oppositely charged ions, held together by strong electrostatic forces. Ionic compound

Isotope

Atoms of the same element with the same number of protons but a different number of neutrons. Isotope

Lattice

A repeating regular arrangement of atoms/ions/molecules. This arrangement occurs in crystal structures. Lattice

Liquid

The state of matter where the particles are arranged randomly and close together. The particles are able to move past each other. Liquid

Malleable

Capable of being deformed and moulded into various shapes. Metals are malleable since the uniform layers of atoms can slide over each other. Malleable

Mass number

The total number of protons and neutrons in the nucleus. Mass number

Metallic bond

The bonds present in metals between the positive metal ions and negatively charged delocalised electrons. Metallic bond

Metals

Elements that react to form positive ions. Found to the left and towards the bottom of the periodic table. Metals

Mixture

Contains at least two different elements or compounds which are not chemically bonded together. Mixtures may melt or boil over a range of temperatures. Mixture

Mobile phase

The fluid (gas or liquid) which moves through the chromatography system, carrying the mixture which is to be separated. Mobile phase

Mole

Standard unit for measuring amounts of chemicals. The mole is the unit for amount of substance. The symbol for the unit mole is mol. Mole

Molecular formula

The actual ratio of atoms of each element present in a compound. Molecule

Noble gases

The elements in Group 0 of the periodic table. They have a stable full outer shell of electrons which makes them very unreactive. Noble gases

Neutron

Neutral subatomic particle present in the nucleus of the atom. Relative mass of 1. Neutron

Non-metals

Elements that react to form negative ions. Found towards the right and top of the periodic table. Non-metals

Oxidation

A reaction involving the gain of oxygen. Oxidation is the loss of electrons. Oxidation

Paper chromatography

A type of chromatography which uses paper as the stationary phase and a solvent as the mobile phase. The solvent carries the mixture up the paper where the substances in the mixture then separate, depending on how soluble they are in the mobile phase. Paper chromatography

Percentage yield

The percentage ratio of the actual yield of product from a reaction compared with the theoretical yield. Percentage yield

Period (periodic table)

A row of the periodic table. Elements in the same period have the same number of electron shells. Period (periodic table)

Periodic table

Table of elements arranged in order of increasing atomic number and such that elements with similar properties are in the same column (group). Periodic table

Proton

Positively charged subatomic particle present in the nucleus of the atom. Relative mass of 1. Proton

Pure substance

The chemistry definition of a pure substance is a substance which contains only one compound or element. Pure substances have fixed melting and boiling points. Pure substance

Reduction

A reaction involving the loss of oxygen. Reduction is the gain of electrons. Reduction

Relative atomic mass

An average value that takes account of the abundance of the isotopes of the element. Relative atomic mass

Relative formula mass

The sum of the relative atomic masses of the atoms in the numbers shown in the formula. It is numerically equal to the mass of one mole of a substance in grams. Relative formula mass

Rf value

A value used in chromatography which is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent. It can be used to identify substances within a mixture. Rf value

Saturated solution

A solution which contains the maximum amount of solute that can be dissolved in the solvent. If any more solute is added, it will not dissolve in the solution. Saturated solution

Simple distillation

A separation technique used to separate a liquid from a solution. The solution is heated so that only the liquid with the lowest boiling point evaporates. This gas is then condensed in a condenser before being collected as a liquid. Simple distillation

Simple molecules

Molecules containing a fixed number of atoms covalently bonded together. Simple molecular structures have low boiling points since they have weak intermolecular forces which are easy to overcome. Simple molecules

Solid

The state of matter where the particles hold a regular arrangement and have the least amount of energy. Solid

Solute

A substance which is dissolved in a solvent to create a solution. Solution

Solubility

The maximum mass of a solute which can be dissolved in 100 g of solvent. Solubility

State symbols

The symbols used in chemical equations to denote the states of the chemicals reacting

Stationary phase

The nonmoving phase which the mobile phase passes over during chromatography. Stationary phase

Subatomic particles

Particles smaller than an atom. Protons, neutrons and electrons are the three most common subatomic particles. Subatomic particles

Water of crystallisation

Water molecules forming part of a crystal structure. Water of crystallisation

Acid

Produces hydrogen ions (H+) in aqueous solutions. They have a pH range between 0 and 7. Acids are known as proton donors. Acid

Alkali

Produces hydroxide ions (OH-) in aqueous solutions. They have a pH range between 7 and 14. Alkalis are soluble bases. Alkali

Alkali metals

The elements in Group 1 of the periodic table. They are typically soft and have relatively low melting points. Alkali metals

Base

A substance which reacts with an acid in a neutralisation reaction to form a salt and water only. Bases are known as proton acceptors. Base

Climate change

A change in global climate patterns largely believed to be caused by the increase in concentration of carbon dioxide in the atmosphere. Climate change

Combustion

The burning of a substance in oxygen. If there is sufficient oxygen present for the substance to burn then it is called complete combustion. Energy is transferred to the surroundings as heat and light. Combustion

Corrosion

The destruction of materials by chemical reactions with substances in the environment, e.g. rusting occurs when iron is exposed to water and oxygen. Corrosion

Displacement

A chemical reaction in which a more reactive element displaces a less reactive element from its compound. Displacement

Flame test

Qualitative test used to identify metal ions (cations). Carried out by inserting a nichrome wire loop with the unknown compound on into a flame and observing the colour. Flame test

Galvanise

A process used to protect against corrosion by coating the metal with a protective layer of zinc. Galvanise

Greenhouse effect

The increase in the temperature of the Earth’s atmosphere due to the greenhouse gases in the atmosphere trapping infra-red radiation from the surface. Greenhouse effect

Greenhouse gases

Gases in the atmosphere which maintain temperatures on Earth high enough to support life. Greenhouse gases include water vapour, carbon dioxide and methane. Greenhouse gases

Halides

The ions formed by halogen atoms by gaining an electron. They have a 1- charge. E.g. Cl-, Br- and I-. Halides

Halogens

The elements in Group 7 of the periodic table. The halogens gain an electron to form halide ions with a 1- charge. Down the group the halogens get less reactive and have higher melting and boiling points. Halogens

Indicators

A chemical used to distinguish between acidic and alkaline solutions. Examples include phenolphthalein, litmus and methyl orange. They are also used in titration reactions to indicate the end point of the reaction by a clear colour change Indicators

Neutralisation

The reaction in which an acid and a base react to form a salt and water. Neutralisation

Oxidation

A reaction involving the gain of oxygen. Oxidation is the loss of electrons. Oxidation

Oxidising agent

A species which brings about oxidation by gaining electrons. The oxidising agent is itself reduced. Oxidising agent

pH scale

A measure of the acidity or alkalinity of a solution ranging from 0 to 14. The pH can be measured using universal indicator or a pH probe. pH scale

Reactivity series

A series in which metals are arranged in order of their reactivity. This can be used to predict products from reactions. Reactivity series

Redox reaction

A reaction in which both oxidation and reduction occur simultaneously. Redox reaction

Reducing agent

A species which brings about reduction by losing electrons. The reducing agent is itself oxidised. Reducing agent

Reduction

A reaction involving the loss of oxygen. Reduction is the gain of electrons. Reduction

Rust

A reddish coating of iron oxide which forms on iron when it undergoes oxidation. Iron rusts when it is in the presence of oxygen and water. Rust

Sacrificial protection

The protection of iron or steel against corrosion by using a more reactive metal. Zinc is often used as a sacrificial metal. Sacrificial protection creates a physical barrier to oxygen and water, preventing corrosion of the metal. Sacrificial protection

Thermal decomposition

The breaking down of a compound by heating. Thermal decomposition

Universal indicator

A mixture of dyes that changes colour gradually over a range of pH and is used in testing for acids and alkalis. Universal indicator

Activation energy

The minimum amount of energy that particles must collide with to react. Activation energy

Calorimetry

The process used to measure the amount of heat energy released or absorbed during a chemical reaction. Calorimetry

Catalyst

Increases the rate of reaction by providing a different reaction pathway with a lower activation energy. They are not used up during the reaction. Catalyst

Collision theory

Chemical reactions only occur when colliding particles collide with the correct orientation and with sufficient energy (activation energy). Collision theory

Displacement

A chemical reaction in which a more reactive element displaces a less reactive element from its compound. Displacement

Effect of concentration on reaction rate

Increasing the concentration of reactants in solution means the reacting particles will be closer together. This means they will collide more often so there will be a higher rate of successful collisions and a faster rate of reaction. Effect of concentration on reaction rate

Effect of pressure on reaction rate

Increasing the pressure of gaseous reactants means the reacting particles will be closer together. This means they will collide more often so there will be a higher rate of successful collisions and a faster rate of reaction. Effect of pressure on reaction rate

Effect of surface area on reaction rate

Increasing the surface area of the reactants means there are more exposed reacting particles. This means there are more frequent successful collisions, so the rate of reaction increases. Effect of surface area on reaction rate

Effect of temperature on reaction rate

Increasing the temperature means the particles will have more kinetic energy and so will move faster. If the molecules are moving faster they will collide more often and, since they’ve gained kinetic energy, a larger proportion of the particles will have at least the activation energy. For both these reasons the rate of reaction increases. Effect of temperature on reaction rate

Endothermic reaction

A reaction in which energy is taken in from the surroundings so the temperature of the surroundings decreases. *[Bond breaking is an endothermic process.] Endothermic reaction

Enthalpy change

The amount of heat energy given out or absorbed during a reaction. Enthalpy change

Exothermic reaction

A reaction in which energy is transferred to the surroundings so the temperature of the surroundings increases. *[Bond making is an exothermic process.] Exothermic reaction