Cambridge IGCSE Chemistry: States of Matter and Reactions

Matter

A physical substance that occupies space and has mass.

States of matter

The three categories of matter: solid, liquid, and gas.

Solid

A state of matter with fixed volume and fixed shape.

Liquid

A state of matter with fixed volume but indefinite shape, taking the shape of its container.

Gas

A state of matter with indefinite volume and shape, taking the shape of its container.

Compressibility of solids

Solids cannot be compressed due to particles being packed close together.

Compressibility of liquids

Liquids can be compressed slightly due to particles being further apart.

Compressibility of gases

Gases can be compressed significantly due to large inter-particle distances.

Melting

The process when a solid turns into a liquid.

Boiling/Evaporation

The process when a liquid turns into a gas.

Condensation

The process when a gas turns into a liquid.

Freezing

The process when a liquid turns into a solid.

Kinetic particle theory

The theory stating that all matter is made up of tiny particles which are always moving randomly.

Heating

The process that increases the average kinetic energy of particles, causing them to move faster.

Cooling

The process that decreases the average kinetic energy of particles, allowing intermolecular forces to hold them in a fixed position.

Heating curves

Graphs showing how a substance's temperature increases as it is heated.

Cooling curves

Graphs showing how a substance's temperature decreases as it cools.

Effect of temperature on gases

Higher temperature causes gases to expand and increase in volume.

Effect of pressure on gases

Higher pressure causes the volume of gases to decrease as it compresses the gas particles together.

Diffusion

The movement of particles from areas of higher concentration to lower concentration, equalizing the distribution over time.

Relative molecular mass

The sum of the masses of the atoms in a molecule, giving the molecule's weight.

Diffusion of a gas

The process by which gas molecules spread from areas of high concentration to areas of low concentration.

Elements

Substances that consist of only one type of atom and cannot be broken down into simpler substances by chemical means.

Symbol of an element

A one or two-letter abbreviation used to represent an element (for example, H for hydrogen, O for oxygen).

Metals

Elements that usually have high melting and boiling points.

Non-metals

Elements that usually have low melting and boiling points.

Compounds

Substances formed when two or more different elements chemically combine in fixed ratios to form a new substance.

Separation of compounds

Compounds can only be separated into their constituent elements through chemical reactions.

Mixtures

Combinations of two or more substances (elements or compounds) not chemically bonded together.

Separation of mixtures

Mixtures can be separated by physical methods, such as filtration or distillation, without breaking chemical bonds.

Properties of mixtures

The properties of mixtures are based on the proportions of its constituents.

Atomic structure

The arrangement of protons, neutrons, and electrons in an atom.

Nucleus

The central part of an atom that contains protons and neutrons.

Protons

Positively charged particles found in the nucleus of an atom.

Neutrons

Neutral particles found in the nucleus of an atom.

Electrons

Negatively charged particles that orbit the nucleus of an atom.

Atomic number

The number of protons in one atom of an element, also known as the proton number.

Mass number

The total number of protons and neutrons in the nucleus of one atom of an element.

Electronic configuration

The arrangement of electrons in the electron shells of an atom.

Group VIII elements

Elements known as noble gases that have a full outer shell, meaning they are unreactive.

Outer shell electrons

The number of outer shell electrons is equal to the group number in Groups I to VII.

Period number

The number of electron shells in an atom is equal to the period number.

Isotopes

Different atoms of the same element that have the same number of protons but different numbers of neutrons.

Isotopes

Isotopes of the same element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration.

Isotope Notation

To write isotopes, we use the following notation: X ZA where A is the mass number, Z is the proton number and X is the symbol of the element itself.

Relative Atomic Mass Calculation

To calculate the relative atomic mass (A r) of an element from its isotopes, we can use the relative mass of its isotopes and the abundance of each isotope (percentage).

Relative Atomic Mass Formula

Using this information, we can use the following formula: A r = (mass of isotope 1 × abundance) + (mass of isotope 2 × abundance) / 100.

Ions

Ions are atoms or molecules that have a charge (are not neutral; number of protons is not equal to number of electrons) due to the gain or loss of electrons.

Cations

Positive ions are known as cations. They are formed when atoms lose electrons, resulting in a positive charge due to more protons than electrons.

Anions

Negative ions are known as anions. They are formed when atoms gain electrons, resulting in a negative charge due to more electrons than protons.

Ionic Bond

An ionic bond is a strong electrostatic attraction between oppositely charged ions.

Ionic Compounds

Ionic bonds are usually found in compounds that contain metals combined with non-metals.

Electron Transfer in Ionic Bonding

Electrons are transferred from the metal atoms to the non-metal atoms, making the atom stable because of their full outer shell.

Dot and Cross Diagram

We use a dot and cross diagram to show these bonds.

Structure of Ionic Compounds

Ionic compounds have a repeating, closely packed structure where positively charged ions (cations) and negatively charged ions (anions) alternate, held together by strong electrostatic forces.

Lattice Structure

This arrangement is known as a lattice, where many millions of ions would be arranged to make up the giant ionic lattice structure.

Properties of Ionic Compounds

Properties of ionic compounds include high melting points and boiling points, good electrical conductivity when aqueous or molten, and poor when solid.

Melting and Boiling Points

These compounds have strong electrostatic forces of attraction between oppositely charged ions throughout the entire lattice, which require a lot of energy to break/overcome, leading to higher melting and boiling points.

Electrical Conductivity of Ionic Compounds

Solid ionic compounds are held firmly in place in the lattice and hence ions in them cannot move, making them a bad conductor.

Dissociation of Ionic Compounds

When this ionic compound is dissolved in water or molten, the ions in the substance disassociate and are able to move, allowing them to conduct electricity.

Charge Cancellation in Ionic Compounds

To form a stable compound, the charges on the opposing ions must always cancel out each other so that the overall charge of the compound is 0.

Example of Ionic Compound Formation

For example, Na+ + Cl− forms NaCl.

Magnesium Chloride Formation

One Mg2+ ion + two Cl− ions forms MgCl2.

Oxidation Number

To figure out the charge of the ion of an element, we can refer to its oxidation number.

Transition Elements

Transition elements have multiple oxidation numbers, and they have names like copper(ii) which means that its oxidation number is 2.

Valence Electrons

To find out the oxidation number for other elements, we use the number of valence electrons (the number of electrons in the last shell of an atom of that element).

Valence Electrons Rule

If the number of valence electrons is more than 4 (so 5, 6, 7, 8), then we subtract the number of valence electrons by 8.

Valence Electrons

8

Oxidation Number of Group I

1

Oxidation Number of Group II

2

Oxidation Number of Group VI

-2

Covalent Bonds

Bonds in which atoms share pairs of electrons to form stable compounds.

Hydrogen Gas

A simple example of covalent bonds where 2 hydrogen atoms share electrons.

Oxygen Gas

A molecule that also has covalent bonds.

Chlorine Gas

A molecule that has covalent bonds.

Water

A molecule that has covalent bonds.

Methane

A molecule that has covalent bonds.

Ammonia

A molecule that has covalent bonds.

Hydrochloric Acid

A molecule that has covalent bonds.

Methanol

A molecule that has covalent bonds.

Ethylene

A molecule that has covalent bonds.

Carbon Dioxide

A molecule that has covalent bonds.

Properties of Simple Covalent Compounds

Low melting points and boiling points, poor electrical conductivity.

Weak Intermolecular Forces

Forces that hold atoms together within a molecule, requiring less energy to break.

Giant Covalent Structures

Networks of atoms bonded together by strong covalent bonds.

Graphite Structure

Consists of many layers of carbon atoms arranged in a hexagonal lattice.

Conductivity of Graphite

Graphite conducts electricity due to delocalized electrons between layers.

Diamond Structure

A network of carbon atoms, each bonded to four other carbon atoms in a tetrahedral arrangement.

Hardness of Diamond

Diamond is one of the hardest substances known due to its strong covalent bonds.

Melting Point of Diamond

Diamond has an exceptionally high melting point.

Conductivity of Diamond

Diamond does not conduct electricity because it does not have free electrons or ions.

Silicon(IV) Oxide Structure

Each silicon atom is bonded to four oxygen atoms through strong covalent bonds in a tetrahedral arrangement.

Hardness of Silicon Dioxide

Silicon dioxide has a hard and rigid structure with a high melting point.

Conductivity of Silicon Dioxide

Silicon dioxide is not a conductor of electricity as there are no free ions or electrons.

Hardness

A property of materials that indicates their resistance to deformation.

High melting points

The temperature at which a solid becomes a liquid, typically high for metals.

Do not conduct electricity

Materials that do not allow the flow of electric current.

Metallic bonding

The electrostatic attraction between the positive ions in a giant metallic lattice and a sea of delocalised, mobile electrons.

Properties of metals

Metals have good electrical conductivity, are ductile, and malleable.

Good electrical conductivity

The ability of a material to allow the flow of electric current due to free electrons.