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Chemistry AQA GCSE

What is a molecule?

It is a combination of 2 or more non-metals.

What is an element?

It is a substance made of only one type of atom.

What is the overall charge of an atom?

An atom contains equal amount of protons and electrons, they have no overall charge and neutral.

What is an isotope?

They are different forms of the same element that has same number of protons but different number of neutrons.

Isotopes have same atomic number but different mass number.

What can sometimes occur to the nucleus of isotopes?

**Sometimes**, the extra neutron make the nucleus unstable, so it is unstable.

What are the chemical and physical properties of isotopes?

Isotopes have different physical properties but have different physical properties but have the same chemical properties.

sum of isotopes(isotope abundance x isotope mass __no).__ /

sum of abundance

What do chemical equations show?

Chemical equations show the reactants (substances at the beginning) and the new products (substances at the end).

What is the law of conservation?

The total mass of the products formed in a reaction is equal to the total mass of the reactants.

Symbol equations

Symbol equations must always be balanced.

To balance, only the big numbers at front can be changed, not the powers.

What is an ion?

Is is an atom with a different number of electrons, it can be both positively or negatively charged.

What is a compound?

A compound is when 2 or more elements are chemically combined in fixed proportions.

What is a mixture?

A mixture is when elements and compounds are mixed together and intermingled but they aren’t chemically joined.

Difference between compounds and mixtures (COMPOUNDS)

• They have fixed compositions, the ratio of the element present is always the same in any particular compound.

• Chemical reactions must be used to separate the elements.

• There are chemical bonds between the atoms of the different atoms of the elements in the compound.

Difference between compounds and mixtures (MIXTURES)

• They have no fixed composition, the proportions vary depending on the amount of each substance.

• The different elements or compounds are separated more easily.

• There are no chemical bonds.

What are the ways to separate mixtures?

• filtration

• Crystallisation

• Distillation

• Fractional distillation

• Paper chromatography

What is Fractional distillation?

Fractional distillation is an effective way of separating miscible liquids, using a fractional column. The separation is possible due to the different boiling points of the liquids in the mixture.

What is paper chromatography?

Paper chromatography separates mixtures of substances dissolved in a solvent as they move up a piece of chromatography paper. The different substances are separated because of their different solubility in the solvent used.

What is a solvent?

The liquid in which a solute dissolves.

What is a solute?

The substance that dissolves in a liquid to form a solution.

What is a solution?

It is the mixture formed when a solute has dissolved in a solvent.

What is soluble used to describe?

Describes a substance that will dissolve.

What is insoluble used to describe?

Describes a substance that will not dissolve.

Hist of the atom: early 1800s

Before the discovery of electrons, John Dalton’s experiments led to the idea that atoms were tiny spheres that couldn’t be divided.

Hist of the atom: end 1800

The electron was discovered by JJ Thomson. Scientist believed that atoms were spheres of positively charge with negative charge spread throughout - the “plum pudding model”.

Hist of the atom: 1903 - 1913

Ernest Rutherford designed an experiment carried out by Geiger and Marsden. They fired alpha particles at a very thin gold foil which scattered, leading to the conclusion that the mass of an atom was concentrated at the centre in a nucleus, which was positively charged. It was proposed that electrons orbited around the nucleus.

Hist of the atom: 1914

Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at a specific distance in certain fixed energy levels (shells).

Later experiments led to the belief that the positive charge of the nucleus could be subdivided into a whole number of smaller particles called protons.

Hist of the atom: 1932

James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of neutral particles, called neutrons.