Chemistry IGCSE- Principles of Chemistry

1.1 States of Matter

The three States of Matter

• The three states are solids, liquids and gases

• The state changes occur at the meting point and boiling point

Changes of state

• The amount of energy needed for interconversion of state depends on the strength of forces between particles —>

This is a physical change involving changes in forces between the particles of the substances

Evaporation vs Boiling:

Both happen when a liquid changes into a gas

• Evaporation only occurs at the surface of liquids; where high energy particles can escape from liquids at low temperatures

• Evaporation occurs over a range of temperatures

• Boiling requires heat energy which transforms into kinetic energy

• It only occurs at a specific temperature which is unique to each pure liquid

Melting:

• Requires heat energy which transforms into kinetic energy, allowing particles to move

• Occurs at a specific temperature that is unique to each pure solid

Freezing:

• Is the reverse of melting and happens at the exact same temperature as the melting point

Condensation:

• When a gas is cooled, particles lose energy so when they bump together, they lack the energy to bounce away and instead group together to form a liquid

Diffusion and Dilution:

Diffusion and Dilution experiments support a theory that all matter (solids, liquids and gases) is made up of tiny, moving particles; the kinetic theory of matter

Diffusion-

• Movement of particles from an area of high concentration to an area of low concentration

• Therefore, particles must be able to move freely (in fact, diffusion cannot occur in solids)

Solutions:

1.2 Elements, compounds and mixtures

Pure substances and mixtures:

Element:

A substance made up from only one type of atom (eg. Sodium)

Compound:

A substance made from two or more elements that are chemically bonded together (eg. Carbon Dioxide)

Mixture:

Two or more elements or compounds physically combined together (eg. Salty water)

Chemical properties of each substance are not altered

Separation and Purification Techniques

Simple distillation:

• Separating a solvent from a solution (eg. Water from salty water)

Fractional distillation:

• Separating a liquid from a mixture of liquids (eg. Ethanol from ethanol and water or fractions from crude oil)

• Works as liquids have different boiling points so, when heated, liquid with lower boiling point will evaporate first, condense and be collected.

Paper chromatography:

• Separating mixtures of soluble substances (eg. Food colourings, inks, dye)

• Used to help identify substances

• Separation depends of the solubility of substance

Practical- Investigating the compositions of inks using chromatography

Method:

1. Draw a start line in pencil across the chromatography paper (approx 1-2 cm from bottom)

2. Use a pipette to add small dots of ink across the line and label them

3. Place the paper into a container with water (solvent) reaching just underneath the start line

4. Allow the solvent to move through the paper, remove the chromatogram before it reaches the top

5. Draw a line in pencil marking the solvent front

Analysis and results:

• Measure distance travelled by each spot and record in a table

• Calculate Rf value

1.3 Atomic Structure

• All substances are made of atoms

• An atom is made up of protons, neutrons and electrons

Atomic and Mass number

Atomic number:

• Number of protons in atom’s nucleus

• Unique for every element

Mass number:

• Total number of protons and neutrons in atom’s nucleus

• Usually different but can be the safe across different elements

Isotopes

• Isotopes are atoms of an element that have the same number of protons, but different number of neutrons

Relative atomic mass

• It is the weighted average of the relative atomic masses of the isotopes in that element

• Their symbol is Ar

Calculating Ar