Chemistry: Molecular Structure and Diagrams

Chemistry

• A molecule of carbon dioxide has an outer electron arrangement represented by O=C=O with dots around O and X's around C.
• A compound is made up of two or more different elements chemically bonded together and cannot be broken down into simpler substances by physical processes.
• Compounds and their elements have different properties.

Diagrams

• A diagram showing the outer electron arrangement in a molecule of carbon dioxide is O=C=O with dots around O and X's around C.
• A diagram showing atoms and molecules can be represented by: W: (O—O—O), X: (O—O), Y: (O), Z: (O—O—O—O).
• A chromatogram shows the separation of four different substances.

Chemical Bonding

• Atoms are the smallest parts of elements, and when atoms of the same type chemically join together, a molecule is formed.
• When different types of atoms chemically join together, they form compounds.

Separation Techniques

• Filtration and crystallisation are used to separate a mixture of insoluble chalk and a soluble salt from water.
• Chromatography is used to separate a mixture of coloured compounds and to separate the colours in an ink.
• Distillation is used to separate a mixture of liquids with different boiling points.

Chemical Reactions

• Cryolite is a mineral which contains aluminium, sodium, and fluorine, with a formula of Na₃AlF₆.
• Sodium phosphate, Na₃PO₄, contains sodium ions, Na⁺, and aluminum sulfate, Al₂(SO₄)₃, contains sulfate ions, SO₄²⁻.
• The formula of aluminum phosphate is Al₂(PO₄)₃.

Physical Chemistry

• The purity of an unknown solid substance can be assessed by measuring its melting point.
• The efficiency of a machine can be calculated using the formula: (useful output energy / (useful output energy + wasted energy)) x 100%.

Electricity

• Electric charge can be calculated using the formula: Q = I x t, where Q is the charge, I is the current, and t is the time.
• The unit of charge is the coulomb (C), which is equivalent to an ampere-second (A s).
• The efficiency of a machine can be calculated using the formula: (useful output energy / (useful output energy + wasted energy)) x 100%.

Circuits

• Lamps in a lighting circuit in a house are connected in parallel, which allows them to be switched separately.
• A fuse is used to protect a circuit from excessive current, and a faulty fuse can be replaced with a new one of the same rating.
• A variable resistor is used to control the brightness of a lamp.

Resistance

• The resistance of a piece of wire depends on its length and cross-sectional area, with resistance being directly proportional to length and inversely proportional to cross-sectional area.
• The current in a circuit can be calculated using Ohm's law: I = V / R, where I is the current, V is the voltage, and R is the resistance.

Static Electricity

• When a plastic rod is rubbed with a dry woollen cloth, the rod becomes positively charged as electrons move from the cloth to the rod.