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In Science, the test of a theory or model is how well it explains lacts. In this Chapitge, properties of tricity. conduct electricity.
7.2 Structure and Physical Properties of Simple Molecular Substances
Simple molecular substances consist of small covalent molecules. This includes elements such as hydrogen, oxygen and bromine, and compounds such as water - and carbon dioxide (Figure 7.1).

Figure 6.17 from Chapter 6 shows how the molecules are arranged in bromine, In this model, the atoms within the molecules are held together by strong covalent bonds. Weak forces (called intermolecular forces) exist between the molecules.
Physical Properties of Simple Molecular Substances
‘1. Physical state
Most substances with simple molecular structures are liquids or gases at room temperature. As the forces between the molecules are weak, the molecules are not held together tightly like particles in a solid, but are free to move.
2. Melting and boiling points

Substances with a simple molecular structure usually have low melting and boiling points (usually below $200^{\circ} \mathrm{C}$ ). See Figure 7.2 for the example on water. Water can be easily changed into a gas by heating. It has a relatively low boiling point of $100^{\circ} \mathrm{C}$.

Only the weak intermolecular forces between the molecules are overcome when water boils. During melting or boiling, the molecules of water separate. The atoms in the molecules do not (Figure 7.2).

The covalent bonds that hold the hydrogen and oxygen atoms together are strong.

The forces of attraction between the water molecules are weak. These forces are easily overcome.

Figure 7.2 Intermolecular forces holding water molecules together are easily overcome when water boils.

Sandith
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