October 1 2010 -- New Laws and the Heating/Cooling Curve of a Pure Substance (notes)

On Friday, we started a new topic about matter and the heating/cooling curve of a pure substance.

Law of Definite Composition
Compounds will have a definite composition. For example, water (H2O) will be water anywhere (it always have 2 hydrogens and 1 oxygen).

Law of Multiple Proportions – when two or more compounds with different properties of the same elements can be made. For example, carbon dioxide (CO2) x 2 = dicarbon tetraoxide/oxalate (C2O4).


A : At this point, the pure substance is in the solid state, because the pure substance is below the melting point. The particles are packed together closely in an orderly manner. Particles can only vibrate at a fixed position and the forces between the particles are very strong.

A-B : When the pure substance is heated, the heat energy is converted to kinetic energy. The molecules vibrate faster in their fixed positions and the temperature increases when the kinetic energy increases.

B : The pure substance is still solid, but the melting stage has begun – the solid begins to change into a liquid. The temperature remains the same.

B-C : The pure substance exists in both solid and liquid states. The heat that is supplied to it is used to overcome the forces of attraction that holds the particles together because the temperature remains constant. This constant temperature is called the melting point. The latent heat of fusion is the heat energy absorbed to overcome the intermolecular forces.

C : The pure substance has completely melted, and the solid has turned to liquid.

C-D : The pure substance is in liquid state, and as the liquid is heated and the temperature is increased, the particles move faster because the kinetic energy is increasing.

D : The pure substance is still in liquid state, and the molecules have gained enough kinetic energy to overcome the forces of attraction between the particles. Some of the molecules begin to change into gas.

D-E : The pure substance is in both liquid and gaseous states. The temperature is unchanged, and the heat energy absorbed is used to overcome the molecular forces between the particles of the liquid rather than increase the temperature. This constant temperature is called the boiling point.

E : All of the pure substance liquid has turned into gas.

E-F : The gas particles continue to absorb more energy and move faster. The temperature increases as heating continues.