Melting and Boiling Points Solubility Science February 10th and 16th

Melting Point We can classify substances by comparing the temperatures at which they change phase. Melting point is….? Water melting point = 0° C Iron melting point = 1,538° C Nitrogen melting point = -210° C Nitrogen boils at -195° C This is why you will see nitrogen on earth as a solid or liquid only artificially made in the lab.

Critical Thinking Think about how particles act during phase change from solid to liquid. Suggest why iron’s melting point is so much higher than water’s, and why nitrogen’s is so much lower than either of them.

Boiling Point Boiling point is….? When we add heat to a liquid the particles exert a pressure called vapor pressure. When the vapor pressure is equal to the air pressure the liquid starts boiling and the particles escape and form a gas.

Critical Thinking At sea level, water boils at 100° C. On top of Mount Everest, water boils at 71° C. Explain the difference!

Solubility Some substances dissolve when placed in other substances. Dissolving is simply the pulling apart into individual particles. Solute- The substance that is dissolved. Solvent- The substance that does the dissolving. If the particles of the solute spread out equally throughout the solvent then we have what is called a solution (Homogeneous). Think of salt water…? If you drop a grain of salt into water, the water molecules will steadily pull the sodium and chloride particles away from each other. Eventually, the sodium and chloride particles will spread out equally throughout the water.

Critical Thinking In the salt water example, which is the solute and which is the solvent?

Solubility cont. Solubility is the extent to which a solute will dissolve a solvent. Example… 100 g of water at 0° C will dissolve 35.7 g of salt. If you add 0.1 g of salt more, it will not dissolve. A solution in which no more of a solute can be dissolved is said to be saturated.

Thinking… How much salt will completely dissolve in 200 g of water at 0° C?

Temperature Temperature affects the ability of a solvent to dissolve a solute. Typically, in cases where the solvent is a liquid and the solute is a solid, the solvent can usually dissolve more of the solute as temperature increases.