Raven lays two tables on the Bench. One lists how much of a solid dissolves in water at different temperatures.
"Look at this one," she says. "At 0 degrees it takes 14 grams. At 60 degrees it takes 107."
Rocket whistles. "Warm water holds way more. So hot tea could hold more sugar than iced tea."
Nova hovers over the salt row. "Careful," she says. "Not every solid behaves the same way. Compare the two rows."
"Salt barely changes," Rocket says, surprised. "Warm or cold, about the same."
Raven slides over the second table, full of parts per million. "What do you notice about ocean water here?"
"Thirty five thousand," Rocket reads. "That is a lot of solute." Nova brightens. "Now read it all."
Solubility is how much of a solute can dissolve in a given amount of solvent. The temperature must be stated, because solubility varies with temperature.
This table comes from a chemistry text hosted by the University of California, Davis. It gives grams of solid that dissolve in 100 grams of water.
Table salt is sodium chloride. The other solid in the table, potassium nitrate, is a different salt that chemists measured the same way.
| Solid | Temperature (degrees Celsius) | Grams that dissolve in 100 grams of water |
|---|---|---|
| Table salt (sodium chloride) | 20 | 36.0 |
| Potassium nitrate | 0 | about 14 |
| Potassium nitrate | 30 | about 48 |
| Potassium nitrate | 60 | about 107 |
The solubility of most solid substances increases as the temperature increases. Potassium nitrate shows this dramatically.
The effect is hard to predict and varies widely from one solute to another. For sodium chloride, a rise in temperature has relatively little effect.
Gases are the opposite. The solubility of a gas decreases as the temperature increases, because the gas particles escape the liquid more easily.
The U.S. Geological Survey reports dissolved salt in parts per million.
Fresh water has less than 1,000 parts per million. Ocean water contains about 35,000 parts per million of salt.
The ocean is a solution. Its solvent is water, and its solute is salts, dissolved in the same proportion all through the sea.
| Kind of water | Dissolved salt (parts per million) |
|---|---|
| Fresh water | less than 1,000 |
| Slightly saline water | 1,000 to 3,000 |
| Moderately saline water | 3,000 to 10,000 |
| Highly saline water | 10,000 to 35,000 |
| Ocean water | about 35,000 |
| Statement | True or false? |
|---|---|
| Solubility values must state a temperature. | ? |
| At 20 degrees Celsius, 100 grams of water dissolves 36.0 grams of table salt. | ? |
| Warming water makes a dissolved gas stay in the water better. | ? |
| Water with 500 parts per million of salt is fresh water. | ? |
Excellent data reading. Tomorrow you will take mixtures apart again, using the properties their parts kept.