Raven spreads a printed table across the Bench. Six substances, two columns of temperatures.
"Melting points and boiling points," she says. "What do you notice? Some are below room temperature and some are way above."
Rocket runs his finger down the list. "Sodium melts at about 98 degrees Celsius. That is a metal, and it would melt in hot water?"
"Only a grown-up with proper gear would ever try," Raven says. "But yes, the number says so."
Nova projects a thermometer onto the wall. "Every substance has its own pair of numbers," she says. "What could you do with that?"
"Identify a mystery substance," Rocket says. "Measure, then match."
Raven nods. "Properties are fingerprints."
Each pure substance has characteristic physical and chemical properties that can be used to identify it. That is the idea behind this week's standard.
The table below is drawn from the Chemistry WebBook of the National Institute of Standards and Technology. It lists temperatures in kelvins.
To change kelvins to degrees Celsius, subtract 273. Water boils at about 373 kelvins, which is 100 degrees Celsius.
Water melts at 0 degrees Celsius and boils at 100 degrees Celsius. Acetic acid is the acid in vinegar.
| Substance | Melting point (kelvins) | Melting point (about, Celsius) | Boiling point (kelvins) | Boiling point (about, Celsius) |
|---|---|---|---|---|
| Water | 273 | 0 | 373 | 100 |
| Acetic acid | 290 | 16 | 391 | 118 |
| Mercury | 234 | minus 39 | 630 | 357 |
| Sodium | 371 | 98 | 1156 | 883 |
| Aluminum | 933 | 660 | 2793 | 2520 |
| Copper | 1358 | 1085 | not listed | not listed |
Mercury is a metal, yet its melting point is far below room temperature. That is why it is a liquid in old thermometers.
Copper does not melt until 1358 kelvins. A copper pan on a stove never gets close.
Density is a property too. The U.S. Geological Survey gives water a density of roughly 1 gram per milliliter, and ice is less dense, which is why it floats.
Here is how properties decide a verdict. Compare the properties before a change with the properties after.
Before: baking soda, a white solid, and vinegar, a liquid. After: a gas that fizzed out of the cup. A gas has very different properties from a solid or a liquid.
Before: an ice cube. After: liquid water that boils at 100 degrees Celsius and has the same density as any water. Same fingerprint, same substance.
| Statement | True or false? |
|---|---|
| Every pure substance has its own characteristic properties. | ? |
| Mercury is a solid at room temperature. | ? |
| Water boils at 373 kelvins. | ? |
| Copper melts at a lower temperature than sodium. | ? |
| A gas has very different properties from a solid. | ? |
Excellent reading. Tomorrow you will find physical and chemical changes all over your home and review the week.