These questions ask for the physical state of a familiar element at 25 degrees Celsius and 1 atmosphere. Check the stated conditions and make sure you are considering the pure elemental substance, not a compound, solution or high-temperature sample.
Use substance, conditions and state in that order
First name the substance precisely. Elemental oxygen and an oxide are different substances, just as elemental carbon and carbon dioxide are different. Next confirm the conditions: a material can melt, freeze or boil when temperature or pressure changes. Only then retrieve the room-temperature state. For example, elemental aluminium is solid under these conditions even though it can be melted at a much higher temperature.
| State at 25 degrees Celsius and 1 atmosphere | Elements to recognise |
|---|---|
| Liquid | Mercury and bromine |
| Gas | Hydrogen, nitrogen, oxygen, fluorine and chlorine |
| Gas | Helium, neon, argon, krypton, xenon and radon |
| Solid | Carbon, sulfur, iodine and iron |
The short list of room-temperature liquids is especially useful: among the elements here, mercury and bromine are liquid. Within the halogens on this list, fluorine and chlorine are gases, bromine is liquid and iodine is solid. This is a sequence to learn, not permission to infer every state from family membership or atomic mass alone.
Where state mistakes come from
| Tempting route | How to reject it |
|---|---|
| Giving every member of a family the same state, or making heavier atoms automatically liquid or solid | State depends on the particular substance and conditions. Recall the named element instead of extending one family member's state to the rest. |
| Transferring a compound's state to an element within it | A compound has its own properties. Water, an oxide, a salt or another compound cannot establish the room-temperature state of its constituent elements. |
| Calling a solid sample gaseous because some vapour is visible | A small vapour phase can exist above a solid. Classify the bulk elemental sample at the stated temperature and pressure. |
| Using an image of a molten material as room-temperature evidence | Melting requires conditions different from those named here. Return to 25 degrees Celsius and 1 atmosphere before classifying the element. |
Try it
At 25 degrees Celsius and 1 atmosphere, select the liquid elemental sample from these cards: mercury with 80 protons, sodium with 11, copper with 29, and magnesium with 12. Enter the proton count on the selected card.
Which statements correctly describe nitrogen at 25 degrees Celsius and 1 atmosphere? Select all that apply.
- Elemental nitrogen is a solid fertilizer grain.
- Elemental nitrogen is gaseous.
- Every nitrogen compound must also be gaseous.
- A solid nitrogen-containing compound does not set the state of elemental nitrogen.
Which physical state belongs to elemental fluorine at 25 degrees Celsius and 1 atmosphere?
- A solid under these conditions
- A liquid under these conditions
- An aqueous substance
- A gas under these conditions