I do not agree with the statement.
The "substance" can be a compound. It's "pure"
as long as there's nothing else in it but its name.
'Pure' water is 100% H₂O with nothing else in it.
'Pure' table salt is 100% NaCl with nothing else in it.
'Pure' carbon dioxide is 100% CO₂ with nothing else in it.
These example substances are all compounds, not elements.
A: what is it called when a solid jumps straight to a gas?
B: what is it called when a liquid becomes a gas?
C: what is it called when a solid changes into a liquid?
Im sure you can at least figure out the last one! (hint: this happens when ice becomes water)
X=r-p. Maybe I don't understand, but I am assuming that you need to isolate for X? you simply subtract p from both sides.<span />