★ Must-know
A pure substance can occur as a solid, liquid, or gas depending on temperature and pressure.
The state-change names are:
Further detail
A solid is compact and ordered, a liquid has contacting but disordered entities, and a gas is disordered and dispersed.
A pure substance may have several crystalline solid forms called polymorphs, a phenomenon known as polymorphism.
1. A sample consists of a single chemical constituent made of two different elements. Which description fits it?
2. A sealed sample of a pure substance is observed at different temperatures and pressures. Which set of physical states may it occupy under suitable conditions?
3. A pure substance changes directly from a solid to a gas without becoming liquid. What is this change called?
What defines a pure substance?
It contains a single chemical constituent.
What distinguishes a simple pure substance from a compound pure substance?
A simple substance, such as O₂, contrasts with a compound such as H₂O.
What three states can a pure substance occupy?
Solid, liquid, or gas.
What is the state change from solid directly to gas called?
Sublimation.
At fixed pressure, how does heating a pure substance change its order?
It moves from a more ordered state toward a more disordered state.
What is the triple point of a pure substance?
The pressure and temperature at which its solid, liquid, and gas phases coexist.
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