What is the difference between a mixture and a pure substance? Give an example of each.

A pure substance (an element or a compound) is made up of only one type of atom, or molecule.

Examples: water (H2O molecules), oxygen (O2 molecules), argon (Ar atoms).

A mixture is made of up of more than one type of atom or molecule.

Examples: salt water (H2O molecules, Na+ ions, Cl- ions),

air (N2 molecules, O2 molecules, Ar atoms, CO2 molecules)

Note: the composition of a pure substance is fixed, but the composition of a mixture can vary. Two salt water solutions might not have the same concentration of salt. Or, if you prefer, one blueberry muffin might have more blueberries in it than another blueberry muffin.

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d. air pressure decreases as altitude increases.