the idea
Matter sorts into two questions: is it pure, and can it be separated physically?
Matter — anything with mass that takes up space
Pure substance
One fixed composition everywhere. Can't be separated by physical means.
One fixed composition everywhere. Can't be separated by physical means.
Mixture
Two or more substances, variable ratio. Can be separated physically by filtering, settling, evaporating, or a membrane.
Two or more substances, variable ratio. Can be separated physically by filtering, settling, evaporating, or a membrane.
Element
One kind of atom: Cu, O₂, C
One kind of atom: Cu, O₂, C
Compound
Elements chemically bonded in a fixed ratio: H₂O, NaCl
Elements chemically bonded in a fixed ratio: H₂O, NaCl
Homogeneous
Uniform throughout (a solution): sugar water, air, brine
Uniform throughout (a solution): sugar water, air, brine
Heterogeneous
Visibly uneven parts: fruit-on-the-bottom, dressing
Visibly uneven parts: fruit-on-the-bottom, dressing
Plant version: UF is the whole idea in one machine. Milk is a mixture, so a membrane can physically split it into retentate (protein and fat) and permeate (water, lactose, minerals). No chemical bond is broken. Getting hydrogen out of water, on the other hand, takes electrolysis, a chemical change, because water is a compound.
sort it
Element, compound, or mixture?
sort it
Physical or chemical change?
A chemical change makes a new substance with a new identity. A physical change only alters form or state, and the substance stays the same.
Clues that a chemical change happened: gas forms that wasn't there before, a lasting color change, heat or light given off, a precipitate, or a new smell. These are clues, not proof. Boiling also makes gas, and that's physical.
density
Density is mass per volume, and it decides what floats.
d = m ÷ V. Less dense things rise and denser things sink. That's why cream separates from unhomogenized milk. Fat globules (~0.93 g/mL) are less dense than skim (~1.035 g/mL), so they float up.
calculate
Solve d = m ÷ V for anything
Density is a conversion factor. 1.032 g/mL means "1.032 g = 1 mL" for this liquid. That's how milk gets to ≈ 8.6 lb/gal: 1.032 g/mL × 3785 mL/gal ÷ 453.6 g/lb = 8.61 lb/gal.
practice
Density problems
exam traps
Common mistakes
- Calling O₂ a compound. It's a molecule, but there's only one element in it, so it's an element.
- Calling dissolving a chemical change. Sugar in water is still sugar. Evaporate the water and you get the sugar back.
- Calling a state change chemical. Melting, freezing, boiling, and condensing are all physical.
- Calculating density with volume in L and mass in g, then comparing to a table in g/mL. Match the units first.
- Mixing up intensive and extensive properties. Density, boiling point, and color don't depend on amount (intensive). Mass and volume do (extensive).