High School Chemistry  ·  Practice by Skill

Classifying Matter Practice Problems

Classifying matter means deciding what a sample is made of and how its particles are arranged. A lot of later chemistry depends on telling elements, compounds, and mixtures apart quickly. You can practice that sorting below, along with reading formulas and spotting physical changes.

What you’ll practice

  • Pure substance or mixture: decide whether a sample has one kind of particle or several.
  • Element or compound: use the formula to tell whether a pure substance can be broken down chemically.
  • Homogeneous or heterogeneous: classify mixtures by whether they look uniform throughout.
  • Counting atoms in a formula: read subscripts and parentheses to find the total atoms in one unit.

Learn the method first →

A quick example: counting atoms in a formula

How many atoms are in one formula unit of Al2(SO4)3?

The subscript outside the parentheses multiplies everything inside: 2 Al, 3 S, and 3 × 4 = 12 O.

Add them up: 2 + 3 + 12 = 17 atoms.

3 free classifying matter problems

Give each one a real try before you open the solution. Your answer is checked as soon as you submit it, and the worked solution shows every step so you can see exactly where a setup went wrong.

#1Formula Counting
How many total atoms are in one formula unit of Ca(NO3)2?
#2Pure Substances
Which of the following is a pure substance?
#3Classification
A sample contains two kinds of particles spread evenly throughout the container. One kind is O2 molecules and the other is CO2 molecules. What is the best classification?

Questions you might have about classifying matter

What is the difference between an element and a compound?
An element has only one kind of atom and cannot be broken into simpler substances by chemical means. A compound has two or more kinds of atoms bonded in a fixed ratio, and it can be broken down chemically.
What is the difference between a homogeneous and a heterogeneous mixture?
A homogeneous mixture looks the same all the way through, like salt water or air. A heterogeneous mixture has parts you can tell apart, like granite or a bowl of cereal with milk.
How do I tell a physical change from a chemical change?
In a physical change the substance stays the same and only its form or appearance changes, like melting ice. In a chemical change new substances form, and clues include gas bubbles, a color change you cannot reverse, or a precipitate.
How do I count the atoms in a chemical formula?
Subscripts multiply the atom right before them. A subscript after a closing parenthesis multiplies every atom inside, so Ca(NO3)2 has 1 Ca, 2 N, and 6 O.
High School Chemistry · Classifying Matter Practice Problems