8th grade · Science · Unit 3 of 9
Particles & Mixtures
Pure substances and mixtures
What your child practices
The particle theory of matter, the states of matter, pure substances versus mixtures, and homogeneous versus heterogeneous mixtures.
Arizona Academic Standards learning standard: 8.P1U1.2 · pure substances and mixtures, and how scientists tell them apart
How we explain it
Children can open this short lesson before they practice.
- All matter is made of tiny particles that are always moving and have spaces between them.
- A pure substance has one kind of particle. A mixture has two or more.
- A homogeneous mixture looks the same all through; a heterogeneous one has parts you can see.
Example: Is salad dressing with oil floating on vinegar homogeneous or heterogeneous? You can see the separate layers. Visible parts mean heterogeneous. Answer: heterogeneous
Particles & Mixtures in Arizona
In the Arizona Academic Standards, this is unit 3 of 9 in 8th grade Science, after Mixtures & Solutions and before Work, Energy & Efficiency.
Gradelings describes a child's practice on this unit as Beginning, Approaching, Meeting or Exceeding (Four practice steps). That reflects practice, not a report-card mark: the teacher decides the mark.
Source: Arizona Department of Education
Sample questions
A taste of this unit. In the app, questions change every time and get easier or harder to match your child.
Question 1
Homogeneous or heterogeneous? Sort each mixture.
Sort into: 🥛 homogeneous (looks the same throughout) · 🍕 heterogeneous (different parts show)
- 🧂salt dissolved in water
- 🥗a salad
- 🥣cereal in milk
- 🍕a pizza
- 🫙vinegar
- 🌬️air in a clean room
🥛 homogeneous (looks the same throughout): salt dissolved in water, vinegar, air in a clean room · 🍕 heterogeneous (different parts show): a salad, cereal in milk, a pizza
Hint: In a homogeneous mixture the parts are evenly mixed and look like one substance. In a heterogeneous mixture you can see different parts.
Question 2
A diagram shows two different kinds of particles mixed together but not joined. This is…
- an element
- a pure substance
- a mixture
- a single atom
a mixture
Hint: Two or more kinds of particles mixed together make a mixture.
Question 3
Which would be a homogeneous mixture?
- sugar dissolved in water
- gravel
- a salad
- sand and water
sugar dissolved in water
Hint: Dissolved sugar is evenly mixed and cannot be seen as separate parts.
Question 4
In the particle theory, the particles of matter are…
- moving only in gases
- still when matter is cold
- always moving
- moving only when you look at them
always moving
Hint: Particles move all the time. They move more slowly when it is colder and faster when it is warmer.
Question 5
Water as ice, as liquid water and as water vapor has the same kind of particle. What differs in each state?
- Whether the water is pure
- The number of different substances
- How the particles are arranged and how they move
- The kind of particle
How the particles are arranged and how they move
Hint: Changing state changes the arrangement and motion, not the kind of particle.
Question 6
Cooking oil and water do not stay mixed. Both are pure, but together they form…
- a new pure substance
- a single element
- a heterogeneous mixture
- a homogeneous mixture
a heterogeneous mixture
Hint: Oil floats in a separate layer, so different parts are visible.