7th grade · Science · Unit 5 of 10
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.
Alaska content standards learning standard: MS-PS1-4 · particle models of pure substances and mixtures
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 Alaska
In the Alaska content standards, this is unit 5 of 10 in 7th grade Science, after Cycles & Succession and before How Plants Work.
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: Alaska Department of Education and Early Development
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
Pure substance or mixture? Sort each one.
Sort into: 💎 pure substance · 🥣 mixture
- 💨oxygen gas
- 🟡a gold bar
- 💧distilled water
- 🥜trail mix
- 🌬️air
- 🌊salt water
- 🪴garden soil
- 🧂pure table salt
💎 pure substance: oxygen gas, a gold bar, distilled water, pure table salt · 🥣 mixture: trail mix, air, salt water, garden soil
Hint: A pure substance is one kind of matter (an element like gold, or a compound like water). A mixture is two or more substances physically combined.
Question 2
Homogeneous or heterogeneous? Sort each mixture.
Sort into: 🥛 homogeneous (looks the same throughout) · 🍕 heterogeneous (different parts show)
- 🧂salt dissolved in water
- 🥗a salad
- 🍵tea with no leaves in it
- 🫙vinegar
- 🍕a pizza
- 🥣cereal in milk
🥛 homogeneous (looks the same throughout): salt dissolved in water, tea with no leaves in it, vinegar · 🍕 heterogeneous (different parts show): a salad, a pizza, cereal in milk
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 3
A bottle of perfume is opened at one end of a room, and soon people at the other end can smell it. Why?
- The gas gets pushed by gravity
- The perfume turns into a solid
- Smell particles are heavier than air
- Gas particles move randomly and spread out
Gas particles move randomly and spread out
Hint: Particles of a gas move in every direction until they are spread throughout the room.
Question 4
A diagram shows only one kind of particle, all identical. This is probably…
- a solution
- a pure substance
- a mixture
- a heterogeneous mixture
a pure substance
Hint: One kind of particle means one pure substance.
Question 5
Brass is copper and zinc mixed evenly throughout a solid. It is a…
- heterogeneous mixture
- gas
- pure substance because it is a solid metal
- homogeneous mixture
homogeneous mixture
Hint: Brass is an alloy: a mixture that looks the same throughout.
Question 6
In the particle theory, the particles of matter are…
- moving only when you look at them
- still when matter is cold
- moving only in gases
- always moving
always moving
Hint: Particles move all the time. They move more slowly when it is colder and faster when it is warmer.