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7th grade · Science · Unit 7 of 14

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.

New York State Learning 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.

  1. All matter is made of tiny particles that are always moving and have spaces between them.
  2. A pure substance has one kind of particle. A mixture has two or more.
  3. 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 New York

In the New York State Learning Standards, this is unit 7 of 14 in 7th grade Science, after How Plants Work and before Density & Buoyancy.

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: New York State Education Department (NYSED)

Sample questions

A taste of this unit. In the app, questions change every time and get easier or harder to match your child.

  1. Question 1

    Water as ice, as liquid water and as water vapor has the same kind of particle. What differs in each state?

    • The kind of particle
    • Whether the water is pure
    • How the particles are arranged and how they move
    • The number of different substances

  2. Question 2

    In which state of matter are the particles farthest apart?

    • solid
    • gas
    • liquid

  3. Question 3

    A diagram shows only one kind of particle, all identical. This is probably…

    • a mixture
    • a pure substance
    • a solution
    • a heterogeneous mixture

  4. Question 4

    Brass is copper and zinc mixed evenly throughout a solid. It is a…

    • heterogeneous mixture
    • pure substance because it is a solid metal
    • homogeneous mixture
    • gas

  5. Question 5

    A bottle of perfume is opened at one end of a room, and soon people at the other end can smell it. Why?

    • Gas particles move randomly and spread out
    • Smell particles are heavier than air
    • The gas gets pushed by gravity
    • The perfume turns into a solid

  6. Question 6

    Homogeneous or heterogeneous? Sort each mixture.

    Sort into: 🥛 homogeneous (looks the same throughout) · 🍕 heterogeneous (different parts show)

    • 🍎clear apple juice
    • 🥗a salad
    • 🍵tea with no leaves in it
    • 🧂salt dissolved in water
    • 🟤muddy water
    • 🍕a pizza

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