8th grade · Science · Unit 2 of 9
Mixtures & Solutions
Mix it, dissolve it, separate it
What your child practices
Telling pure substances from mixtures, mechanical mixtures from solutions, solutes and solvents, comparing concentration, and choosing separation methods like filtering, evaporation, magnets and distillation.
Arizona Academic Standards learning standard: 8.P1U1.2 · identifying substances by their physical properties
How we explain it
Children can open this short lesson before they practice.
- A pure substance has one kind of particle. A mixture has more than one.
- In a mechanical mixture you can see the parts. In a solution, the solute dissolves in the solvent.
- Separate by filtering, evaporating, using a magnet or distilling.
Example: Cup A has 10 g of sugar in 100 mL of water. Cup B has 20 g in 100 mL. Which is more concentrated? Both have the same amount of water. Cup B has more sugar in it. Answer: Cup B
Mixtures & Solutions in Arizona
In the Arizona Academic Standards, this is unit 2 of 9 in 8th grade Science, after Chemical Reactions and before Particles & Mixtures.
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
Mechanical mixture or solution? Tap an item, then tap its basket.
Sort into: 🥣 mechanical mixture · 🧪 solution
- 🍪chocolate chip cookie
- 🌬️air
- 🎺brass (copper and zinc)
- 🍵sugar dissolved in tea
- 🏖️sand stirred into water
- 🥜trail mix
🥣 mechanical mixture: chocolate chip cookie, sand stirred into water, trail mix · 🧪 solution: air, brass (copper and zinc), sugar dissolved in tea
Hint: In a mechanical mixture you can see the different parts. In a solution, one substance dissolves into another, so it looks the same all the way through.
Question 2
Which cup of sugar water is the least concentrated?
Sugar water cups Cup Sugar Water A 5 g 100 mL B 6 g 100 mL C 20 g 200 mL D 6 g 200 mL - Cup A
- Cup D
- Cup B
- Cup C
Cup D
Hint: Concentration is how much solute is dissolved in an amount of solvent. Work out the grams of sugar in each 100 mL of water, then compare.
Question 3
Panning for gold works because gold is…
🪙- lighter than sand, so it floats
- dissolved in the river water
- magnetic, so it sticks to the pan
- denser than sand, so it sinks to the bottom of the pan
denser than sand, so it sinks to the bottom of the pan
Hint: Swirling the pan washes the lighter sand away while the heavy gold settles.
Question 4
How could you get the salt back out of salt water?
☀️- pour it through a filter
- pick it out with tweezers
- use a magnet
- let the water evaporate
let the water evaporate
Hint: Dissolved salt passes right through a filter. But when the water evaporates, the salt is left behind as crystals.
Question 5
Muddy water stands for an hour, and the mud sinks to the bottom. This kind of mixture is a…
- solution
- solvent
- pure substance
- suspension
suspension
Hint: In a suspension, the pieces are mixed in but not dissolved, so they settle out over time.
Question 6
What is a pure substance?
- any liquid you can see through
- matter made of only one kind of particle
- anything found in nature
- two substances stirred together
matter made of only one kind of particle
Hint: Distilled water, table salt and gold are pure substances. Each is made of just one kind of particle.