Subscribe

7th grade · Science · Unit 3 of 18

Solutions & Separating

Dissolving and splitting mixtures

What your child practices

Solutes, solvents and solubility, what makes things dissolve faster, saturated and unsaturated solutions, concentration, why water is called the universal solvent, ways to separate mixtures, and some environmental effects of industrial separation.

Texas Essential Knowledge and Skills (TEKS) teks student expectation: 7.6(D); 7.6(E) · solutes, solvents and concentration, and what speeds up dissolving

How we explain it

Children can open this short lesson before they practice.

  1. In a solution, the solute dissolves in the solvent. Water dissolves so much it is called the universal solvent.
  2. Stirring, heating and smaller pieces make things dissolve faster. A saturated solution can’t dissolve any more.
  3. Separate mixtures by filtering, evaporating, using a magnet or distilling.

Example: How can you get the salt back from salt water? Salt is dissolved, so a filter won’t catch it. Heat the water until it evaporates. The salt is left behind. Answer: evaporation

Solutions & Separating in Texas

In the Texas Essential Knowledge and Skills (TEKS), this is unit 3 of 18 in 7th grade Science, after Physical or Chemical? and before Motion, Forces & Newton’s Laws.

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: Texas Education Agency (TEA)

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

    Mining leaves 'tailings', waste rock and water kept in ponds after the valuable minerals are separated. Why must tailings ponds be monitored?

    • They make the ground magnetic
    • They remove all pollutants from water
    • They turn into fuel
    • They can leak harmful substances into water and soil

  2. Question 2

    What does chromatography do?

    • It removes magnetic metals
    • It makes mixtures more concentrated
    • It changes a liquid into a solid
    • It separates dissolved substances, such as the dyes in ink, as they travel at different speeds

  3. Question 3

    At 20 °C, at most 60 g of Substance X dissolves in 100 mL of water. You stir 75 g of Substance X into 100 mL of water at 20 °C. How many grams stay undissolved?

    answer boxg

  4. Question 4

    Why does water being a good solvent matter for living things?

    • It stops all chemical reactions
    • It turns food into sunlight
    • It keeps cells from needing food
    • It carries nutrients and wastes in blood and cells

  5. Question 5

    Oil is heated and separated into gasoline, diesel and other products in a tall tower. What is this process?

    • evaporation of a dissolved salt
    • magnetic separation
    • fractional distillation, because the parts boil at different temperatures
    • filtration, because the parts have different sizes

  6. Question 6

    Which method separates dissolved salt from salt water and also collects the pure water?

    • sieving
    • using a magnet
    • distillation
    • filtration

More 7th grade