Why water dissolves so many things

Answer Key - Grade 8 Science Worksheet - Building Confidence

Circle the best answer, or write your answer on the line.

1. Read this first

   Potassium nitrate in 100 g of water (approximate reference data)

   water temperature (°C)	grams that dissolve
   0	13
   20	32
   40	64
   60	110

   A teacher handles potassium nitrate.

2. 1.

   Ari's teacher dissolves potassium nitrate in 400 g of water at 20 °C until no more dissolves. How many grams dissolve?

   128 g
3. 2.

   A teacher dissolves potassium nitrate in 250 g of water at 60 °C until no more dissolves. How many grams dissolve, Bo?

   275 g

4. Table salt in 100 g of water (approximate reference data)

   water temperature (°C)	grams that dissolve
   0	36
   20	36
   40	36
   60	37

5. 3.

   Esme stirs table salt into 300 g of water at 20 °C until no more dissolves. How many grams dissolve?

   108 g

6. Four stir tests (example data)

   substance	what its particles are like	after 1 minute of stirring in water
   Table salt	charged ions	dissolves, clear liquid
   Sugar	polar molecules with slightly charged ends	dissolves, clear liquid
   Olive oil	molecules with no charged ends	floats in a layer
   Sand	atoms joined in a strong network	settles to the bottom

7. 4.

   Using the table, which explanation fits the particle model for salt dissolving, Jun?

   • Water's slightly charged ends attract the salt ions and pull them apart into the water
   • The water is warm enough to melt the salt, so the salt turns into a liquid that mixes in
   • Water dissolves any solid it touches, so salt is just one more solid in the long list
   • The salt particles disappear when the water touches them, so nothing is left to see in the cup
8. 5.

   Using the table, why does olive oil float in a layer instead of dissolving, Kira?

   • Oil does dissolve, but water needs far longer than one minute to pull its particles apart
   • Oil particles are larger than sugar particles, so they cannot fit between the water particles
   • Oil is lighter than water, and lighter liquids cannot dissolve in a heavier liquid like water
   • Oil's particles have no charged ends, so water has little to attract and pull away
9. 6.

   Using the table, why does sand settle in the water, Luz?

   • Sand needs warmer water to melt, and the water in the cup was too cool to melt it
   • Sand's atoms are joined in a strong network that water cannot pull apart
   • Sand is heavier than water, and heavy solids cannot dissolve in a lighter liquid
   • Sand grains are too big to fit among water particles, even though water attracts them strongly
10. 7.

    The jar stayed sealed. What does the table show about mass when salt dissolves, Milo?

    • The mass stayed 280 g, so the dissolved salt is still in the jar with its mass
    • The mass fell, since dissolved salt is spread so thin that it no longer adds to the weight
    • The mass rose, since the dissolved salt spreads out and takes up more room in the jar
    • Nothing can be said, since the salt cannot be seen once it has dissolved in the water

11. Filtering a mixture (example data)

    what is measured	grams
    Sand on the filter paper	15
    Salt dissolved in the water	30
    Water in the mixture	200
    Liquid that passed through	230

12. 8.

    Pia filters a mixture of sand, salt and water. What passes through the filter paper?

    • The water and the dissolved salt pass through, and the sand grains stay on the paper
    • The sand, salt and water pass through, because the liquid that came through weighs 230 g
    • The water passes through, and the dissolved salt is destroyed so it appears in neither place
    • Just the water passes through, because the dissolved salt is a solid and stays on the paper with the sand

13. Sugar in 100 g of water at 20 °C (example data)

    stirring	seconds for 10 g to dissolve	most sugar that dissolves (g)
    none	120	204
    gentle	60	204
    hard	30	204

14. 9.

    What does stirring change in this table, Ren?

    • How much sugar is left over, since harder stirring makes the sugar disappear faster
    • How much sugar the water can hold, since harder stirring made 10 g dissolve sooner
    • Neither, since the most sugar that dissolves is the same each time, so stirring changes nothing
    • How fast the sugar dissolves, but not how much sugar the water can hold
15. 10.

    All three were at 20 °C. Why does the powder dissolve fastest, Tala?

    • Powder has more surface touching the water, so water reaches more sugar at once
    • Powder is warmer than the cube, so its particles move faster and it dissolves sooner
    • Powder lets more sugar dissolve in total, because each tiny grain holds extra sugar
    • Powder is a different substance from the cube, with a higher solubility in water