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
   • 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
   • The water is warm enough to melt the salt, so the salt turns into a liquid that mixes in
8. 5.

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

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

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

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

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

    • The mass rose, since the dissolved salt spreads out and takes up more room in the jar
    • The mass fell, since dissolved salt is spread so thin that it no longer adds to the weight
    • The mass stayed 280 g, so the dissolved salt is still in the jar with its mass
    • Nothing can be said, since the salt cannot be seen once it has dissolved in the water
11. 8.

    A teacher heated the dish. Where did the other 100 g go when the water evaporated, Noah?

    • The water is still in the dish, hidden inside the sugar, which is why it weighs 25 g
    • The water and some sugar were destroyed, and the 25 g in the dish is what was left over
    • The water turned into sugar, which is why 25 g of solid was left in the dish
    • The water left as vapor into the air, while the 25 g of sugar stayed in the dish

12. 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

13. 9.

    What does stirring change in this table, Ren?

    • Neither, since the most sugar that dissolves is the same each time, so stirring changes nothing
    • 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
    • How fast the sugar dissolves, but not how much sugar the water can hold

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

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

15. 10.

    Does warming the water help a lot to dissolve more table salt, Zee?

    • No: warmer water dissolves less table salt than colder water does, by a small amount
    • Yes: warm water dissolves salt faster, so a greater amount of salt dissolves
    • No: about 1 g more salt dissolves between 0 °C and 60 °C, a very small change
    • Yes: 60 °C water dissolves far more salt than 0 °C water, as it does for most solids