Why water dissolves so many things

Answer Key - Grade 8 Science Worksheet

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

1. 1.

   A teacher evaporated the water. Ari says salt melts in water. Does the table support that?

   • No: salt sinks in water because it is heavier, and sinking solids cannot melt or dissolve
   • No: the salt was destroyed in the water, so the recovered salt must be new salt
   • No: the salt came back unchanged, so it dissolved; melting salt would take 801 °C
   • Yes: the salt turned clear in the water, which is what happens when a solid melts

2. Read this first

   Sugar and sand in water (example data)

   mixture	after stirring	after 1 hour	does the solid pass the filter paper?
   sugar in water	clear	clear	passes through
   sand in water	cloudy	sand on the bottom	stays on the paper

3. 2.

   What does the table show about dissolved sugar particles, Bo?

   • They have been destroyed, since the sugar is gone when it dissolves, and that is why the paper catches no sugar
   • They have melted into a liquid in the water, so the filter paper cannot catch the sugar
   • Sugar particles are large, but water dissolves any solid, so they pass through the paper too
   • They are too small to see, settle out or catch in a filter, unlike sand grains
4. 3.

   A teacher warms the oil. Which statement does the table support, Esme?

   • Water dissolves salt and sugar, so water dissolves any solid, and wax is just slow
   • Oil dissolves wax since oil floats on water, and floating liquids dissolve solids
   • Water dissolves salt and sugar but not wax, so different liquids dissolve different solids
   • Oil dissolves salt and sugar more slowly than water does, so the liquids work the same way

5. Sugar dissolving (example data)

   setup	water temperature (°C)	stirring	seconds for 10 g to dissolve
   A	20	none	150
   B	50	hard	20

6. 4.

   Jun wants to test only the effect of temperature. Which change fixes the table?

   • Remove setup A, since a test with no stirring cannot be compared with any other test
   • Time setup A and setup B once more each, keeping no stirring in A and hard stirring in B
   • Rerun setup A with hard stirring, so only the temperature differs between the setups
   • Rerun setup B with gentle stirring at 80 °C, so both settings differ from the first run
7. 5.

   Stirring and the amount of sugar were the same each time. Which conclusion do the data support, Luz?

   • The 20 °C times differ from each other, so these data are too uneven to support a conclusion
   • The 70 s trial shows warm water is sometimes slower than cold water, which weakens the claim
   • More sugar can dissolve in 50 °C water than in 20 °C water, since it dissolved sooner
   • Sugar dissolved faster at 50 °C in each trial, and the gap is much larger than the spread

8. 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.

9. 6.

   A teacher already dissolved 60 g of potassium nitrate in 300 g of water at 20 °C. How many more grams can dissolve for Milo?

   36 g
10. 7.

    Noah's teacher already dissolved 40 g of potassium nitrate in 50 g of water at 60 °C. How many more grams can dissolve?

    15 g
11. 8.

    In a teacher's demonstration, 110 g of potassium nitrate is dissolved in 250 g of water at 40 °C. How many more grams can dissolve, Pia?

    50 g

12. Solubility in 100 g of water (approximate reference data)

    water temperature (°C)	potassium nitrate (g)	table salt (g)	sugar (g)
    0	13	36	179
    20	32	36	204
    40	64	36	238
    60	110	37	287

    A teacher handles potassium nitrate.

13. 9.

    Tala needs to dissolve 50 g of a solid in 100 g of water at 20 °C. Which solids can do it?

    • Sugar can, since 204 g dissolves in 100 g at 20 °C; the other two hold 32 g and 36 g
    • Sugar and table salt can, since both are kitchen solids that dissolve easily in water
    • Table salt can, since it dissolves the same amount at each temperature, which is enough
    • Potassium nitrate can, since it dissolves more as the water warms, which makes up the difference
14. 10.

    Zee says warming water greatly increases how much of any solid dissolves. Which solid is the best counterexample?

    • Sugar: 287 g dissolves at 60 °C, far more than at 0 °C, which goes against the claim
    • Table salt: about 1 g more dissolves between 0 °C and 60 °C, so warming barely helps
    • No solid is a counterexample, since each column goes up at least a little with temperature
    • Potassium nitrate: it rises from 13 g to 110 g, which shows the claim has exceptions