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: 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
   • No: the salt was destroyed in the water, so the recovered salt must be new salt
   • No: salt sinks in water because it is heavier, and sinking solids cannot melt or dissolve

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?

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

4. Sugar dissolving (example data)

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

5. 3.

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

   • Rerun setup A with hard stirring, so only the temperature differs between the setups
   • Remove setup A, since a test with no stirring cannot be compared with any other test
   • Rerun setup B with gentle stirring at 80 °C, so both settings differ from the first run
   • Time setup A and setup B once more each, keeping no stirring in A and hard stirring in B
6. 4.

   Kira wants to know how much sugar dissolves at 20 °C and at 50 °C, not how fast. Which plan is best?

   • Stir sugar into 100 g of water at 20 °C and 200 g of water at 50 °C, and record the grams added
   • Time how long 10 g of sugar takes to dissolve at each temperature, since faster dissolving means more dissolves
   • Stir 10 g of sugar into 100 g of water at each temperature and record whether it dissolved or not
   • Stir sugar into 100 g of water at each temperature until some stays undissolved, and record the grams added
7. 5.

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

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

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.

    A teacher cools 100 g of water, saturated at 60 °C, to 0 °C. Which solid forms the most crystals, Ren?

    • Table salt, since a saturated salt solution at 60 °C holds the least and crystallizes first
    • The three form the same amount, since each solution is saturated at the same 60 °C
    • Sugar, since its solubility falls by 108 g and potassium nitrate's falls by 97 g
    • Potassium nitrate, since its solubility changes the most compared with its starting amount
14. 10.

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

    • 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
    • Sugar: 287 g dissolves at 60 °C, far more than at 0 °C, which goes against the claim