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
   • No: salt sinks in water because it is heavier, and sinking solids cannot melt or dissolve
   • 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

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 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
   • 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?

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

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?

   • 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
   • 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
7. 5.

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

   • 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
   • 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
8. 6.

   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
   • 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
   • The 70 s trial shows warm water is sometimes slower than cold water, which weakens the claim
9. 7.

   A teacher handles potassium nitrate. 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

10. 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.

11. 8.

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

    • Potassium nitrate, since its solubility changes the most compared with its starting amount
    • 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
    • Table salt, since a saturated salt solution at 60 °C holds the least and crystallizes first
12. 9.

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

    • 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
    • Sugar and table salt can, since both are kitchen solids that dissolve easily in water
    • Sugar can, since 204 g dissolves in 100 g at 20 °C; the other two hold 32 g and 36 g
13. 10.

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

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