Why water dissolves so many things

Answer Key - Grade 8 Science Worksheet - Challenge Me

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.

   A teacher cools a saturated potassium nitrate solution made with 200 g of water from 60 °C to 20 °C. How many grams of crystals form, Ari?

   156 g
3. 2.

   Bo's teacher cools a saturated potassium nitrate solution made with 50 g of water from 60 °C to 40 °C. How many grams of crystals form?

   23 g
4. 3.

   A teacher evaporated the water. Jun claims dissolving destroys the sugar. Which reasoning best uses the data to answer?

   • The sealed mass stayed 180 g, which shows the sugar was destroyed but its mass moved into the water
   • The sealed mass stayed 180 g and 30 g of sweet solid came back, so the sugar was still there
   • Mass changes during dissolving, since 180 g in the jar became just 30 g in the dish
   • The solid in the dish weighs 30 g, which shows the sugar turned into a new substance as it dissolved

5. Salt in 100 g of water at 20 °C (example data)

   grams of salt added	grams dissolved	grams left on the bottom
   20	20	0
   30	30	0
   40	36	4
   50	36	14

6. 4.

   Kira says water at 20 °C cannot dissolve salt because some stayed on the bottom. What does the table show?

   • Water dissolved up to 36 g and then stopped: the solution is saturated, not unable to dissolve salt
   • Stirring longer would make the extra 14 g dissolve, since the salt just needs more time
   • The extra 14 g would dissolve in warmer water, since warm water holds much more salt
   • Water stopped dissolving salt at 30 g, since 30 g was the last amount with nothing left over on the bottom

7. Solids in two liquids (example data)

   solid	what its particles are like	in water	in warm cooking oil
   Table salt	charged ions	dissolves	does not dissolve
   Sugar	polar molecules	dissolves	does not dissolve
   Candle wax	molecules with no charged ends	does not dissolve	dissolves

   A teacher warms the oil.

8. 5.

   A new solid X behaves like table salt in water. What do you predict about X in warm oil, Milo?

   • X does not dissolve in warm oil, like table salt, since oil has no charged ends to attract it
   • X dissolves in warm oil too, since liquids dissolve any solid if they are given enough time
   • X melts in warm oil, since a liquid that is warm enough to melt a solid will mix with it
   • X dissolves in warm oil, since oil floats on water and lighter liquids dissolve solids
9. 6.

   Solid Y does not dissolve in water but dissolves in warm oil. What do you predict about its particles, Noah?

   • They are probably very hot, since oil can melt the solid but cold water cannot melt it
   • They are probably heavier than water, since heavy solids sink in water and so cannot dissolve in it
   • They are probably molecules with no charged ends, like wax, so water has little to attract
   • They are probably charged ions, like table salt, since a solid that dissolves in oil has charges
10. 7.

    A drop of water-based food coloring falls into a jar of water topped by a layer of oil. Where does the color end up, Pia?

    • Nowhere, since the oil layer coats the water and blocks the dye from entering it
    • Spread through both layers, since a dye dissolves in any liquid that it touches in the jar
    • In the oil layer, since oil is the top layer and colors float up to lighter liquids in the jar
    • In the water layer, since the dye's charged particles are attracted to water, not to oil
11. 8.

    Ren wants the most salt that dissolves in 100 g of water at 20 °C. Which plan is most reliable?

    • Stir 10 g of salt into the water and time how quickly it dissolves, since slower dissolving means a lower limit for the salt
    • Add measured salt in small steps, stirring until a little stays undissolved, record the total, and repeat three times
    • Add salt in small steps to 100 g of water at 20 °C, then repeat in 150 g of water at 30 °C to check the total again
    • Add measured salt in small steps while stirring until a little stays undissolved, and record the total from one run, which saves time
12. 9.

    Tala will compare how much potassium nitrate and table salt dissolve at 40 °C, with a teacher handling the potassium nitrate. Which plan is fair?

    • Use 100 g of water at 40 °C for the salt and 100 g of water at 60 °C for the potassium nitrate, then compare the grams
    • Use 100 g of water at 40 °C for each solid, add measured amounts until some stays undissolved, and compare the grams
    • Use 100 g of water at 40 °C for each solid, time how long 10 g of each takes to dissolve, and compare the times
    • Skip the test and assume the solid that dissolves more at 0 °C also dissolves more at 40 °C, with no test needed

13. Sugar dissolving, one trial each (example data)

    stirring	seconds for 10 g to dissolve
    none	120
    gentle	70
    hard	30

14. 10.

    Zee wants a claim about stirring. Which statement do these results best support?

    • Stirring lets more sugar dissolve in the water, since the sugar dissolved sooner with harder stirring
    • Harder stirring seemed to dissolve sugar faster, but one trial per setup is thin evidence, so repeat each setup
    • Each level of stirring cut the time in half, so the next level of stirring would take 15 seconds
    • Harder stirring dissolves sugar faster, as the three times show, so the test needs no repeats before the claim is made