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 saturated potassium nitrate solution made with 300 g of water is cooled by a teacher from 40 °C to 0 °C. How many grams of crystals form, Esme?

   153 g
5. 4.

   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 solid in the dish weighs 30 g, which shows the sugar turned into a new substance as it dissolved
   • 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

6. 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

7. 5.

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

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

8. Oxygen dissolved in water (approximate reference data)

   water temperature (°C)	milligrams of oxygen in 1 liter
   0	14.6
   10	11.3
   20	9.1
   30	7.6

9. 6.

   Luz says warmer water always dissolves more of a substance. What does the table show about oxygen gas?

   • The same amount of oxygen dissolves at each temperature, because oxygen does not depend on water
   • Oxygen dissolves at each temperature, so water dissolves any gas the same way it dissolves any solid
   • Less oxygen dissolves as water warms, so the idea fits some solids but not this gas
   • More oxygen dissolves as water warms, because the particles move faster and mix better

10. 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.

11. 7.

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

    • 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
    • X does not dissolve in warm oil, like table salt, since oil has no charged ends to attract it
12. 8.

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

    • 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
    • They are probably heavier than water, since heavy solids sink in water and so cannot dissolve in it
    • They are probably very hot, since oil can melt the solid but cold water cannot melt it
13. 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
    • Skip the test and assume the solid that dissolves more at 0 °C also dissolves more at 40 °C, with no test needed
    • 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

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

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

15. 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 dissolves sugar faster, as the three times show, so the test needs no repeats before the claim is made
    • 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