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?

   • 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
   • The sealed mass stayed 180 g and 30 g of sweet solid came back, so the sugar was still there
   • The sealed mass stayed 180 g, which shows the sugar was destroyed but its mass moved into the water

6. 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

7. 5.

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

   • 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
   • Oxygen dissolves at each temperature, so water dissolves any gas the same way it dissolves any solid
   • The same amount of oxygen dissolves at each temperature, because oxygen does not depend on water

8. 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.

9. 6.

   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 does not dissolve in warm oil, like table salt, since oil has no charged ends to attract it
   • X dissolves in warm oil, since oil floats on water and lighter liquids dissolve solids
   • X melts in warm oil, since a liquid that is warm enough to melt a solid will mix with it
10. 7.

    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 heavier than water, since heavy solids sink in water and so cannot dissolve in it
    • They are probably charged ions, like table salt, since a solid that dissolves in oil has charges
    • They are probably very hot, since oil can melt the solid but cold water cannot melt it
11. 8.

    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?

    • In the oil layer, since oil is the top layer and colors float up to lighter liquids in the jar
    • Spread through both layers, since a dye dissolves in any liquid that it touches in the jar
    • In the water layer, since the dye's charged particles are attracted to water, not to oil
    • Nowhere, since the oil layer coats the water and blocks the dye from entering it
12. 9.

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

    • 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
    • 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 measured salt in small steps while stirring until a little stays undissolved, and record the total from one run, which saves time

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?

    • 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
    • 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