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.

   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
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 stopped dissolving salt at 30 g, since 30 g was the last amount with nothing left over on the bottom
   • 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

7. 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

8. 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
   • 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
   • More oxygen dissolves as water warms, because the particles move faster and mix better
9. 6.

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

   • 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
   • X dissolves in warm oil too, since liquids dissolve any solid if they are given enough time
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
    • 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
11. 8.

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

    • 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
    • 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 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
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?

    • 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 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, time how long 10 g of each takes to dissolve, and compare the times
    • Use 100 g of water at 40 °C for each solid, add measured amounts until some stays undissolved, and compare the grams

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