Solids, liquids and gases as moving particles

Answer Key - Grade 8 Science Worksheet

Circle the best answer, or write your answer on the line.

1. Read this first

   Ice on a hot plate (example data)

   minute	temperature (°C)
   0	−10
   1	−5
   2	0
   4	0
   6	0
   8	0
   9	6
   10	12

2. 1.

   The hot plate adds energy at a steady rate the whole time. Where does the energy go between minutes 2 and 8, Ari?

   • It makes each particle larger and softer, which melts the ice while the temperature stays put
   • It escapes into the room, so the ice receives no energy and its temperature stays put during these minutes
   • It pulls particles out of their fixed places, so the ice melts while the temperature stays put
   • Its particles stop moving while the ice melts, so there is no motion left to raise the temperature

3. Heating 10 g of ice (example data)

   minute	temperature (°C)
   0	0
   3	0
   4	24
   7	96
   8	100
   20	100
   30	100

4. 2.

   A hot plate adds energy at a steady rate. Why is the flat part at 100 °C so much longer than the flat part at 0 °C, Bo?

   • The thermometer responds more slowly at 100 °C than at 0 °C, so the flat part looks longer than it is
   • There is more water at 100 °C than there was ice at 0 °C, since melting adds mass, so it takes longer
   • The hot plate delivers less energy each minute once the ice has melted, so the water heats more slowly
   • Pulling particles completely apart into a gas takes much more energy than loosening them into a liquid

5. Cooling a pure liquid (example data)

   minute	temperature (°C)
   0	90
   2	78
   4	66
   6	63
   8	63
   10	63
   12	63
   14	56
   16	48

6. 3.

   A pure liquid cools in the lab. What is happening to the sample from minute 6 to minute 12, Esme?

   • It is boiling, since a flat part on a temperature graph means the sample is changing into a gas
   • It has already frozen and is now a solid, and the temperature stays level because solids cannot cool
   • It is freezing: particles slow into fixed places and release energy at a steady temperature
   • It has stopped losing energy to the room, so its particles have stopped slowing down for a while
7. 4.

   Each beaker held 200 mL of water and 1 drop of dye. Jun reads the dye test. Which conclusion do the data support?

   • Each 20 °C rise cut the time in half, so the time at 70 °C would be 45 seconds, half of 90
   • The rule holds for any liquid and any dye, since each temperature was timed once and the three results agree
   • Dye spread faster in warmer water, which fits particles moving faster at higher temperatures
   • The dye spread faster in warmer water because the dye particles grew larger as the water heated
8. 5.

   Each setup used 1 drop of dye. Kira wants to fix this table so the test is fair. Which change should the class make?

   • Drop the 50 °C setup, since hot water is too different from cold water to compare fairly
   • Time setup A and setup B once more each, leaving 200 mL of water in A and 100 mL in B as they were
   • Run setup A again with 100 mL of water, so that only the temperature differs between the setups
   • Run setup B again with 300 mL of water, so that the volumes differ by even more than before
9. 6.

   Water evaporates at a steady rate. Water evaporates at a steady rate from Milo's open beaker. What is the mass of the beaker and water on day 7?

   378 g
10. 7.

    Pia leaves a bowl of water on a windowsill. Water evaporates at a steady rate. What is the mass of the bowl and water on day 10?

    150 g

11. Sealed jar of ice that melts (example data)

    state	mass (g)	volume (cm³)
    Ice	90	98
    Liquid water	90	90

12. 8.

    A sealed jar of ice melts. The mass stays the same but the volume falls. Why, Ren?

    • The liquid water has more mass than the ice had, so the same jar holds it in less space overall
    • Some particles were destroyed in the melting, so fewer particles are left to fill the jar's space
    • The same particles take up less space once the open ice pattern breaks and they pack closer
    • Each particle shrank when it melted, so the same particles take up less room

13. Sealed syringe of air (example data)

    plunger	volume (mL)	mass of air (g)
    Start	60	0.07
    Pushed in	20	0.07

14. 9.

    Tala pushes the plunger of a sealed syringe and squeezes the air. How does the density of the air change, and why?

    • It stays the same: the mass is unchanged, so the density is unchanged
    • It triples: the same mass of air is squeezed into one third of the volume
    • It falls to one third: pressing particles together makes each one lighter
    • It triples: pushing the plunger forces more air particles in from outside

15. Colored liquid in a thin tube (example data)

    temperature (°C)	height of the liquid (mm)
    10	40
    30	52
    50	64

16. 10.

    The level of a colored liquid rises in a thin tube as it warms. What does the particle model say, Zee?

    • Each particle grows as it warms, so the same particles fill a taller column of the tube
    • Faster particles push a little farther apart, so the same particles fill more volume
    • Warmer particles stop moving and settle upward, so the column grows taller
    • Warming adds liquid particles to the tube from the air around the bulb, so the column grows