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

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

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 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
   • It has already frozen and is now a solid, and the temperature stays level because solids cannot cool
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?

   • Dye spread faster in warmer water, which fits particles moving faster at higher temperatures
   • The rule holds for any liquid and any dye, since each temperature was timed once and the three results agree
   • Each 20 °C rise cut the time in half, so the time at 70 °C would be 45 seconds, half of 90
   • 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
   • 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
   • Time setup A and setup B once more each, leaving 200 mL of water in A and 100 mL in B as they were
9. 6.

   Luz plans a fair test of water temperature on how fast dye spreads. Which plan is best?

   • Use 200 mL of water and 1 drop of dye in each beaker, change just the temperature, and time one beaker at each temperature, since it saves time
   • Use 200 mL of water and 1 drop of dye in each beaker, change just the temperature, and time three beakers at each temperature
   • Use 200 mL of water in each beaker, change the temperature, and add extra dye to the hot beakers so the color shows up better against the water
   • Use 200 mL of water and 1 drop of dye in each beaker, then time three beakers at 10 °C and one beaker at each hotter temperature
10. 7.

    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
11. 8.

    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

12. Sealed syringe of air (example data)

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

13. 9.

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

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

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

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

15. 10.

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

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