Temperature, particle motion, and how much the temperature changes

Answer Key - Grade 8 Science Worksheet - Challenge Me

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

1. Read this first

   Two samples with the same energy (rounded values)

   sample	material	mass (g)	energy to warm 1 g by 1 °C (J)	temperature rise (°C)
   A	sand	100	0.84	15
   B	water	200	4.2	?

2. 1.

   Samples A and B get the same energy. How many °C does sample B rise, Ari?

   1.5 °C
3. 2.

   Samples A and B get the same energy. How many °C does sample B rise, Esme?

   12 °C
4. 3.

   Equal masses of two liquids get energy from identical heaters. Which liquid needs more energy per degree, Jun?

   • Liquid A: it reached the higher temperature, so it needed more energy to get there
   • Neither: both lines start at 20 °C, so the two liquids need equal energy per degree
   • Liquid B: the same energy raised its temperature less, so each degree takes more energy
   • Liquid A: its steeper line shows it absorbed more energy during the ten minutes of heating
5. 4.

   Two 200 g samples of water are heated by identical heaters. What does the graph show, Luz?

   • Line B warms faster, since water that starts out hotter takes in energy from its heater more quickly than cooler water does
   • Line B gained more thermal energy from its heater than line A did, since it ends at a higher temperature on the graph
   • Line A warms faster, since water that starts out cooler has more room to take in energy from its heater than warmer water has
   • They warm at the same rate, so the starting temperature does not change the rise from a given energy

6. Pots of water poured onto ice (example data)

   pot	mass of water (g)	temperature (°C)	grams of ice melted
   Small	200	60	150
   Big	2000	60	1500

7. 5.

   Milo says the big pot has more thermal energy because it is hotter. Which reply uses the table?

   • The big pot is hotter, since it melted more ice than the small pot did from the same starting temperature of 60 °C
   • Both pots hold equal thermal energy, since both pots started at 60 °C and both melted ice that was at 0 °C
   • Both pots are at 60 °C, but the big pot has ten times the mass and melted ten times the ice, so it holds more energy
   • The small pot's particles move faster, since less water gave the ice a larger share of the energy per gram melted

8. Three 100 g samples with the same energy (example data)

   sample	temperature rise (°C)
   X	10
   Y	18
   Z	25

9. 6.

   Noah claims material Z has the greatest specific heat because it reached the highest temperature. Does the table support that?

   • No: Z rose the most for the same energy, so it needs the least energy per degree, and X has the greatest
   • Yes: Z rose the most, so each degree of Z's rise took in more energy than each degree of X's
   • Yes: Z reached the highest temperature, and a higher temperature means more thermal energy in each gram
   • No: the three samples cannot be compared with each other, since each of them warmed by a different amount

10. Two samples with the same energy (example data)

    sample	mass (g)	temperature rise (°C)
    Sand	200	30
    Water	100	20

11. 7.

    Both samples get the same energy. Pia says water has the greater specific heat because it warmed less. Which reply is best?

    • No: the samples have different masses, so they cannot be compared until a new test makes both masses match exactly
    • No: sand warmed more, so sand absorbed more energy and has the greater specific heat of the two samples, even though its mass is larger
    • Yes: water warmed 20 °C and sand warmed 30 °C, so water needs 30 ÷ 20 = 1.5 times the energy per degree, which still makes it the greater
    • Yes, once mass is allowed for: water had half the mass and two thirds of the rise, so it needs about 3 times the energy per gram per degree
12. 8.

    A sparkler spark is at about 1000 °C yet barely stings when it lands on skin. Which explanation fits, Ren?

    • A spark at 1000 °C holds a huge amount of thermal energy, but skin absorbs it too fast to feel any of it
    • The spark's temperature is really low, since it holds so little thermal energy to pass on to the skin it touches
    • The spark has a tiny mass, so its fast-moving particles still hold very little thermal energy to pass on
    • Sparks are far cooler than 1000 °C once they leave the sparkler, since moving air cools them down a great deal
13. 9.

    The same energy warms 50 g of water by 12 °C but 150 g by only 4 °C. Which explanation fits the particle model, Tala?

    • Particles in the larger sample are bigger than those in the small one, so each needs more energy before it speeds up
    • The larger sample loses its extra energy to the air before the thermometer has time to show any warming
    • The energy is shared among three times as many particles, so each speeds up by about a third as much
    • The extra particles absorb the energy faster, but each one still gains the same speed as in the small sample
14. 10.

    Two cups of water, each at 50 °C, are poured together. Why is the mixture at 50 °C and not 100 °C, Zee?

    • The particles slow down as they collide, so each cup gives up half of its particle speed to the other cup
    • Temperature is an average particle speed, so equal samples keep the same average while the thermal energy adds
    • Temperature adds up like mass does, but the mixture loses half of its energy to the air as the two cups mix
    • Temperature measures total thermal energy, which stays equal here since the two cups' energies cancel each other out