Reactions that release or absorb energy

Answer Key - Grade 8 Science Worksheet - Building Confidence

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

1. 1.

   A reaction in a teacher's cup changes the water from 21 °C to 29 °C. Change = end − start. What is the temperature change, Ari?

   8 °C
2. 2.

   Bo reads that a reaction changes its water from 24 °C to 19 °C. Change = end − start. What is the temperature change? (A cooling is negative.)

   -5 °C
3. 3.

   A cold pack's mix takes the water from 25 °C down to 12 °C. Change = end − start. What is the temperature change, Esme? (A cooling is negative.)

   -13 °C
4. 4.

   The water in Jun's cup changes by −5 °C during a reaction. What did the reaction do?

   • Released cold into the water
   • Released energy into the water, since the number is a change
   • Absorbed energy from the water: it was endothermic
   • Nothing, since a negative number means no change
5. 5.

   The water in Kira's cup changes by +12 °C during a reaction. What did the reaction do?

   • Made energy out of nothing for the water
   • Nothing, since a positive number means no change
   • Absorbed energy from the water, since the water gained heat
   • Released energy to the water: it was exothermic
6. 6.

   A reaction makes Luz's beaker feel cold. What is happening to the energy?

   • The reaction gives out cold, which flows into the beaker
   • Glass feels cold whatever the reaction does
   • The reaction gives energy to the beaker and the hand
   • The reaction takes energy from the beaker and the hand
7. 7.

   Which statement fits the energy bars for Noah's reaction?

   • Neither: the totals before and after are the same
   • Endothermic: the surroundings gave 30 kJ to the chemicals
   • Exothermic: the chemical bar grew, so it released energy
   • Exothermic: 30 kJ of thermal energy was created
8. 8.

   The thermal bar grows in this chart. Where did that extra energy come from, Pia?

   • The reaction made new energy while it ran
   • From the walls of the cup, which lost energy
   • From the chemical store, which fell by 30 kJ
   • From the cold surroundings, which gave their energy away
9. 9.

   On a snowy day, iron in Ren's hand warmer reacts with air and the packet heats up. What does the reaction do?

   • Stores sunlight and gives it off slowly
   • Releases energy as heat that warms the hands
   • Absorbs heat from the hands to make the packet hot
   • Makes cold leave the hands
10. 10.

    Tala squeezes an instant cold pack, and it turns cold. What is happening inside the pack?

    • Ice forms in the pack from the squeezing
    • The reaction makes cold and gives it to the arm
    • The reaction releases energy that makes the pack cold
    • The reaction absorbs energy from around it