Science 9 · Unit 3: Forces and energy · Cambridge Lower Secondary Science Stage 9

Questions

1. 1. What is the difference between evaporation and boiling?

   • Evaporation needs heating; boiling does not.
   • Evaporation happens only at the surface and at any temperature; boiling happens all through the liquid at its boiling point.
   • They are exactly the same thing.
   • Boiling happens at any temperature; evaporation only at 100 °C.
   Check

2. 2. Hot water at 80 °C is placed in a beaker and left in a room at 20 °C. Which of these causes the temperature of the water to decrease?

   • condensation
   • freezing
   • evaporation
   • boiling
   Check

3. 3. Isopropanol is a liquid that evaporates quickly. Why does isopropanol at room temperature feel cold when it is put on the skin?

   • It stops sweat from evaporating.
   • It evaporates quickly, taking thermal energy from the skin.
   • It adds thermal energy to the skin.
   • It is colder than room temperature.
   Check

4. 4. Equal volumes of liquid soap and water at the same temperature are put on your skin. Liquid soap evaporates much more slowly than water. Which feels colder?

   • the liquid soap, because it evaporates more slowly
   • the liquid soap, because it has stronger forces between its particles
   • the water, because it evaporates faster and removes thermal energy faster
   • They feel the same, because they are at the same temperature.
   Check

5. 5. A water cooler is made from porous clay, so water slowly soaks through to the outside. Why does this keep the water inside cool?

   • The pot reflects all the Sun's radiation.
   • The water evaporates from the outside of the pot, taking thermal energy from the pot and the water inside.
   • Clay is a good conductor of heat.
   • Clay makes the water colder by reacting with it.
   Check

6. 6. Which describes evaporation?

   • a liquid changing to a gas
   • a liquid changing to a solid
   • a solid changing to a liquid
   • a gas changing to a liquid
   Check

7. 7. Which particles escape from a liquid when it evaporates?

   • the particles with the most energy
   • any particles at random, whatever their energy
   • the particles with the least energy
   • the particles at the bottom of the liquid
   Check

8. 8. The boiling point of water is 100 °C. One girl says: 'Water can evaporate at temperatures much lower than 100 °C.' The other says: 'Water can only evaporate at 100 °C.' Who is correct?

   • neither, because water never evaporates
   • the girl who says water can evaporate at much lower temperatures, because water evaporates at temperatures such as 15 °C
   • the girl who says only at 100 °C, because water below 100 °C has no energy
   • the girl who says only at 100 °C, because 100 °C is the boiling point
   Check

9. 9. How does sweat cool the skin?

   • Sweat is made at a lower temperature than the skin.
   • Sweat evaporates, adding thermal energy to the skin.
   • Sweat evaporates, taking thermal energy away from the skin.
   • Sweat reflects the Sun's radiation.
   Check

10. 10. A liquid is evaporating and is not being heated. What happens to the average energy of the particles left behind?

    • It becomes zero.
    • It stays the same.
    • It increases.
    • It decreases.
    Check

11. 11. A liquid is evaporating and is not being heated. What happens to its temperature?

    • It stays the same.
    • It rises to its boiling point.
    • It falls.
    • It rises.
    Check

12. 12. Arun has sweat on his face. The air temperature is 25 °C. He uses a fan to blow air onto his face. The fan does not change the temperature of the air. Why does the fan help him cool faster?

    • It blows the sweat off his face.
    • It stops the sweat from evaporating.
    • It carries away the water vapour, so the sweat evaporates faster.
    • It blows cold air onto his face.
    Check

13. 13. Perfume evaporates faster than water. Sofia puts equal volumes of perfume and water, at the same temperature, separately on her hand. Which feels colder?

    • the water, because water is colder
    • They feel the same, because they are at the same temperature.
    • the water, because it stays on the skin longer
    • the perfume, because it evaporates faster and removes thermal energy faster
    Check

14. 14. In very humid air, sweat hardly evaporates. Why doesn't sweat cool you in humid conditions?

    • The sweat evaporates too quickly.
    • Humid air makes more sweat.
    • The air already holds so much water vapour that the sweat can't evaporate, so no energy is removed.
    • Humid air is always hotter than your skin.
    Check

15. 15. An air cooler blows room air through a sponge soaked in water. Why is the air that comes out cooler?

    • The water in the sponge is frozen.
    • The sponge absorbs the heat and keeps it.
    • Water evaporates from the sponge, taking thermal energy from the air passing through.
    • Wet air is always cold.
    Check