Understanding Concepts
1. Identify each of the following as a physical, chemical, or nuclear change, with reasons
for your choice:
(a) a gas barbecue operating
(b) an ice cube melting in someone’s hand
(c) white gas burning in a camping lantern
(d) wax melting on a hot stove
(e) zinc metal added to an acid solution in a beaker
(f) ice applied to an athletic injury
2. Identify the system and surroundings in each of the examples in the previous question.
3. Identify the following as examples of open or isolated systems and explain your iden
tification:
(a) gasoline burning in an automobile engine
(b) snow melting on a lawn in the spring
(c) a candle burning on a restaurant table
(d) the addition of baking soda to vinegar in a beaker
(e) a gas barbecue operating
4. A thimbleful of water at 100°C has a higher temperature than a swimming pool full of
water at 20°C, but the pool has more thermal energy than the thimble. Explain.
5. Identify each of the following as an exothermic or endothermic reaction:
(a) hydrogen undergoes nuclear fusion in the Sun to produce helium atoms;
(b) the butane in a lighter burns;
(c) the metal on a safety sprinkler on the ceiling of an office melts when a flame is
brought near it.
Making Connections
6. (a) List five changes that you might encounter outside your school laboratory. Create
a table to classify each change as physical, chemical, or nuclear; endothermic or
exothermic; and occurring in an open or an isolated system.
(b) What are the most commonly encountered types of chemical reactions in terms
of energy flow?
7. The energy content of foods is sometimes stated in “calories” rather than the SI unit of
joules. Physical activity is described as “burning calories”. Research the answers to
the following questions:
(a) What are the relationships among a calorie, a Calorie, and a joule?
(b) Are calories actually burned? Why is this terminology used?
1. Classify the changes as physical, chemical, or nuclear, with reasons:
(a) A gas barbecue operating: Chemical change
- Combustion of gas produces new substances (CO₂, water vapor) and releases energy.
(b) An ice cube melting in someone’s hand: Physical change
- Change in state from solid to liquid without altering the substance's chemical structure.
(c) White gas burning in a camping lantern: Chemical change
- Combustion reaction forms new substances (CO₂, water vapor) and releases heat and light.
(d) Wax melting on a hot stove: Physical change
- Change in state (solid to liquid) without changing the chemical composition of the wax.
(e) Zinc metal added to an acid solution in a beaker: Chemical change
- A chemical reaction occurs, producing hydrogen gas and forming a new compound (zinc salt).
(f) Ice applied to an athletic injury: Physical change
- The ice melts (solid to liquid) due to heat transfer, with no chemical changes involved.
2. Systems and Surroundings:
- (a) System: Burning gas in the barbecue; Surroundings: Air and barbecue grill.
- (b) System: Ice cube; Surroundings: Hand.
- (c) System: Burning white gas; Surroundings: Lantern and air.
- (d) System: Wax; Surroundings: Stove and air.
- (e) System: Zinc and acid solution; Surroundings: Beaker and air.
- (f) System: Ice; Surroundings: Skin and body heat from the injury.
3. Open or Isolated Systems:
(a) Gasoline burning in an automobile engine: Open system
- Energy and matter (exhaust gases) are exchanged with the surroundings.
(b) Snow melting on a lawn in the spring: Open system
- Heat from the environment and water exchange with surroundings occur.
(c) A candle burning on a restaurant table: Open system
- Heat, light, and gases exchange with the surroundings.
(d) The addition of baking soda to vinegar in a beaker: Open system
- Gas (CO₂) escapes into the surroundings during the reaction.
(e) A gas barbecue operating: Open system
- Heat and gases exchange with the surroundings.
4. Thimble vs. Swimming Pool:
- The thimbleful of water at 100°C has a higher temperature because the molecules in the thimble have more kinetic energy per unit of mass.
- The swimming pool has more thermal energy overall because it contains a much larger number of water molecules, even though the temperature is lower.
5. Exothermic or Endothermic Reactions:
(a) Hydrogen fusion in the Sun: Exothermic
- Releases an enormous amount of energy in the form of heat and light.
(b) Butane in a lighter burns: Exothermic
- Combustion releases heat and light.
(c) Metal on a sprinkler melts: Endothermic
- Absorbs heat from the flame to melt the metal.
6. Changes Outside the Laboratory:
(a) Table of Examples:
| Change | Physical/Chemical/Nuclear | Endothermic/Exothermic | Open/Isolated System |
|---|---|---|---|
| Leaves burning in a pile | Chemical | Exothermic | Open |
| Ice melting on the ground | Physical | Endothermic | Open |
| Nuclear power plant reaction | Nuclear | Exothermic | Isolated |
| Rusting of an iron fence | Chemical | Exothermic | Open |
| Boiling water in a kettle | Physical | Endothermic | Open |
(b) Common Chemical Reactions in Energy Flow:
- Combustion reactions are the most common exothermic reactions, as they release energy. Endothermic reactions, like photosynthesis, are also significant for absorbing energy.
7. Calories and Energy Content of Foods:
(a) Relationships Between Calorie, calorie, and Joule:
- 1 calorie (small calorie) = 4.18 joules (J).
- 1 Calorie (large calorie, or kilocalorie) = 1000 calories = 4180 J.
(b) Are calories "burned"?
- Calories are not physically burned. The term refers to the process where stored chemical energy in food is released during metabolism, similar to combustion.
(c) Laboratory Methods to Determine Energy Content of Foods:
- Calorimetry: Food is burned in a bomb calorimeter to measure the heat released.
- The heat raises the temperature of water in the calorimeter, and the energy content is calculated using the heat formula.
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