Heating and cooling curves

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课程笔记

Heating and Cooling Curves

A heating curve shows how temperature changes as a substance is heated.
A cooling curve shows how temperature changes as a substance cools.
For a pure substance, temperature remains constant during a state change (melting or boiling).
For an impure substance, melting and boiling occur over a range of temperatures.

Pure vs Impure Substances

A pure substance is made from only one element or compound.
An impure substance contains more than one element or compound (has impurities).
Pure substances have a single, sharp melting/boiling point; impure substances melt/boil over a range.

Internal Energy

Internal energy is the total energy stored in the particles of a substance.
It consists of kinetic energy (movement) and potential energy (bonds/forces) stores.
As thermal energy is transferred to a substance, its internal energy increases.

Changes During Heating

When temperature rises, the kinetic energy of particles increases.
In a solid, particles vibrate more as temperature increases.
During melting, bonds break and temperature stays constant; potential energy increases.
The thermal energy absorbed during a state change is called latent heat (from Latin for 'hidden').
In liquids and gases, particles move faster as temperature rises.

Changes During Cooling

As a gas cools, particles slow down and kinetic energy decreases.
During condensation, bonds form and temperature stays constant; potential energy decreases.
During freezing, bonds form and temperature stays constant; potential energy decreases.

Key Points

Temperature remains constant during state changes (melting, boiling, freezing, condensing).
The kinetic energy of particles changes only when temperature changes.
The potential energy of particles changes only during state changes.
Internal energy always increases when heating and decreases when cooling.

Particle arrangement in solid, liquid, and gas. In solids, particles vibrate in fixed positions; in liquids, they slide past each other; in gases, they move rapidly and are far apart.

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练习题

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1.What is a pure substance?
Easy
- AA substance made from only one element or compound
- BA substance that contains no impurities
- CA substance that has a fixed melting point
- DAll of the above
2.What is internal energy?
Easy
- AThe total kinetic energy of particles
- BThe total potential energy of particles
- CThe sum of the kinetic and potential energy stores of particles
- DThe energy transferred to a substance when heated
3.During which process does the temperature of a pure substance remain constant?
Easy
- AHeating a solid
- BMelting and boiling
- CCooling a gas
- DHeating a liquid
4.What happens to the kinetic energy of particles when a solid is heated and its temperature rises?
Easy
- AIt decreases
- BIt stays the same
- CIt increases
- DIt is converted to potential energy
5.Which statement about the cooling curve of a pure substance is correct?
Medium
- ATemperature decreases at a constant rate throughout
- BTemperature remains constant during condensation and freezing
- CThe gas phase has the highest temperature at the start
- DThe curve is identical to the heating curve
6.An impure substance melts over a range of temperatures. What does this indicate?
Medium
- AThe substance has a high specific heat capacity
- BThe substance contains more than one element or compound
- CThe substance is a pure compound
- DThe substance is in the gas phase
7.What happens to the potential energy of particles during melting?
Medium
- AIt decreases
- BIt remains constant
- CIt increases
- DIt is converted to kinetic energy
8.On a heating curve for a pure substance, which section shows an increase in kinetic energy without a change in state?
Medium
- AThe horizontal plateau
- BThe sloping line before a plateau
- CThe sloping line after a plateau
- DBoth sloping sections

更多题目

9.Why is the term 'latent' used for the heat absorbed during melting?
Medium
- ABecause it causes a large temperature change
- BBecause it is 'hidden' and does not cause a temperature rise
- CBecause it is released during cooling
- DBecause it is measured in joules
10.Which of the following best describes the particle arrangement in a solid?
Easy
- AParticles are far apart and move quickly
- BParticles are close together in a regular pattern and vibrate about fixed positions
- CParticles are close together but can slide past each other
- DParticles are randomly arranged and move freely
11.What happens to the motion of gas particles as they cool?
Easy
- AThey move faster
- BThey move slower
- CThey stop moving
- DThey vibrate more
12.A student measures the temperature of a pure substance as it cools from gas to solid. Which sequence of states is correct?
Easy
- AGas → liquid → solid
- BSolid → liquid → gas
- CGas → solid → liquid
- DLiquid → gas → solid
13.The cooling curve of a pure substance shows a horizontal section at 80°C. What is happening to the substance during this section?
Medium
- AIt is condensing from gas to liquid.
- BIt is freezing from liquid to solid.
- CIt is boiling from liquid to gas.
- DIt is melting from solid to liquid.
14.A student heats a pure substance and records the temperature every 30 seconds. The graph shows a horizontal line at 100°C for 2 minutes. Which statement explains why the temperature does not rise during this time?
Medium
- AThe substance is boiling and the added energy is used to overcome intermolecular forces, increasing potential energy.
- BThe substance is boiling and the added energy increases the kinetic energy of the particles.
- CThe substance is melting and the added energy is used to break bonds, increasing kinetic energy.
- DThe substance is condensing and the added energy is released to the surroundings.
15.A pure substance has a melting point of 0°C and a boiling point of 100°C. What is the state of the substance at 50°C?
Easy
- ASolid
- BLiquid
- CGas
- DSolid and liquid mixture
16.The heating curve for a pure substance shows two horizontal sections. The first is at 0°C and the second at 100°C. What is the substance?
Medium
- AWater
- BEthanol
- COxygen
- DIron
17.On a cooling curve, the temperature of a pure substance remains constant during which process?
Easy
- ACondensation
- BHeating
- CCooling of a liquid
- DExpansion
18.An impure substance melts over a range of temperatures. Which graph shape would you expect for its heating curve?
Medium
- AA diagonal line sloping upward with no horizontal sections.
- BA diagonal line with a single horizontal section at the melting point.
- CA diagonal line with a rounded, sloping section instead of a sharp horizontal line.
- DA horizontal line at all temperatures.
19.A student measures the temperature of a pure substance as it cools from 120°C to 20°C. The cooling curve shows a horizontal section at 100°C. Which change of state is occurring at 100°C?
Medium
- ACondensation
- BFreezing
- CMelting
- DBoiling
20.During the melting of a pure solid, the temperature remains constant. What happens to the internal energy of the substance?
Medium
- AIt decreases.
- BIt remains constant.
- CIt increases.
- DIt first increases then decreases.
21.What is the term for the thermal energy absorbed during a state change without a temperature change?
Easy
- ALatent heat
- BSpecific heat capacity
- CKinetic energy
- DThermal conductivity
22.A pure substance is heated from -10°C to 110°C. Its heating curve shows a horizontal section at 0°C. Which change of state occurs at 0°C?
Easy
- AMelting
- BFreezing
- CBoiling
- DCondensation
23.The cooling curve of a pure substance shows a horizontal section at 0°C. What is the substance likely to be?
Medium
- AWater
- BEthanol
- CMercury
- DSodium chloride
24.What happens to the motion of particles in a liquid as it cools?
Easy
- AThey move more slowly.
- BThey move more quickly.
- CThey stop moving.
- DThey move at the same speed.
25.In an experiment to determine the melting point of a pure solid, a student heats the solid and records its temperature every 30 seconds. The graph shows a horizontal section at 78°C. Which statement explains why the temperature remains constant?
Medium
- AThe thermal energy supplied is used to break bonds between particles, increasing potential energy.
- BThe thermal energy supplied is used to increase the kinetic energy of the particles.
- CThe substance is losing heat to the surroundings at the same rate it is gaining heat.
- DThe substance has reached its maximum temperature and cannot get hotter.
26.A student records the cooling curve of an unknown pure substance. The substance cools from 120°C to 20°C. The graph shows a horizontal section at 80°C. What is the state of the substance at 90°C?
Medium
- AGas
- BLiquid
- CSolid
- DGas and liquid
27.A pure substance is heated from -20°C to 120°C. Its heating curve shows a horizontal section at 0°C and another at 100°C. Which change of state occurs at 0°C?
Easy
- AMelting
- BBoiling
- CCondensation
- DFreezing
28.A student heats an impure substance and records its temperature over time. Which graph shape would you expect for its heating curve?
Medium
- AA curve with a single horizontal section at the melting point.
- BA curve with a sloping section during melting, not a sharp horizontal line.
- CA curve with two horizontal sections at different temperatures.
- DA curve with no horizontal sections at all.
29.During the cooling of a pure liquid, the temperature remains constant at 80°C for several minutes. What is happening to the substance during this time?
Medium
- AThe liquid is freezing into a solid.
- BThe liquid is boiling into a gas.
- CThe gas is condensing into a liquid.
- DThe solid is melting into a liquid.
30.What is the term for the thermal energy absorbed or released during a change of state without a change in temperature?
Easy
- ALatent heat
- BSpecific heat capacity
- CInternal energy
- DKinetic energy
31.A student measures the temperature of a pure substance as it cools from 150°C to 30°C. The cooling curve shows a horizontal section at 100°C. Which change of state occurs at 100°C?
Medium
- ACondensation
- BFreezing
- CMelting
- DBoiling
32.A pure substance has a melting point of -10°C and a boiling point of 60°C. What is the state of the substance at 25°C?
Easy
- ASolid
- BLiquid
- CGas
- DSolid and liquid
33.The graph shows the heating curve of a pure substance. Which section of the curve represents an increase in the kinetic energy of the particles without a change in state?
Medium
- AThe horizontal sections only
- BThe sloping sections only
- CBoth sloping and horizontal sections
- DNeither sloping nor horizontal sections
34.A student plots a cooling curve for a pure substance. The curve shows a horizontal section at 0°C for 3 minutes. Which of the following is true about the internal energy of the substance during this time?
Hard
- AInternal energy decreases because potential energy decreases.
- BInternal energy remains constant because temperature is constant.
- CInternal energy decreases because kinetic energy decreases.
- DInternal energy increases because potential energy increases.
35.A pure substance is heated from solid to gas. On its heating curve, which change of state occurs at the higher temperature?
Easy
- AMelting
- BBoiling
- CCondensation
- DFreezing
36.What is an impure substance?
Easy
- AA substance that contains more than one element or compound
- BA substance that is dirty and contains visible particles
- CA substance that has a fixed melting point
- DA substance that is made from only one element
37.On a cooling curve of a pure substance, the temperature remains constant during which process?
Medium
- ACondensation and freezing
- BMelting and boiling
- CHeating a solid
- DCooling a gas
38.What happens to the internal energy of a substance as it increases in temperature?
Easy
- AThe internal energy increases
- BThe internal energy decreases
- CThe internal energy remains constant
- DThe internal energy first increases then decreases
39.Why does the temperature of a pure substance stay constant while it melts?
Medium
- AThe thermal energy supplied is used to break bonds, increasing potential energy, not kinetic energy
- BThe substance is not absorbing any thermal energy during melting
- CThe particles stop moving completely during melting
- DThe thermal energy is used to increase the kinetic energy of the particles
40.How does the heating curve of an impure substance differ from that of a pure substance?
Medium
- AMelting and boiling occur over a range of temperatures, not at a single temperature
- BThe curve has no horizontal sections
- CThe curve is steeper for an impure substance
- DThere is no difference; both have the same shape
41.What is a pure substance made from?
Easy
- AOnly one element or compound
- BA mixture of two elements
- CA compound and an element
- DAny substance found in nature
42.During a state change, the temperature of a pure substance remains constant.
Easy
True or false?
43.While a pure substance is increasing in temperature, which energy store of the particles increases?
Medium
- AKinetic energy
- BPotential energy
- CBoth kinetic and potential energy
- DNeither kinetic nor potential energy
44.During melting, bonds are broken and the temperature remains constant. Which energy store increases?
Medium
- AKinetic energy
- BPotential energy
- CBoth kinetic and potential energy
- DNeither kinetic nor potential energy
45.An impure substance melts at a single, sharp temperature.
Easy
True or false?
46.The word 'latent' in 'latent heat' comes from Latin. What does it mean?
Easy
- AHidden
- BVisible
- CChange
- DHeat
47.As a liquid is boiled, bonds are broken and the kinetic energy store of particles increases.
Medium
True or false?
48.Which of the following is an example of a pure substance?
Easy
- ADistilled water
- BSea water
- CAir
- DMilk
49.Match each section of a heating curve for a pure substance to the correct description.
Medium
- Section A (rising line)
- Section B (horizontal line)
- Section C (rising line after B)
- Melting / boiling
- Heating solid / liquid / gas
- Heating liquid / gas
50.Put the following events in the correct order as a pure solid is heated to become a gas.
Medium
- Solid particles vibrate more and temperature rises.
- Solid melts; temperature stays constant.
- Liquid particles move faster and temperature rises.
- Liquid boils; temperature stays constant.
- Gas particles move rapidly and temperature rises.
51.Complete the sentence:
Easy
Internal energy consists of the ____ energy and ____ energy stores of the particles.
52.A pure substance is heated from -20°C to 120°C. Its heating curve shows horizontal sections at 0°C and 100°C. How many degrees does the temperature rise while the substance is in the liquid state?
Medium
53.Describe why the temperature of a pure substance stays constant while it melts.
Medium
54.A pure substance is heated from -20°C to 120°C. Its heating curve shows horizontal sections at 0°C and 100°C. How many degrees does the temperature rise while the substance is in the liquid state?
Medium
- A100 °C
- B110 °C
- C120 °C
- D90 °C

复习笔记

## Heating and Cooling Curves - **Pure substance**: made from only one element or compound. It has a single melting/boiling point. - **Heating curve**: temperature rises until a state change, then stays constant during the change (e.g., melting, boiling). - **Cooling curve**: temperature falls until a state change, then stays constant during the change (e.g., condensing, freezing). - **Internal energy**: sum of kinetic energy and potential energy of particles. - As temperature rises, kinetic energy increases (particles move/vibrate faster). - During a state change, temperature is constant; bonds break/form, potential energy changes (latent heat). - **Impure substance**: contains more than one element/compound; melts/boils over a range of temperatures. ### Key Points - **Melting**: solid → liquid; bonds break, potential energy increases, temperature constant. - **Boiling**: liquid → gas; bonds break, potential energy increases, temperature constant. - **Latent heat**: thermal energy supplied during state change, 'hidden' because temperature doesn't change. - For impure substances, the heating/cooling curve shows a sloped region instead of a flat plateau.

Pure substance: A substance made from only one element or compound.
Impure substance: A substance containing more than one element or compound; melts/boils over a range of temperatures.
Internal energy: The total kinetic energy and potential energy stores of the particles in a substance.
Latent heat: Thermal energy supplied to a material during a state change, where temperature remains constant; from Latin for 'hidden'.
Heating curve: A graph showing temperature change as a substance is heated; plateaus indicate state changes.
Cooling curve: A graph showing temperature change as a substance is cooled; plateaus indicate state changes.

闪卡

What is a pure substance?

A pure substance is made from only one element or compound.

What happens to temperature during a state change for a pure substance?

The temperature remains constant during a state change.

What is internal energy?

Internal energy consists of the kinetic energy and potential energy stores of the particles.

What happens to the kinetic energy of particles when a substance increases in temperature?

The kinetic energy store of the particles increases.

What happens to particles in a solid as it increases in temperature?

The vibrations of the particles in the solid increase.

What happens to bonds during melting?

Some bonds are broken during melting.

What happens to the potential energy of particles during melting?

The potential energy store of the particles increases.

What is latent heat?

Latent heat is the thermal energy supplied to a material during a state change, where temperature remains constant. 'Latent' comes from Latin for 'hidden'.

What happens to particles in a liquid or gas as temperature increases?

Particles move more quickly and their kinetic energy store increases.

What happens to bonds during boiling?

Bonds are broken during boiling.

What is an impure substance?

An impure substance contains more than one element or compound (i.e., contains impurities).

How does the melting/boiling of an impure substance differ from a pure substance?

For an impure substance, melting and boiling happen over a range of temperatures, not at a single temperature.

What happens to gas particles as they cool?

Gas particles slow down and their kinetic energy decreases.

What happens to the internal energy of a substance as it increases in temperature?

The internal energy increases.

Why does temperature stay constant during melting of a pure substance?

Because the thermal energy supplied is used to break bonds and increase potential energy, not to increase kinetic energy.

What are the three states of matter in order of increasing temperature on a heating curve?

Solid, liquid, gas.

On a cooling curve, what state changes occur as temperature decreases?

Gas condenses to liquid, then liquid freezes to solid.

What is the difference between a heating curve and a cooling curve?

A heating curve shows temperature increase over time as energy is added; a cooling curve shows temperature decrease over time as energy is removed.

What does a flat section on a heating curve represent?

A state change (melting or boiling) where temperature remains constant.

What does a flat section on a cooling curve represent?

A state change (condensing or freezing) where temperature remains constant.

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