Change of State — Part 2

E1 · ENGAGE

What happens when we stop heating?

In Part 1 we heated water from ice to steam. Now we ask the reverse question — what happens when we let things cool down, or never heat them at all? Every object left alone will eventually reach the temperature of the room around it.

🧊 Live Cooling Simulation

Room temperature: 20°C

CURRENT TEMP

120°C

CURRENT STATE

Gas (Steam)

Gas cooling

Condensation plateau (100°C)

Liquid cooling

Solidification plateau (0°C)

Solid cooling → room temp

🌡️

Room Temperature Rule

Any object left without heating or cooling will eventually reach the temperature of its surroundings. This is called thermal equilibrium. In Shatila camp, room temperature is about 20°C in most seasons.

📉

The Cooling Curve

A cooling curve is a temperature-vs-time graph that shows what happens when a substance loses heat. It is the mirror image of the heating curve — same plateaus, same transition temperatures, but going downward.

Q1 1 pt Obj 6 A cup of hot tea at 75°C is left on the table in a room at 22°C. After many hours with no heating, what temperature will the tea reach?

A0°C — it will freeze eventually

B75°C — it keeps the temperature it started with

C22°C — it equilibrates with the room

D50°C — halfway between start and room temperature

🟡 Hint 1: Think about what "no heating" means for a system.

🟣 Hint 2: Objects always exchange heat with the environment until they match it.

🔵 Hint 3: Thermal equilibrium = object temperature = room temperature.

Q2 1 pt Obj 6 On a cooling curve for water starting at 120°C, the temperature stops decreasing for a while at 100°C. What is happening during this plateau?

AThe water is absorbing heat from the surroundings

BThe steam is condensing — releasing heat energy while changing state

CThe temperature sensor is broken

DThe water has reached room temperature

How do you feel about the cooling curve concept?

E2 · EXPLORE

Build Your Own Cooling Curve

A scientist gives you a table of temperature readings taken every minute while a substance cools from 110°C to room temperature. Your job: read the table, identify each time interval's state, then watch your curve come to life.

📋 Experimental Data — Substance X cooling from 110°C (Melting point: 80°C · Boiling point: ??°C → you'll find out!)

Time (min)	Temperature (°C)	Observation
0	110	Still very hot — no bubbles
2	104	Cooling slowly, vapor visible
4	100	⚠ Temperature STOPPED changing
6	100	⚠ Still 100°C — plateau continues
8	96	Temperature dropping again
10	88	Liquid visible in container
12	80	⚠ Temperature STOPPED AGAIN
14	80	⚠ Still 80°C — second plateau
16	60	Substance has solidified
18	35	Cooling toward room temp
20	20	Reached room temperature

States identified: 0 / 11

📈 Your Cooling Curve — builds as you answer the table

Data points you identified

Full curve (hidden until revealed)

🔬 Discovery Questions

Q11 ptObj 6 How many plateaus appear on the cooling curve of Substance X?

AZero — temperature always changes smoothly

BTwo — at 100°C (condensation) and 80°C (solidification)

CThree — at 100°C, 80°C, and 20°C

Q21 ptObj 5+6 Based on the data table, what is the melting point of Substance X?

A100°C

B80°C

C20°C

D110°C

Q31 ptObj 6 At t = 10 min, Substance X is at 88°C. Its melting point is 80°C and boiling point is 100°C. What state is it in?

ASolid

BLiquid (between melting and boiling point)

CGas

DMixture of liquid and solid

Can you read a cooling data table and draw a curve?

E3 · EXPLAIN

Reading the Graph — Expert Level

Scientists must extract exact information from temperature-time graphs. Let's practice with three different scenarios — a heating curve, a cooling curve, and a mystery substance.

📊 Graph A — Heating Curve for Water

Study the labeled graph, then answer the questions below

GA-11 pt On Graph A, which segment shows the liquid water heating up?

ASegment AB — from -15°C to 0°C

BSegment BC — the flat plateau at 0°C

CSegment CD — from 0°C to 100°C

DSegment DE — the flat plateau at 100°C

GA-21 pt On Graph A, segment DE is the boiling plateau. What is the substance doing during this interval?

AGaining temperature as it turns to gas

BAbsorbing heat energy to break intermolecular bonds — changing state at constant temperature

CLosing heat to the surroundings

GA-31 pt A point on graph A is at t=8 min. Temperature reads 55°C. Which state is the water in?

ASolid

BLiquid — 55°C is between 0°C (MP) and 100°C (BP)

CGas

DMelting mixture

📊 Graph B — Cooling Curve for Substance Y (MP = 45°C · BP = 95°C)

Room temperature = 20°C

GB-11 pt Graph B shows substance Y cooling. What state is substance Y in at 70°C?

ASolid (70°C > 45°C MP, but wait...)

BLiquid — 70°C is between 45°C (MP) and 95°C (BP)

CGas

GB-21 pt On Graph B, after cooling stops (substance reaches room temperature), the curve becomes a flat horizontal line. What does this mean?

AThe substance froze solid and can't cool further

BThe substance reached thermal equilibrium with its surroundings — no more heat exchange

CThe experiment ended and data stopped being recorded

🔢 Numeric Exercise 2 pts

Substance Y has a melting point of 45°C and boiling point of 95°C.
Question: At 30°C, what state is substance Y in? At 100°C, what state is it in?

State at 30°C:

State at 100°C:

Can you read heating AND cooling graphs?

E4 · ELABORATE

🕵️ Temperature Detective

Five rounds. Each round shows you a mystery graph with a glowing dot at a specific point. Your mission: identify the state of the substance at that point. Earn points, keep your lives!

Round 1 / 5

❤️❤️❤️

Score: 0

Loading round...

🧪 Bonus Challenge — The Mixture Moment 2 pts

A pure substance is at exactly its melting point. Both solid AND liquid are present at the same time. This is a mixture of states. The temperature stays fixed until all of one state converts to the other.

B11 pt Water is being cooled slowly. You notice both liquid water and ice cubes in the same glass. What temperature must the water be at?

A-10°C

B0°C — exactly at the melting/freezing point

C50°C

D100°C

B21 pt After the last piece of ice melts and only liquid remains, but no more heat is added, what happens to the water's temperature if room temperature is 22°C?

AStays at 0°C forever

BRises from 0°C toward 22°C (room temp) — thermal equilibrium

CDrops to -10°C as ice forms again

Can you identify states from any graph point?

E5 · EVALUATE

Final Assessment

This is your exam-style challenge. All skills from both Part 1 and Part 2 are tested here. Take your time — read each question carefully.

Part A — Graph Interpretation

Questions 1–5 · 5 points

11 pt The graph above shows a heating curve. What state is the substance in during segment BC (the first flat region)?

APure solid

BPure liquid

CMixture of solid and liquid (at melting point)

DGas

21 pt In the graph, segment EF rises steeply after the boiling plateau. What state is the substance in along EF?

ALiquid

BGas (steam/vapor)

CSolid

31 pt Why does the temperature remain constant at 0°C during segment BC?

ANo heat is being added during this segment

BAll heat energy goes into breaking bonds for the state change — none raises temperature

CThe substance is releasing heat equal to what it gains

41 pt You are given this data about Substance Z: Melting Point = 62°C · Boiling Point = 182°C · Room Temp = 25°C. In which state is Substance Z at room temperature?

ASolid — 25°C is below the melting point (62°C)

BLiquid — 25°C is between MP and BP

CGas

51 pt A cooling curve for water starts at 120°C. The curve shows two plateaus. Which temperatures do these plateaus occur at?

A50°C and 25°C

B100°C (condensation) and 0°C (solidification)

C0°C and -15°C

Part B — Table to Graph

Questions 6–9 · 4 points

Substance W has: Melting Point = 30°C · Boiling Point = 78°C. The following temperature readings were recorded as it was heated from -10°C.

61 pt Based on Substance W data, what state is it in at 20°C?

ASolid — 20°C is below the melting point (30°C)

BLiquid

CGas

71 pt Substance W is at 50°C. What state is it in?

ASolid

BLiquid — 50°C is between MP (30°C) and BP (78°C)

CGas

81 pt Substance W is heated to 90°C. What state is it in?

ALiquid

BGas — 90°C is above the boiling point (78°C)

CSolid

91 pt Substance W is at exactly 30°C. What is observed?

AOnly solid is present

BOnly liquid is present

CBoth solid and liquid coexist — melting/freezing point transition

DBoiling begins

Part C — Cooling Curve Analysis

Questions 10–12 · 3 points

101 pt The cooling curve above ends with a flat horizontal line at 22°C. The room temperature is 22°C. What does this final horizontal segment represent?

AThe substance has frozen completely

BThermal equilibrium — substance reached room temperature and stops changing

CAnother phase change is occurring at 22°C

111 pt The graph above shows a cooling curve with ONE plateau. What type of state change is occurring at that plateau?

AMelting (fusion)

BEvaporation

CSolidification (freezing) — liquid releasing heat to become solid

121 pt A teacher gives you this information about a substance: "At 25°C the substance is liquid. At -5°C it is solid." What can you conclude about the melting point?

AThe melting point is somewhere between -5°C and 25°C

BThe melting point is exactly 10°C

CThe melting point is 25°C

Part D — Written Response

Question 13 · 2 pts (teacher reviewed)

Question 13: A scientist measures the temperature of a beaker of water every minute as it cools from 100°C to room temperature (20°C) with no heating. Describe what the temperature-time graph would look like. Include: where the plateau is, what happens after the plateau, and what temperature the curve ends at.

💡 Key words to include: plateau · 0°C · solidification · room temperature · thermal equilibrium

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