Heating Curve Diagram illustrating phase changes from solid to liquid to gas, with key points indicating state transitions and temperature plateaus.

Let's solve the problem step by step using the heating curve provided in the image. The graph shows temperature vs. time for a substance being heated, with distinct phases and phase changes.

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🔍 Understanding the Heating Curve

A heating curve illustrates how the temperature of a substance changes as it is heated over time. Key features:

- Sloped lines: Temperature increases → substance is absorbing heat and increasing kinetic energy.
- Horizontal (flat) lines: Temperature stays constant → phase change is occurring (solid ↔ liquid ↔ gas). Energy goes into breaking intermolecular bonds, not increasing temperature.

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## ✔ Activity #1: Heating Curves – Answers & Explanations

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Part 1: At point A, the beginning of observations...

> At point A, the substance exists in a solid state.
> Material in this phase has a fixed volume and shape.
> With each passing minute, heat/energy is added to the substance.
> This causes the molecules of the substance to gain more kinetic energy and hence move faster which we detect by a temperature rise in the substance.

✔ Explanation:
- Point A is at the start of the graph, where temperature is low and rising steadily.
- The substance is solid because no phase change has occurred yet.
- Solids have fixed volume and shape due to strong intermolecular forces.
- As heat is added, molecules vibrate more, gaining kinetic energy → temperature rises.

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Part 2: At point B and C...

> At point B, the temperature of the substance is 70°C and at Point C the temperature is 70°C.
> This first change in state is where melting occurs as the temperature remains constant at the melting point of the substance.
> The substance is now in the liquid phase.
> Material in this phase has variable volume and no fixed shape.
> The energy put to the substance between minutes 5 and 9 was used to convert the substance from a solid to a liquid.

✔ Explanation:
- From the graph:
- Point B is at 70°C, and the flat line from B to C continues at 70°C.
- This horizontal segment (B to C) represents the melting process.
- During melting, temperature remains constant even though heat is added — that energy breaks bonds to turn solid into liquid.
- Melting point = 70°C.
- After melting (at point C), the substance becomes liquid.
- Liquids have definite volume but take the shape of their container (so "no fixed" shape).
- Time interval from 5 to 9 minutes corresponds to the melting phase (B to C).

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📌 Summary of Key Phases on the Graph:

| Segment | Phase Change | Temperature | State |
|--------|--------------|-------------|-------|
| A → B | Solid heating | Rising | Solid |
| B → C | Melting | Constant (70°C) | Solid → Liquid |
| C → D | Liquid heating | Rising | Liquid |
| D → E | Boiling | Constant (140°C) | Liquid → Gas |
| E → F | Gas heating | Rising | Gas |

(Note: Although not asked, D→E is boiling at 140°C.)

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✔ Final Answer Fill-ins:

#### First paragraph:
At point A, the beginning of observations, the substance exists in a solid state. Material in this phase has a fixed volume and shape. With each passing minute, heat/energy is added to the substance. This causes the molecules of the substance to gain more kinetic energy and hence move faster which we detect by a temperature rise in the substance.

#### Second paragraph:
At point B, the temperature of the substance is 70°C and at Point C the temperature is 70°C. This first change in state is where melting occurs as the temperature remains constant at the melting point of the substance. The substance is now in the liquid phase. Material in this phase has variable volume and no fixed shape. The energy put to the substance between minutes 5 and 9 was used to convert the substance from a solid to a liquid.

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💡 Key Concept Recap:

- During phase changes, temperature stays constant.
- Added heat during phase changes is called latent heat.
- Melting point = 70°C
- Boiling point = 140°C (from D to E)

Let me know if you'd like help with the cooling curve or further questions!