To Determine Melting and Boiling Points of a Pure Substance (Class 9 Chemistry Practical)
To experimentally determine the melting point and boiling point of a pure substance using heating and cooling methods.
TL;DR: To experimentally determine the melting point and boiling point of a pure substance using heating and cooling methods.
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Aim: To experimentally determine the melting point and boiling point of a pure substance using heating and cooling methods.
Materials Required
- Pure substance (naphthalene or paradichlorobenzene)
- Thermometer (0-110°C)
- Test tube
- Beaker
- Water bath/Bunsen burner
- Clamp and stand
- Stirring rod
Theory
Melting point is the temperature at which a solid changes to liquid at constant pressure. Boiling point is the temperature at which a liquid changes to vapor. For a pure substance at constant pressure, these temperatures are constant. Heat is added during melting and boiling without temperature change (latent heat).
Procedure
- For melting point: Take a test tube containing the pure solid substance
- Insert a thermometer into the substance without touching the tube walls
- Heat the test tube gently in a water bath, stirring occasionally
- Note the temperature when the first drop of liquid appears (melting starts)
- Continue heating and note the temperature when the last crystal melts (melting ends)
- The melting point is the average of these two temperatures
- For boiling point: Heat the test tube further until the liquid boils
- Record the temperature at which steady boiling occurs (boiling point)
Observation
The solid naphthalene appears white and opaque. Upon heating, it begins to soften at [temperature]. The first liquid drop appears, and the substance gradually melts. At [boiling point]°C, the liquid begins to boil steadily with vapor evolving.
Result
The melting point of the substance is [value]°C and the boiling point is [value]°C. These values match the known values for the pure substance, confirming its purity.
Precautions
- Use a water bath for gentle, uniform heating
- Stir the substance occasionally for uniform heating
- Do not allow the thermometer to touch the tube walls
- Record observations carefully at the exact moment of melting/boiling
- Cool before removing the tube from the bath
Viva Questions
What is the melting point?
Melting point is the temperature at which a solid changes to liquid at constant atmospheric pressure.
Why is a water bath preferred over direct flame?
A water bath provides uniform, gentle heating and prevents decomposition of the substance at high local temperatures.
What is latent heat of fusion?
Latent heat of fusion is the heat energy required to change a solid to liquid at constant temperature.
How does pressure affect boiling point?
Higher pressure increases the boiling point; lower pressure decreases it.
More Class 9 Chemistry Practicals
- To Prepare a True Solution and Observe the Tyndall Effect
- To Find the pH of Samples Using Indicators
- To Study a Double Displacement (Precipitation) Reaction
- To Test Acids and Bases with Indicators
- To Verify the Law of Conservation of Mass
- To Study the Effect of Temperature on Reaction Rate
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