CBSE Chemistry Experiment Viva Questions with Answers on a Determination of Melting Point

Introduction

To determine an object's purity, the melting points of several elements are measured. The melting range of an incredibly pure substance will be one or two degrees. The melting range is higher and smaller than the initial impure one and is typically depressed and widened by impure chemicals, which have a considerably higher range. A certain temperature change causes the melting to happen, but the molecule's structure also plays a role. Varying compounds have different melting characteristics precisely because of this.

Table of Contents

• Aim 

• Apparatus required 

• Theory Procedure 

• Observations 

• Result 

• Precautions 

Aim

To figure out the melting point of a given solid substance.

Apparatus Required

• Given solid substance

• Aluminium block

• Clamped stand

• Capillary tube

• Tripod 

• Thermometer

• Benzoic Acid

• Kerosene burner

Theory

Melting is the process of a compound changing from a solid to a liquid state when heated, and the melting point is the temperature at which a solid melts in its purest form. The determination of the melting point aids in the identification of the compound because every pure solid has a characteristic melting point. The melting point of the solid is lowered by the presence of impurities. Thus, a compound's melting point can be used to determine its purity.

Procedure

• Take a 5- to 6-cm-long fine capillary. Seal the capillary tube's one end by placing the end horizontally into the extreme edge of a small, steady Bunsen flame for a short period of time while rotating the capillary.

• On a porous plate, place a tiny amount of the substance whose melting point needs to be determined and grind it with a spatula.

• By inserting the capillary tube's open end into the powdered compound and gently spinning it, you can introduce the powdered compound into the tube.

• To make the compound sink into the closed end, gently tap the capillary tube against the porous plate. Three to four times, repeat the introducing and tapping steps.

• To attach the capillary to the thermometer's lower end, moisten the bulb with liquid paraffin or concentrated sulphuric acid.

• Place the thermometer and capillary tube in the melting point device such that the closed end of the capillary tube stays below the liquid paraffin's surface. The melting point apparatus should contain at least two thirds of its volume of liquid paraffin.

• Now, gently heat the beaker and record the temperature every so often. Finally, record the temperature at which the compound begins to melt and finishes melting.

• With a new capillary tube and new amounts of the material, repeat the experiment.

Determination of Melting Point 

Observation

The temperature at which the unknown substance starts to melt is denoted as 119 °C

The temperature at which a substance will totally melt is denoted as 124 °C

Unknown substance's melting point is \[\dfrac{{119 + 124}}{2} = 121.5^\circ C.\]

Results

The melting point of the given compound =121.5 °C.

Precautions

1. Make sure the benzoic acid and naphthalene samples are dry and pulverised.

2. Keep the thermometer and capillary tube at the same height.

3. Avoid leaving any air spaces when packing the powder into the capillary tube.

Lab Manual Questions

1. Ionic compounds have higher melting points for what reason?

Ans. Because of the strong electrostatic forces that interact between ions, ionic compounds have high melting temperatures.

2. How can we characterise a substance using melting points?

Ans. The melting point of a pure substance will be sharp, meaning that the compound melts fully within a range of 2 °C.

3. Ionic chemicals crystallise for what reason?

Ans. Ions are grouped in precise three-dimensional ways in ionic compounds, which makes them crystalline in nature.

4. Compounds that are nonpolar are volatile. mention a reason.

Ans. Non-polar substances are volatile because there are weak van der Waals interactions between their molecules.

Viva Questions

1. What is the melting point?

Ans. It is described as the constant temperature at which a substance can exist in both its solid and liquid states.

2. How can finding the melting point inform us of the purity of the compound?

Ans. The melting point reveals a substance's purity. A substance's melting point is typically lowered if it contains moisture or another contaminant. A pure substance is one that has a narrow melting point.

3. The acute melting point is what?

Ans. A solid's melting point is considered acute if it entirely melts within a 1 °C range.

4. Why is the melting point of pure solids so high?

Ans. A pure solid melts at a constant temperature because it has the same force of attraction between particles everywhere.

5. How do impurities affect the melting point of solids?

Ans. Solids with impurities have lower melting points.

6. Can we directly heat the capillary tube to find its melting point?

Ans. No, direct heating would provide a quick and uneven heating.

7. Can any other liquid be used to calculate the melting point of liquid paraffin?

Ans. Yes, you may find the melting point using concentrated H2S04 or silicone oils.

8. Why does benzamide have a higher melting point than acetamide?

Ans. The functional group of benzamide and acetamide is the same, however benzamide has a larger molecular mass than acetamide. Because of this, benzamide has greater melting point and stronger intermolecular pressures.

9. Why do various substances' melting points vary?

Ans. Intermolecular forces that occur in the solid-state affect melting point. Since the intensity of the intermolecular interactions varies among different substances, so do their melting points.

10. What temperature does water reach its maximum density?

Ans. The maximum density of water is reached at 4 °C.

Practical Based Questions:

1. The compound's purity is verified by _______.

1. Its melting and boiling point

2. Chromatographic technique

3. Spectroscopy

4. All of the above

Ans. The compound's purity is verified by its melting and boiling point, chromatographic technique, and spectroscopy.

2. Among the following assertions, pick the ones that are true.

1. Melting is the process of turning a solid into a liquid, and freezing is the opposite.

2. Freezing is the process of turning a solid into a liquid, and melting is the opposite.

3. Melting is the process of turning a liquid into a solid, whereas freezing is the opposite.

4. None of the above

Ans. Melting is the process of turning a solid into a liquid, and freezing is the opposite.

3. The temperature at which solid and liquid coincide in equilibrium is called ______.

1. Melting point of liquid

2. Freezing point of liquid

3. Freezing point of solid

4. All of the above

Ans. The temperature at which solid and liquid coincide in equilibrium is called freezing point of liquid.

4. Among the following statements, pick the ones that are false.

1. Every pure solid crystalline substance has a certain melting point that is typical to it.

2. A sample of impure material has a varied melting point.

3. The melting points of two separate pure substances coincide.

4. The test for a solid substance's purity is its melting point.

Ans. The melting points of two separate pure substances coincide.

5. The temperature _____ when the solid and liquid phases are in equilibrium.

1. Increases gradually

2. Decreases gradually

3. Remains constant

4. None of the above

Ans. The temperature remains constant when the solid and liquid phases are in equilibrium.

6. Ice melts at a temperature of _____.

1. 0 °C

2. 100 °C

3. 4 °C

4. -4 °C

Ans. Ice melts at a temperature of 0 °C.

7. The definition of molar heat of fusion is.

1. Energy needs to melt one gram of solid.

2. Energy needs to melt one mole of solid.

3. Energy needs to melt one kilogram of solid.

4. Energy needs to melt ten moles of solid.

Ans. The definition of molar heat of fusion is energy needed to melt one mole of solid. 

8. At 100 °C, how does water behave?

1. Solid

2. Vapour

3. Liquid

4. Liquid and vapour

Ans. At 100 °C, water behaves as vapour.

9. What is the water's molar heat of fusion?

1. 1.3

2. 0.84

3. 7.61

4. 6.01

Ans. The water's molar heat of fusion is 6.01.

10. What is the mercury's melting point?

1. 357 °C

2. -39 °C

3. 0 °C

4. 100 °C

Ans. The mercury's melting point is -39 °C.

Conclusion

People can better grasp a material's physical and chemical qualities by knowing its melting point. Any substance's melting point is influenced by a wide range of variables, including the force of attraction, any impurities present, and the size and structure of the molecules.