Class 11 Chemistry Notes Chapter 4 (Chapter 4) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 4 from your Lab Manual, "Basic Laboratory Techniques." These fundamental skills are crucial not just for your practical exams but often form the basis for questions in various government competitive exams where basic science knowledge is tested. Pay close attention to the procedures and especially the precautions.

Chapter 4: Basic Laboratory Techniques - Detailed Notes

This chapter covers essential skills required for handling common laboratory apparatus made of glass and cork.

1. Cutting a Glass Tube or Glass Rod

• Aim: To cut a glass tube or rod to a desired length accurately and safely.
• Materials Required: Soda-glass tube or rod, triangular file, metre scale/ruler, wire gauze, Bunsen burner.
• Procedure:
  1. Marking: Place the glass tube/rod on a flat surface. Measure and mark the desired length.
  2. Scratching: Hold the tube/rod firmly. Place the sharp edge of the triangular file on the mark. Press down firmly and make a single, deep scratch perpendicular to the length of the tube/rod. Do not saw back and forth.
  3. Breaking: Hold the tube/rod with the scratch facing upwards and away from you. Place your thumbs on the opposite side of the scratch, close together. Apply gentle outward pressure while pulling the hands apart slightly. The glass should break cleanly at the scratch.
  4. Fire Polishing (Crucial Step): The freshly cut edges are very sharp. To smooth them, hold one end of the cut tube/rod and introduce the edge into the outer edge of a non-luminous (blue) Bunsen burner flame. Rotate the tube/rod continuously until the sharp edges melt slightly and become rounded. Do not overheat, as this can cause the opening to narrow or close. Repeat for the other end if necessary. Place the hot glass on a wire gauze to cool down; do not place hot glass directly on the cold benchtop.
• Principle: Making a scratch creates a point of stress concentration. When pressure is applied, the stress exceeds the tensile strength of the glass at that point, causing a clean break. Fire polishing melts the sharp edges due to heat, and surface tension pulls the molten glass into a rounded shape.
• Precautions:
  • Make only one deep scratch. Multiple scratches weaken the glass unpredictably.
  • Hold the glass firmly but not too tightly to avoid breaking it prematurely.
  • Always break the glass tube/rod away from your body.
  • Wear safety glasses.
  • Fire polish the cut edges immediately to avoid injuries.
  • Rotate the glass continuously during fire polishing for uniform heating.
  • Allow the glass to cool completely on wire gauze before handling.

2. Bending a Glass Tube

• Aim: To bend a glass tube at a required angle (e.g., 60°, 90°, 120°).
• Materials Required: Soda-glass tube (approx. 20-25 cm long), Bunsen burner, wire gauze, (optional: asbestos sheet with angle markings).
• Procedure:
  1. Heating: Hold the glass tube horizontally with both hands. Introduce the portion where the bend is required into a luminous (yellow/bushy) Bunsen burner flame. Use a flame spreader (wing top) if available for wider heating.
  2. Rotating: Rotate the tube continuously and uniformly using your thumb and fingers to ensure even heating over a length of 4-5 cm. Continue heating until the glass becomes soft and pliable (it will start to sag slightly, and the flame may turn orange/yellow due to sodium ions).
  3. Bending: Remove the softened tube from the flame. Hold it for a second, then slowly and smoothly bend it upwards to the desired angle. Allow it to bend under its own weight as much as possible rather than forcing it. Ensure the bend is smooth and rounded, not flattened or kinked.
  4. Cooling (Annealing): Place the bent tube on a wire gauze or asbestos sheet and allow it to cool down slowly. Slow cooling prevents the build-up of internal stress and makes the bend less fragile.
• Principle: Glass (especially soda-lime glass) softens upon heating. Uniform heating prevents localized overheating and potential collapse or thinning of the tube walls. Slow cooling (annealing) allows the glass structure to stabilize without internal stresses, preventing easy breakage later.
• Why Luminous Flame? A luminous flame is less hot but covers a wider area than a non-luminous flame. This allows for uniform heating over a larger section of the tube, which is essential for a smooth, gradual bend without kinks.
• Precautions:
  • Use a luminous flame for uniform heating over a wider area.
  • Rotate the tube continuously while heating.
  • Do not heat too strongly or in one spot, as this can melt the glass or cause the tube walls to collapse.
  • Bend the tube after removing it from the flame.
  • Bend slowly and smoothly; avoid forcing the bend.
  • Cool the bent tube slowly on wire gauze (annealing).
  • Wear safety glasses.

3. Drawing Out a Glass Jet

• Aim: To draw a fine nozzle or jet at the end of a glass tube.
• Materials Required: Soda-glass tube (approx. 15-20 cm long), Bunsen burner, triangular file, wire gauze.
• Procedure:
  1. Heating: Hold the glass tube horizontally. Heat the middle portion strongly in a non-luminous (blue) flame. Rotate the tube continuously for uniform heating until the glass becomes very soft and red hot over a small section (1-2 cm).
  2. Drawing: Remove the tube from the flame. Pause for a second, then pull the ends apart steadily, smoothly, and horizontally. The heated section will narrow down into a capillary. Continue pulling until the capillary is of the desired thickness. Keep the tube aligned while pulling.
  3. Cooling: Hold the tube steady until the glass solidifies. Place it on a wire gauze to cool completely.
  4. Cutting: Once cool, make a fine scratch with a file at the desired point on the capillary section and break it gently.
  5. Finishing: Fire polish the sharp tip of the jet very carefully and briefly in the flame to smooth it without sealing the opening.
• Principle: Strong heating in a concentrated area makes the glass very soft (low viscosity). Pulling the ends apart stretches this softened portion into a thin capillary.
• Why Non-Luminous Flame? A non-luminous flame is hotter and more concentrated, suitable for heating a small section of the glass tube intensely to make it soft enough for drawing out.
• Precautions:
  • Use a non-luminous flame for intense heating.
  • Rotate the tube continuously for uniform heating.
  • Pull the ends smoothly, steadily, and axially (in a straight line).
  • Do not pull too fast or too slow.
  • Be extremely careful while fire polishing the fine tip to avoid sealing it.
  • Wear safety glasses.

4. Boring a Cork

• Aim: To make a smooth hole of the required size in a cork stopper to insert a glass tube or thermometer.
• Materials Required: Cork stopper, set of cork borers, cork borer sharpener, glycerine or water, cloth.
• Procedure:
  1. Selection: Choose a cork of good quality (without deep cracks). Select a cork borer with a diameter slightly smaller than the diameter of the glass tube to be inserted (ensures a tight fit).
  2. Preparation: Ensure the cutting edge of the borer is sharp (use a sharpener if needed). Soften the cork slightly if it's very hard (e.g., by rolling underfoot or using a cork press).
  3. Lubrication: Wet the cutting edge of the cork borer with water or preferably glycerine. This reduces friction and helps in making a smooth hole.
  4. Boring: Place the cork on a flat, protected surface (like a wooden block) or hold it firmly. If the cork is tapered, start boring from the narrow end. Hold the borer perpendicular to the cork surface. Apply steady downward pressure while simultaneously twisting the borer back and forth.
  5. Completion: Continue boring until the borer emerges from the other side. Remove the borer by twisting it in the reverse direction. Push the removed cork piece out of the borer using the metal rod provided or a narrower borer.
  6. Cleaning: Wipe the borer clean.
• Principle: A sharp cutting edge combined with twisting motion cuts the cork material cleanly. Lubrication reduces friction, preventing tearing and ensuring a smoother hole.
• Precautions:
  • Select the correct size of borer.
  • Ensure the borer is sharp.
  • Always lubricate the borer (glycerine is better than water).
  • Keep the borer perpendicular to the cork surface.
  • Apply steady pressure and twist; do not use excessive force.
  • Bore from the narrow end towards the wider end for tapered corks.
  • Keep fingers away from the path of the borer as it emerges from the cork.
  • Never bore towards the palm of your hand.

Multiple Choice Questions (MCQs)

1. What is the primary purpose of fire polishing the cut ends of a glass tube?
   a) To make the tube longer
   b) To remove sharp edges and prevent injury
   c) To narrow the opening of the tube
   d) To clean the glass surface

2. Which type of Bunsen burner flame is generally preferred for bending a glass tube, and why?
   a) Non-luminous flame, because it is hotter.
   b) Luminous flame, because it heats a wider area uniformly.
   c) Non-luminous flame, because it is cleaner.
   d) Luminous flame, because it is less hot and prevents melting.

3. While cutting a glass tube, how should the scratch be made with the triangular file?
   a) By sawing back and forth multiple times.
   b) By making several shallow scratches around the tube.
   c) By making a single, deep scratch perpendicular to the tube length.
   d) By heating the tube first and then scratching.

4. What is the process of cooling a bent glass tube slowly on a wire gauze called, and what is its purpose?
   a) Quenching; to make the glass harder.
   b) Tempering; to increase flexibility.
   c) Annealing; to prevent internal stress and fragility.
   d) Sintering; to fuse glass particles.

5. When drawing out a glass jet, which type of flame is used and why?
   a) Luminous flame; for gentle heating.
   b) Non-luminous flame; for strong, concentrated heating.
   c) Luminous flame; to heat a large area.
   d) Any flame can be used.

6. When boring a hole in a cork, the diameter of the selected cork borer should ideally be:
   a) Exactly the same as the glass tube to be inserted.
   b) Slightly larger than the glass tube to be inserted.
   c) Slightly smaller than the glass tube to be inserted.
   d) Approximately half the diameter of the glass tube.

7. What substance is commonly used to lubricate a cork borer before use?
   a) Oil
   b) Acid
   c) Alcohol
   d) Glycerine or water

8. What safety precaution is essential immediately after cutting a glass tube?
   a) Washing the tube with water.
   b) Heating the entire tube evenly.
   c) Fire polishing the cut edges.
   d) Measuring the length accurately again.

9. During the bending of a glass tube, continuous rotation is necessary to:
   a) Cool the glass quickly.
   b) Ensure uniform heating and prevent sagging in one spot.
   c) Make the glass tube shorter.
   d) Create sharp angles.

10. For a tapered cork, boring should ideally start from:
    a) The wider end.
    b) The narrow end.
    c) The middle.
    d) Either end.

Answer Key:

1. b) To remove sharp edges and prevent injury
2. b) Luminous flame, because it heats a wider area uniformly.
3. c) By making a single, deep scratch perpendicular to the tube length.
4. c) Annealing; to prevent internal stress and fragility.
5. b) Non-luminous flame; for strong, concentrated heating.
6. c) Slightly smaller than the glass tube to be inserted.
7. d) Glycerine or water
8. c) Fire polishing the cut edges.
9. b) Ensure uniform heating and prevent sagging in one spot.
10. b) The narrow end.

Mastering these basic techniques is fundamental for safe and effective work in a chemistry lab. Ensure you understand the 'why' behind each step and precaution. Good luck with your preparation!