Cracking glass

Class practical

Comparing the resistances to cracking of soda glass and Pyrex (borosilicate) glass when heated.

Apparatus and Materials

• Piece of window glass (approximately 100 mm square)
• Tripod
• Clear silica tubes 125 mm (if available), 2
• Safety screens, 2
For each group of students
• Soda glass tube or length of tubing 125 mm
• Pyrex (borosilicate) glass tube or length of tubing 125 mm
• Bunsen burner
• Beaker

Health & Safety and Technical Notes

Whenever a demonstration is performed with glass which might shatter and hurt someone, you must use a pair of large safety screens of Perspex. The screen between the teacher and the apparatus should be approximately 1 m high by 60 cm wide. This is narrow enough so that you can reach round and manipulate the apparatus and high enough to shield the face. The screen between the apparatus and the class should be approximately 1 m square.

These screens (of 3 mm Perspex or thicker) should not be framed. That would spoil the feeling of full transparency. They can be supported by pairs of slotted bases.

The fractured glass tube must be disposed of safely.

Used engine oil contains carcinogens and must NOT be brought into a school laboratory. New and unused oil is safe to use.

Read our standard health & safety guidance

Procedure

1. Place the piece of window glass on a tripod. Heat it near one corner with a bare flame. When the pane cracks, ask students why this happens. Do not give an explanation yet.
2. Give each group of students a soda glass tube. Ask them to heat it in a Bunsen flame. Then plunge it into water in the beaker. Tell them to repeat the experiment with the Pyrex tubing.
3. Repeat the experiment again, as a demonstration, with the silica tube if it is available.

Teaching Notes

• The window glass cracks because it is not a good thermal conductor. The portion in the flame expands so that the strain between that portion and the cool, unheated part fractures the glass.
• The difference between the Pyrex glass and the soda glass is probably due to the lower expansion of the Pyrex glass. The strain it experiences is lower than soda glass for the same temperature change.

• Students may suggest that the difference in cracking between soda glass and Pyrex is that Pyrex is able to stand a greater strain. You can demonstrate that this is not the case. Put two lengths (1 m or 1.5 m soda glass and Pyrex of equal size) between two stools. Depress the centre of each one in turn with a half-metre rule. Note the position when each breaks, which should be very similar. The greater the depression, the greater the strain. Normally the tubes break with the same amount of bending; the same strain.
• The silica tubing withstands high temperatures very well and does not crack. This is why Nichrome wire is threaded through silica glass tubing to make the element in a bar fire.

This experiment was safety-tested in March 2006