Glass capillaries
 

Making glass capillaries (class practical)
Curriculum relevance:  uses of chemistry: properties of glass: melting, physical change, fibre optics, (total reflection, endoscopes, data transmission) animal cloning, capillary action, weathering.  Everyday practical relevance.  

Soda lime glass: Softening Point: 726°C, Annealing Range: 546°C
Na2CO3 which lowers the melting point of SiO2 from 1700 °C to about 1,000°C. Soda makes the glass water soluble, which is usually undesirable, so lime (calcium oxide, CaO), some MgO and aluminum oxide are added to provide for a better chemical durability. The resulting glass contains about 70 to 72 percent silica by weight.

Materials:
Each student needs: goggles, 3 20 cm lengths of glass tubing (ends ground or fired to prevent cuts)
Each pair needs: heat mat, Bunsen burner, glass scorer
The class needs:  water coloured with food colouring, glass bin, basin with ice-cold water in case of burns

Students do:
1. collect goggles, glass tubing, bunsen burners, heat mats
2. heat centre of glass rod (rotate tube) and pull out capillary (3 x to allow practice)
3. cool glass and then snap capillary by bending
4. test how high the coloured water rises by capillary action
5. score glass and detach capillary
6. seal end of “pipette”, cool and test
7. heat capillary and shatter in cold water
8. dispose of glass

Safety considerations:  hot glass is less dangerous than hot metal (tripod, tongs).  Glass splinters could fly off if very thick capillaries are broken.  Freshly broken glass is very sharp. 

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