Views: 16
Properties of Glass
Glass is classified according to the parent oxide from which they were formed. There are two types of glasses
- Soft glass – soda lime glass or soda glass
- Hard glass – resistant glass or borosilicate glass
Properties of Glass
(a) Soda glass or soft glass
- Soda glass melts on a Bunsen burner flame.
- They are prone to diventrification (forms crystals easily) therefore they need careful introduction on a flame.
- They are very cheap and readily available in any quantity and size.
- Give an intense and persistence yellow sodium flame on heating.
- They are smooth on feeling with finger i.e. the edges will feel smooth on scratching.
- When viewed against a source of light, its edge will have a green tinge.
(b) Borosilicate glass
Borosilicate Glass
Borosilicate glass is a hard colorless glass with a low coefficient of expansion it softens at high temperatures
It is the type of glass most commonly used in the laboratory today is from the borosilicate glass family. Trade names you may be more familiar with are PYREX (Corning), KIMAX (Kimble) and DURAN (Schott).
This type of glass can be put to the flame directly and requires less annealing than soda glass
- They are rough on feeling or the edge will feel rough on scratching
- It’s expensive than soda glass
Relative chemical durability
This refers to the amount of glass removed per square meter surface area of glass after exposure to a chemical or reagent (e.g. NaOH, HCl etc.) In 24 hours at 95oc .
The following table shows the behavior of the two respective reagents
Type of glass |
5% NaOH |
5% HCl |
Soda Borosilicate |
32g 40g |
0.2g 0.05g |
Soda glass loses glass because Na+ move to the surface of the glass. Strong alkali solution attack glass therefore they should not be stored in glass. Acidic solution except phosphoric acid and hydrofluoric acid do not attack glass, hence they can be stored in glass containers.
Effects of heat on glass
Glass is a poor conductor of heat, when it is heated; the surface that is near to the flame undergoes compression strains while the inner surface undergoes tension strains. When it is removed from the flame, the air around it affects the outside layer fat and it therefore undergoes tensile strength while the inner layer undergoes compression strains. I.e. the outer layer cools faster than the inner layer and hence enforces a strain within the inner layer, these causes cracking.
To remove strains from glass, the glass is normally put into an annealing oven at a certain temperature (annealing temperature) and left for some time. Eventually the temperature is reduced stepwise at certain intervals until room temperature is reached. The starting temperature should always be 5oc higher than the annealing temperature.