Borosilicate glass vs Soda Lime glass vs Pyrex – what is the difference?

Borosilicate glass has a higher proportion of silicone dioxide than Soda Lime glass, as shown on the table below;

This difference means that Borosilicate glass does not expand as much on heating, so it is less likely to break when heated.

Pyrex is one particular blend of Borosilicate glass, with a particularly high heating tolerance.

Soda Lime glass is sometimes used for glassware which is not likely to be directly and strongly heated, for example, Petri dishes or TLC chromatography tanks.

Borosilicate glass or Pyrex is usually used for glassware that may be directly heated, such as beakers or boiling flasks.

Pyrex® is borosilicate glass which differs from other glass types as it possesses unique properties of high resistance to chemical exposure, thermal expansion and thermal shock.  This has advantages in laboratory uses, a key one being where glassware is directly heated, in beakers, test tubes or flasks.

Let’s look at some of these properties in more detail and see what advantages they bring.

Glass is an inorganic mixture of metal oxides fused together at high temperatures, which upon cooling, solidifies into the clear, rigid, non-crystalline, versatile material. So what is different with Pyrex® glass?

The typical composition of Pyrex® is (% by weight): 
SiO2 = 80.6%
B2O>3 = 13.0%
Na2O = 4.0%
Al2O3 = 2.3%

Chemical Properties

Pyrex® has a very high resistance to attack from water, acids, salt solutions, halogens and organic solvents. Only hydrofluoric acid, hot concentrated phosphoric acid and strong alkaline solutions can corrode; making it ideally suitable for laboratory experiments.

Hydrolytic Resistance

For many applications, it is important that laboratory glassware has excellent hydrolytic resistance.  One example of this is during steam sterilisation procedures, where repeated exposure to water vapour at a high temperature can leach out alkali (Na+) ions. Pyrex® has a relatively low alkali metal oxide content and consequently a high resistance to attack from water.

Acid Resistance

Glasses with a high percentage weight of silica (SiO2) are less likely to be attacked by acids. Pyrex® is over 80% silica and therefore it has a remarkably resistant to acids (with some exceptions). It is resistant to chemical corrosion, making it perfectly suitable for laboratory experiments.

Alkali Resistance

Alkaline solutions attack all glasses and Pyrex®  can be classified as moderately resistant to such attacks.

Temperature Resistance

Pyrex’s excellent thermal properties at both high and low temperatures is one of its key features.  This is due to lower levels of expansion and contraction as a result of temperature change when compared to other glass types. Naturally, precautions to prevent sudden temperature changes should always be avoided to prevent cracking.

A: Pyrex is suitable for use from -192°C to +500°C

This makes it an ideal choice for lab glassware which will be directly heated. Soda-Lime glass is less suitable for direct heating, so choose Pyrex where possible if you will be applying heat.

Beakers, test tubes and flasks that are made from Pyrex glass are suitable for direct heating by hotplate, heating mantle or bunsen burner.

Remember that heating and cooling should always be slow and steady – Pyrex can withstand huge temperature changes, but if they are too sudden it may still crack due to thermal shock.