What gives Pyrex glassware its advantages?

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%
• B₂O₃ = 13.0%
• Na₂O = 4.0%
• Al₂O₃ = 2.3%

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 their advantages.

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 high temperatures 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 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 <a href="http://camblab.info/index.php/what-are-the-temperature-limits-for-pyrex-glassware/" target="_blank">high and low temperatures</a> 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.