Why does a vacuum flask use glass inside?

A vacuum flask often uses glass for the inner wall due to several advantageous properties of glass that make it ideal for this application:

1. Low Thermal Conductivity: Glass has relatively low thermal conductivity compared to metals. This means it does not easily transfer heat, which helps in maintaining the temperature of the contents inside the flask.
2. Chemical Inertness: Glass is chemically inert, meaning it does not react with most substances. This is particularly important for storing a variety of liquids, including acidic or alkaline beverages, without the risk of contamination or chemical reactions that could alter the taste or safety of the contents.
3. Smooth Surface: The smooth surface of glass is easy to clean and does not harbor bacteria or odors. This makes it hygienic for storing food and beverages.
4. Transparency: Although not always necessary, the transparency of glass can be useful for seeing the contents inside the flask without opening it. This can be particularly handy for checking the level of liquid remaining.
5. Durability: High-quality borosilicate glass, often used in vacuum flasks, is resistant to thermal shock. This means it can withstand sudden changes in temperature without cracking, which is crucial for a container that frequently holds hot and cold liquids.
6. Non-porous: Glass is non-porous, which means it does not absorb liquids or odors. This ensures that the flask does not retain smells or flavors from previous contents, maintaining the purity of whatever is stored in it.

However, glass does have some drawbacks, such as being more fragile and heavier compared to stainless steel. To address these issues, many modern vacuum flasks use stainless steel for the inner wall, which offers similar benefits in terms of thermal insulation and chemical resistance while being more durable and lightweight. Nonetheless, glass remains a popular choice for its superior purity and non-reactive properties.