Why does glass break so easily?

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📓 The short answer

Glass breaks easily because its atoms form a rigid, disordered network rather than an ordered crystal lattice. When under stress, there's no way for planes of atoms to slip past each other, so its molecular bonds break easily and form cracks.

📚 The long answer

Glass is ubiquitous in our everyday life. In fact, you're staring at it right now on your phone or computer screen.

Yet for as long as humans have incorporated glass into our lives, scientists are still puzzling over the details of this solid, yet fragile, substance.

Let's start with the basics:

What is glass?

When you cool a liquid, it hardens. Water becomes ice. Lava becomes rock. Molten metal becomes solid metal. Glass also hardens when it cools, but it looks very different at an atomic level.

Most liquids harden into a solid through crystallization, in which their molecules go from a free-flowing, disordered state to an organized, repeating pattern.

Glass is different. When you cool glass, its disorganized molecules start contracting, crowding a bit together, and eventually stop moving. But they never form the repeating pattern structure that make up crystalized solids. In essence, solid glass looks like a "frozen" liquid at an atomic level.

Glass, however, is not a liquid. If it were a liquid, you could squish it into new arrangements to take the shape of a container, but its molecules are truly rigid. Glass is a special type of solid material: On the outside, it acts like a solid, but on the inside it appears like a liquid.

Why does glass break so easily?

Glass is considered to be a brittle substance. That means it tends to break, without deforming, when under stress. A metal spoon will bend before it snaps, but a glass straw will shatter without any bend. Why is that?

The secret to glass' brittleness lies in its amorphous (i.e. non-crystalline) molecular structure. Unlike most solids which have molecules organized in neat stacks, the silicon and oxygen atoms that make up glass are connected at random angles in an irregular pattern.

While some of the molecules are closely clustered, the clusters themselves don't have as much force holding them together. That means glass is riddled with weak points.

And since glass, unlike a crystalline solid, doesn't have planes of atoms that can slip past each other when met with force, there is no way to relieve stress. Instead of bending, the stress causes the clusters of molecules to separate and form a crack. As the crack grows, more bonds break and the glass fractures even more.

This is why a broken piece of glass is so sharp. Its edges can be as thin as a single molecule. Ouch.

How do you make glass stronger?

Tempered glass undergoes extreme heat and high-pressure cooling to make the outer layers of the glass cool and solidify faster than the inner layers. The surface layer compresses while the interior stays in tension, giving tempered glass its unique strength.

Read more about how tempered glass works in this previous article I've written.

--- Thanks for reading this week's newsletter! If you have any thoughts, questions, or favorite GIFs, my inbox is always open. Just hit reply to send me a note! :) ​ All my best, Caitlin ​Click here to read five facts about Caitlin​

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​Sources for this week's newsletter​

🌐 Wikipedia article of the week

​Mpemba effect​

"The Mpemba effect is the observation that a liquid (typically water) that is initially hot can freeze faster than the same liquid which begins cold, under otherwise similar conditions."

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👀 Catch up on other curious questions

• ​How do laser pointers work?​​
• ​What causes the Earth to spin?​​
• ​Why does a wooden spoon stop water from boiling over in a pot?​​

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