①Is Glass a Liquid? - The State of Fascinating Materials [Reading Science]

Is glass a solid or a liquid?
Photo by YouTube Screenshot

[Asia Economy Reporter Kim Jong-hwa] The forms that matter takes on Earth, that is, the states of matter, are generally known to exist in solid, liquid, and gas states. Matter is made up of very small invisible particles (molecules), and there is an attractive force acting between these molecules.

In solids, the molecules are very close to each other, and the attractive forces between them are very strong. Therefore, the molecules cannot move freely and only vibrate in place. Since the molecules cannot move, the shape does not change. That is why solids can be seen with the eyes and held with the hands.

Liquids have molecules that are somewhat apart, and the attractive forces between them are weaker than in solids, so they have the property of flowing without a fixed shape. Therefore, their shape changes depending on the container they are in. However, their volume does not change even when force is applied. Some liquids mix well with each other, while others do not.

Liquids also tend to gather into the smallest possible surface area, a property called surface tension. This surface tension causes water droplets to form. In other words, droplets are a phenomenon where a small number of molecules clump together.

Gases have molecules that are very far apart and almost no attractive forces between them, so their volume is not fixed and they tend to disperse. Their shape changes depending on the container, they fill the container, and they have weight. They are invisible and cannot be held by hand.

Here, one question arises. Does matter exist only in solid, liquid, and gas states? Scientists say that matter also exists in states such as 'Supercritical liquid', 'supersolid', 'Plasma', and 'Bose-Einstein Condensation' besides solid, liquid, and gas.

If you look at very old stained glass, the glass has flowed down, making the top part thin and the bottom part thick.
[Photo by YouTube screen capture]

Then, where do very fine powders, toothpaste, mayonnaise, and the liquid crystal of a smartphone screen belong among solid, liquid, and gas? How are these ambiguous states of matter defined?

Powder is very small but has the molecular arrangement of a solid, so it can be considered small solids. Substances like toothpaste or mayonnaise have an intermediate state between liquid and solid. Therefore, such states of matter are called 'GEL' or gel.

The liquid crystal of a smartphone screen looks like a solid. However, it cannot be simply called a solid. When electricity passes through the liquid crystal, the molecular arrangement changes from liquid to solid. So it has properties of both liquid and solid. Before electricity flows, it is liquid; after electricity flows, it is correct to call it solid.

There is one substance that we often mistakenly think of as a 'solid' in daily life. That is glass. It is visible, can be held by hand, is hard, and its volume and shape do not change, so it was mistaken to have the properties of a solid. However, glass is a 'liquid'. Glass has the viscosity of a liquid, but it is a very viscous, sticky liquid.

More precisely, glass can be called an 'amorphous solid'. This means it is a solid that is not crystalline. It is called an amorphous solid because its structure differs from that of typical solids. In fact, the properties of glass are closer to liquid than solid.

Glass is said to flow like a liquid. If you look closely at the stained glass of old churches, the glass has flowed down, making the top thinner and the bottom thicker. Glass in the past, when glassmaking technology was not advanced, flowed down over thousands of years. Nowadays, with advanced glass manufacturing technology, glass does not flow at room temperature. However, when heated to high temperatures, it easily turns into a liquid.

Lightning is one of the natural plasmas.
[Photo by YouTube screen capture]

Scientists define the 'fourth state of matter' as 'Plasma'. When matter in the gas state is heated to a very high temperature, the particles that make up the matter split into smaller particles carrying electric charge. This state is plasma.

In space, almost all matter exists in the plasma state. Plasma generated by natural phenomena such as auroras or lightning is called natural plasma, plasma generated by operating nuclear reactors to produce electricity is called high-temperature plasma, and plasma used in everyday life such as fluorescent lamps, neon signs, and PDPs for computer monitors is called low-temperature plasma.

Besides these substances, there are quite a few substances whose states are difficult to define. A representative example is 'fire'. The properties of fire will be examined in the [②Is fire a gas? - The fascinating states of matter] section.