Water can take many forms. At low temperatures (below \(0^\text{o} \text{C}\)), it is a solid. When at "normal" temperatures (between \(0^\text{o} \text{C}\) and \(100^\text{o} \text{C}\)), it is a liquid. While at temperatures above \(100^\text{o} \text{C}\), water is a gas (steam). The state that water is in depends upon the temperature. Each state has its own unique set of physical properties. Matter typically exists in one of three states: solid, liquid, or gas.
Figure \(\PageIndex{1}\): Matter is usually classified into three classical states, with plasma sometimes added as a fourth state. From left to right: quartz (solid), water (liquid), nitrogen dioxide (gas).The state that a given substance exhibits is also a physical property. Some substances exist as gases at room temperature (oxygen and carbon dioxide), while others, like water and mercury metal, exist as liquids. Most metals exist as solids at room temperature. All substances can exist in any of these three states. Figure \(\PageIndex{2}\) shows the differences among solids, liquids, and gases at the molecular level. A solid has definite volume and shape, a liquid has a definite volume but no definite shape, and a gas has neither a definite volume nor shape.
Figure \(\PageIndex{2}\): A Representation of the Solid, Liquid, and Gas States. (a) Solid O2 has a fixed volume and shape, and the molecules are packed tightly together. (b) Liquid O2 conforms to the shape of its container but has a fixed volume; it contains relatively densely packed molecules. (c) Gaseous O2 fills its container completely—regardless of the container’s size or shape—and consists of widely separated molecules.Technically speaking, a fourth state of matter called plasma exists, but it does not naturally occur on earth, so we will omit it from our study here.
A plasma globe operating in a darkened room. (CC BY-SA 3.0; Chocolateoak).
In the solid state, the individual particles of a substance are in fixed positions with respect to each other because there is not enough thermal energy to overcome the intermolecular interactions between the particles. As a result, solids have a definite shape and volume. Most solids are hard, but some (like waxes) are relatively soft. Many solids composed of ions can also be quite brittle.
Solids are defined by the following characteristics:
If we were to cool liquid mercury to its freezing point of \(-39^\text{o} \text{C}\), and under the right pressure conditions, we would notice all of the liquid particles would go into the solid state. Mercury can be solidified when its temperature is brought to its freezing point. However, when returned to room temperature conditions, mercury does not exist in solid state for long, and returns back to its more common liquid form.
Solids usually have their constituent particles arranged in a regular, three-dimensional array of alternating positive and negative ions called a crystal. The effect of this regular arrangement of particles is sometimes visible macroscopically, as shown in Figure \(\PageIndex{3}\). Some solids, especially those composed of large molecules, cannot easily organize their particles in such regular crystals and exist as amorphous (literally, “without form”) solids. Glass is one example of an amorphous solid.
Figure \(\PageIndex{3}\):(left) The periodic crystalline lattice structure of quartz \(SiO_2\) in two-dimensions.
(right) The random network structure of glassy \(SiO_2\) in two-dimensions. Note that, as in the crystal, each Silicon atom is bonded to 4 oxygen atoms, where the fourth oxygen atom is obscured from view in this plane. Images used with permission (public domain).
If the particles of a substance have enough energy to partially overcome intermolecular interactions, then the particles can move about each other while remaining in contact. This describes the liquid state. In a liquid, the particles are still in close contact, so liquids have a definite volume. However, because the particles can move about each other rather freely, a liquid has no definite shape and takes a shape dictated by its container.
Liquids have the following characteristics:
A familiar liquid is mercury metal. Mercury is an anomaly. It is the only metal we know of that is liquid at room temperature. Mercury also has an ability to stick to itself (surface tension)—a property that all liquids exhibit. Mercury has a relatively high surface tension, which makes it very unique. Here you see mercury in its common liquid form.
Video \(\PageIndex{1}\): Mercury boiling to become a gas.
If we heat liquid mercury to its boiling point of \(357^\text{o} \text{C}\) under the right pressure conditions, we would notice all particles in the liquid state go into the gas state.
If the particles of a substance have enough energy to completely overcome intermolecular interactions, then the particles can separate from each other and move about randomly in space. This describes the gas state, which we will consider in more detail elsewhere. Like liquids, gases have no definite shape, but unlike solids and liquids, gases have no definite volume either. The change from solid to liquid usually does not significantly change the volume of a substance. However, the change from a liquid to a gas significantly increases the volume of a substance, by a factor of 1,000 or more. Gases have the following characteristics:
What state or states of matter does each statement, describe?
What state or states of matter does each statement describe?
Matter is everything around us. Matter can be a solid, liquid or gas. Water is the only common substance on Earth that is naturally found as a solid, liquid or gas.
Water is the only common substance that is naturally found as a solid, liquid or gas.
Solids, liquids and gases are known as states of matter.
Matter is everything around us.
Scientists use the word matter to describe anything that takes up space and has mass.
Matter is made up of tiny particles, including;
Atoms are like individual LEGO blocks. They are the smallest unit that anything can be easily broken down into. If atoms are like LEGO blocks, molecules are what you can build with them.
The way atoms and molecules are arranged affects the state in which the matter is.
Right now, you are probably sitting on a chair at a desk, or standing on the floor of your classroom. The chair, desk and floor are all solids. A solid is something that can hold its own shape and is hard to compress (squash).
The particles in most solids are closely packed together. Even though the particles are locked into place and cannot move or slide past each other, they still vibrate a little.
Ice is water in its solid form or state. Ice keeps its shape when frozen, even if it is removed from its container. However, ice is different from most solids because its molecules are less densely packed than in liquid water so ice takes up a little more space than the same amount of liquid water. This is why ice floats. It is also why old pipes can crack and burst when the water inside them freezes.
The easiest way to decide if something is a liquid is to ask this question: If I try and move it from one container to another by trying to pour it, will it take on the shape of the new container?
If you have a glass of water and pour it into another glass, it clearly conforms – it takes on the shape of the glass. If you spill the water, it will go everywhere. Because it isn’t in a container, it conforms to the shape of the floor, making a big puddle!
In most liquids, the particles are less densely packed, so they can move around and slide past each other. While a liquid is easier to compress than a solid, it is still quite difficult – imagine trying to compress water in a container.
Water is an example of a liquid, and so are milk, juice and lemonade.
What other examples of liquids can you come up with?
The atoms and molecules in gases are much more spread out than in solids or liquids. They vibrate and move freely at high speeds. A gas will fill any container, but if the container is not sealed, the gas will escape. Gas can be compressed much more easily than a liquid or solid. (Think about a diving tank – 600L of gas is compressed into a 3L cylinder.) Right now, you are breathing in air – a mixture of gases containing many elements such as oxygen and nitrogen.
Water vapour is the gaseous form or state of water. Unlike ice or water, water vapour is invisible. We exhale water vapour whenever we breathe out. We cannot see the water vapour as we exhale, but if we hold our glass or a smartphone to our mouths, we can see the water vapour condensing (becoming liquid) on these objects as it cools.
Can you find out if there are any other states of matter?