Solids and liquids are collectively called condensed phases because their particles are in virtual contact. The two states share little else, however.

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Solids

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 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 8.7 “Crystalline Arrangement”. 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.


Liquids

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.


Gases

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 further in Section 8.3 “Gases and Pressure”. 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. Figure 8.8 “A Representation of the Solid, Liquid, and Gas States” shows the differences among solids, liquids, and gases at the molecular level, while Table 8.2 “Characteristics of the Three States of Matter” lists the different characteristics of these states.


Figure 8.8 A Representation of the Solid, Liquid, and Gas States. 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.


Table 8.2 Characteristics of the Three States of MatterCharacteristicSolidLiquidGas
shapedefiniteindefiniteindefinite
volumedefinitedefiniteindefinite
relative intermolecular interaction strengthstrongmoderateweak
relative particle positionsin contact and fixed in placein contact but not fixednot in contact, random positions

Example 2

What state or states of matter does each statement, describe?

This state has a definite volume.This state has no definite shape.This state allows the individual particles to move about while remaining in contact.

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Solution

This statement describes either the liquid state or the solid state.This statement describes either the liquid state or the gas state.This statement describes the liquid state.