A liquid is a state of matter that has a definite volume, but no fixed shape. In other words, a liquid takes the shape of its container. Liquids consist of atoms or molecules that are loosely connected by intermolecular bonds. In contrast, the atoms or molecules in a solid are fixed in a rigid shape, while the particles in a gas are separated by great distances. Most liquids resist compression. Liquids have surface tension, so they can wet surfaces.
Examples of Liquids
Here are examples of liquids at room temperature and pressure.
- Soft drinks
- Liquid dishwashing detergent
- Household bleach
Properties of Liquids
Liquids share common properties.
- Liquids take the shape of their container. Particles within a liquid can move. Liquids share this property with gases.
- Liquid flow when poured.
- Liquids have a relatively fixed volume. Changing the temperature or pressure of a liquid does slightly alter its volume.
- Liquids can wet surfaces.
- Most liquids are difficult to compress. Liquids share this property with solids.
- Liquids have a higher thermal conductivity than gases. In other words, they absorb heat better than gases.
- At the right combination of temperature and pressure, liquids become solids or gases.
Common Misconceptions About Liquids
There are common misconceptions about liquids. In particular, students have difficulty distinguishing between certain liquids and solids. As with all classification systems, the states of matter contain some ambiguous cases. Instructors are cautioned to avoid confusing examples and be prepared to answer questions about liquids.
Do All Liquids Contain Water?
No, all liquids do not contain water. For example, oil and mercury contain no water, yet they are liquids. A liquid does not need to be “water” to wet a surface. For example, oily fried food in a paper bag wets the paper. Pure rubbing alcohol wets skin before it evaporates.
Are Soft Substances Liquids?
Many solids are hard, but hardness is not an identifying property of the state of matter. A bar of chocolate is a solid, yet it’s easy to bend or dent. It melts into a liquid. Some students think pillow and fabrics are liquids because they are easy to compress and can change shape. Pillows and fabrics contain fibers. While it may not be easy to see that these fibers are solids using visual inspection, their atoms and molecules are arranged in a fixed order.
Are Sand and Powders Liquids?
Sand and powders take the shape of their container, so you might be tempted to think they are liquids. Yet, they consist of tiny solid particles. Think of it this way: A hard candy is clearly a solid. Yet, if you put a bunch of hard candies into a jar, they fill the container. You can pour them out of a jar. Sand and powders are like the candies, except they consist of smaller particles.
Are Gels and Mayonnaise Liquids?
Some substances could be classified as either liquids or solids. Gels, oobleck, slime, clay, and mayonnaise are poor examples of liquids or solids because they display characteristics of both states of matter, depending on conditions. Mixtures may contain different phases of matter and resist classification.
- Krnel, D.; Watson, R., Glazar, S. A. (2005). “The development of the concepts of ‘matter’: a cross-age study of how children describe materials”. International Journal of Science Education 27: 367-383. doi:10.1080/09500690412331314441
- Stavy, R.; Stachel, D. (1985). “Children’s ideas about ‘solid’ and ‘liquid'”. European Journal of Science Education 7: 407-421. doi:10.1080/0140528850070409