Colored by lights at night, an incredible amount of water plunges continuously over the sheer rock wall of Niagara Falls. If you stand close to the falls, the roar of the water is almost deafening, and the rushing water drenches you with spray. No wonder Niagara Falls was once named one of the seven natural wonders of the world! It’s amazing how such a common substance—water—can be so impressive.
Water and Other Liquids
Water is the most common substance on Earth, and most of it exists in the liquid state. A liquid is one of four well-known states of matter, along with solid, gas, and plasma states. The particles of liquids are in close contact with each other but not as tightly packed as the particles in solids. The particles can slip past one another and take the shape of their container. However, they cannot pull apart and spread out to take the volume of their container, as particles of a gas can. If the volume of a liquid is less than the volume of its container, the top surface of the liquid will be exposed to the air, like the vinegar in the bottle pictured in the Figure below.
Q: Why does most water on Earth’s surface exist in a liquid state? In what other states does water exist on Earth?
A: Almost 97 percent of water on Earth’s surface is found as liquid salt water in the oceans. The temperature over most of Earth’s surface is above the freezing point (0°C) of water, so relatively little water exists as ice. Even near the poles, most of the water in the oceans is above the freezing point. And in very few places on Earth’s surface do temperatures reach the boiling point (100°C) of water. Although water exists in the atmosphere in a gaseous state, water vapor makes up less than 1 percent of Earth’s total water.
A Liquid has intermolecular forces that are weaker than those of a solid.
What happens to ice if you heat it? As the temperature of ice increases, the strong chemical bonds holding the molecules in place give way. The molecules of ice are no longer held in a fixed location: they start to move around, turning into liquid water.
Have you ever seen a water strider bug resting on the surface of water? What keeps the bug from falling in? The intermolecular forces of the water are strong enough to hold the bug on the surface of the water.
Surface Tension and Viscosity
Two unique properties of liquids are surface tension and viscosity. Surface tension is a force that pulls particles at the exposed surface of a liquid toward other liquid particles. Surface tension explains why water forms droplets, like the water droplet that has formed on the leaky faucet pictured in the Figure below. You can learn more about surface tension at this URL: http://io9.com/5668221/an-experiment-with-soap-water-pepper-and-surface-tension.
Water drips from a leaky faucet.
Viscosity is a liquid’s resistance to flowing. You can think of it as friction between particles of liquid. Thicker liquids are more viscous than thinner liquids. For example, the honey pictured in the Figure below is more viscous than the vinegar. You can learn more about viscosity at this URL: http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Viscosity-840.html.
Q: Which liquid do you think is more viscous: honey or chocolate syrup?
A: The viscosity of honey and chocolate syrup vary by brand and other factors, but chocolate syrup generally is more viscous than honey.
- A liquid is a state of matter in which particles can slip past one another and take the shape of their container. However, the particles cannot pull apart and spread out to take the volume of their container.
- Surface tension is a force that pulls particles at the exposed surface of a liquid toward other liquid particles. Viscosity is a liquid’s resistance to flowing.
- liquid: State of matter that has a fixed volume but not a fixed shape.
Table below shows the viscosity of water at different temperatures. Use the data in the table to answer the questions below. The meaning of the units of viscosity is not necessary to appreciate the relationship between temperature and viscosity.
|Temperature [°C]||Viscosity [mPa•s]|
- Describe in words what the data in the table show.
- If you were to draw a line graph of temperature and viscosity, what would it look like? Make a rough sketch to show how it would look. (Assume that the x-axis represents temperature and the y-axis represents viscosity.)
- Write a hypothesis to explain the relationship between temperature and viscosity of water.
- State the properties of matter in the liquid state.
- What property of liquids explains why water beads up on the car surface pictured in the Figure below?
- Predict which liquid has greater viscosity: olive oil or motor oil (SAE 40). Then do online research to find out if your prediction is correct.