Properties of Water
Water covers about three quarters of the earth’s surface and makes up about three quarters of our body weight. In fact, without water, life would not be possible. This simple fact is why scientists are constantly looking for water on other planets – the presence of water could indicate the presence of life.
We have discussed some of the properties of water in previous lessons. This lesson will consider water’s properties in more depth. As you read about the characteristics of water, take some time to think about how these characteristics relate to the fields of water and wastewater treatment.
As you should remember, water is represented by the formula H2O. The picture below can also be used to represent water.
Lesson 5 explained how hydrogen bonds form between water molecules. The ability of water molecules to form hydrogen bonds, as shown below, causes many of water’s unique characteristics. For example, you should recall that hydrogen bonding makes water an excellent solvent.
Water is unique in that it is found as a gas, a liquid, and a solid at natural earth temperatures. In contrast, most other substances are naturally found in only one or two states. This property of water is integral to our daily lives, and is especially important in the hydrologic cycle.
On this page, we will consider how water is influenced by temperature. The surrounding air temperature can change the temperature of water, change water’s state, and change water’s density.
Specific Heat Capacity
Specific heat capacity is the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius. Every substance has its own specific heat capacity, with the specific heat capacity of water being 1 cal/(g°C).
The specific heat capacity of water is much higher than that of other common substances. For the sake of comparison, the specific heat capacity of oil is about 0.5 cal/(g°C) and the specific heat capacity of aluminum is about 0.2 cal/(g°C). This means that it takes a lot more heat to raise the temperature of water compared to the amount of heat it would take to raise the temperature of oil or aluminum.
The high specific heat of water helps the earth’s temperature remain moderate since water traps heat during the day and releases it slowly at night. As a result, the temperature on earth’s surface does not vary very widely, ranging from extremes of 134°F to -129°F. For comparison, the moon has no liquid water and its temperatures can range from 240°F to -290°F. (The lack of atmosphere on the moon, along with other factors, also contributes to the wide range of temperature.)
Water also has a very high boiling point, meaning that liquid water turns into water vapor at a higher temperature (212°F) than would be expected due to the size and weight of the molecule. The high boiling point of water is due to the hydrogen bonds which tend to hold water molecules together, preventing them from breaking apart and entering the gaseous state.
Since it takes such a large amount of energy to change the state of water, sweating is a very effective method of cooling the body. In order to evaporate, the sweat requires the input of a great deal of heat energy, some of which comes from our bodies. So, as our sweat evaporates, we begin to feel cooler.
Density and Expansion
As you should remember from ENV 110, density is the ratio of mass to volume. Dense objects feel heavier and tend to sink while less dense objects feel lighter and tend to float.
The density of most objects changes slightly as the temperature changes. In general, warmer temperatures tend to make substances less dense because the greater random kinetic energy makes the molecules spread out. The amount that objects expand when heated is known as the coefficient of expansion.
The density of water, once again, is a special case. Water is most dense at 39°F, and as it cools or warms from this temperature, the water expands slightly. This means that ice is slightly less dense than cold water, which is why ice floats on the surface of bodies of water. The floating ice slows the freezing process by insulating the water underneath, which contributes to the moderate temperatures on earth. In addition, the layer of ice prevents many lakes from freezing solid, allowing fish and other organisms to survive under the ice.
The changing density of water at different temperatures is also responsible for turnover. Turnover occurs when the water on the surface of a lake cools in the fall. Eventually, this cold water will become more dense than the warmer water beneath, so the cold water will sink to the bottom and the warm water will rise to the surface. When lakes are used as the water source for water treatment plants, turnover can cause abrupt changes in the quality of the raw water.
In a body of water, hydrogen bonds between water molecules are constantly pulling the molecules in many different directions. However, at the water’s surface, the molecules are only being pulled from side to side and down, with no hydrogen bonds pulling them upwards. This results in a skin of water at the surface in which the molecules are held together very tightly.
Surface tension is a measurement of the amount of force required to break this skin on the surface of water. Other liquids have a surface tension as well, but the surface tension in water is quite strong due to the hydrogen bonds. The pictures below show some examples of the results of water’s strong surface tension.
Surface tension is what holds drops of water together in a round shape. Surface tension allows both water striders and paperclips to float on water even though they are more dense than the water. In addition, surface tension allows you to fill a cup slightly over the brim with water.
Surface tension is also responsible for another phenomena known as capillary action. Capillary action occurs when water climbs upward through a small space, defying gravity due to the forces of adhesion and surface tension. The image below shows one example of capillary action – a narrow straw was placed in a cup of water and the water crept upwards through the straw.
What causes the movement of water during capillary action? The first factor is adhesion, the attraction between water and another object. In this case, adhesion attracted the water within the straw to the surface of the straw. Molecules of water which came in contact with the straw tended to move upward along the inside of the straw, as shown below:
Water’s surface tension is so strong that, as water is pulled upward along the straw’s walls, the water in between tends to be pulled upward also. The downward pull of gravity prevents the central water from rising quite as high as the water which is adhered to the straw, so the result is a meniscus, as shown in the first picture in this section.
Capillary action is important in moving water upwards through small spaces. Plants depend on capillary action to move water upward from the roots to the leaves. In the soil, capillary action also tends to move water upward between the soil particles.
Water has many unique properties, many of which are based on its molecules’ ability to form hydrogen bonds. Water is found at earth’s temperatures as a solid, liquid, and gas. It has a high specific heat capacity and boiling point. Water is most dense at 39°F. Water also has a strong surface tension.