http://www.ck12.org Chapter 15. Water
Introduction
In our last section, we took a detailed look at water and its physical properties and characteristics. We examined
properties like surface tension, heat capacity, and density that are significantly higher for water than for many other
liquids. In this lesson, we are going to study water’s ability to act as a solvent. Water is particularly good at dissolving
ionic compounds and very polar molecular substances. By looking at the properties of a given substance, it is often
possible to predict whether it will dissolve in water. This type of analysis is aided by understanding the dissolution
process on a molecular level.
Aqueous Solutions
Anaqueous solutionis a homogeneous mixture in which the most abundant component is water. The other
substance(s) are typically introduced in a solid or liquid form and mixed into the water until they are evenly
distributed. For example, if we placed a sample of ethanol in water and mixed it homogeneously, we would have a
solution. Table salt mixed with water is another example of a solution. More generally, asolutionis a homogeneous
mixture. The solution component that is present in the largest amount is called thesolvent, and all other substances
are referred to assolutes. In aqueous solutions, water is always the solvent.
Experiments
We are going to visualize some simple experiments to explain how an aqueous solution is created. This involves
placing various common substances in water and seeing what happens. The substances we will consider are (1) table
salt (NaCl); (2) cane sugar (C 12 H 22 O 11 ), and (3) sand (SiO 2 ) (Figure15.10).
FIGURE 15.10
Left: Table Salt; Center: Sugar; Right:
SandScenario 1 –Salt Added to Water
What will happen when we add table salt to water? At the macroscopic level? At the molecular level?
If we add a spoonful of table salt to a glass of water, we will notice that the salt crystals immediately begin to
disappear. We can also stir the solution to speed up the process. Over time, the crystals get smaller and smaller, and
eventually, we will be left with a transparent solution, assuming we have enough water to dissolve all of the salt.
This process is calleddissolution. Because dissolution occurs, we say that salt issolublein water. Any substance
that is able to dissolve in water is described as being water soluble.
Where did the salt go? If we were to measure the mass of the salt and the mass of the water before mixing them, we
would find that the mass of the solution is simply the sum of its two parts, as expected by the law of conservation of
mass. The salt is still there, it is just no longer visible. (Interestingly, volume is not additive in the same way, because