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Comparison of Water with Other Liquids




Fig. 3-17. Water, alcohol and oil have different adhesive and cohesive properties 

Photo by Kanesa Seraphin

We see water, alcohol and oil have different adhesive and cohesive properties and also look at the relative ability of selected liquids to dissolve solids and other liquids.

We have explored adhesion and cohesion in water. Now, we look at those same properties in other substances. See Figure 3-17 for a visual comparison of liquids.
A solvent is something capable of dissolving another substance. We call the substance (i.e. salt) that is dissolved in a solvent (i.e. water) the solute.
The same properties that make water cohesive and adhesive also make it a good solvent. However, water is not the only substance that is cohesive or adhesive. Remember that other molecules that have hydrogen covalently bonded to Fluorine (F), Oxygen (O), or Nitrogen (N) can also form hydrogen bonds. 
Even molecules that cannot form hydrogen bonds have some cohesive and adhesive properties resulting from intermolecular attractive forces. The general term given to intermolecular forces is van der Waals forces. These forces include the attraction of polar molecules to other polar molecules as well as the attraction of nonpolar molecules to other nonpolar molecules. For comparison, hydrogen bonds are about 4-100x stronger than van der Waals forces. Ionic an covalent bonds are about 2-100x stronger than hydrogen bonds.
In this section, we look at the relative ability of selected liquids to dissolve solids and other liquids.

Table 3-3 lists four liquid solvents with information on their polarity and the kind of bonding between their atoms. In the activity that follows, we will investigate the capacity of water to dissolve materials compared to other solvents.

Table 3-3: Chemical structure of selected liquids.
Liquid Bonding Type Polarity Drawing
Distilled water* Covalent Highly polar
Alcohol Covalent Slightly polar
Oil** Covalent Nonpolar
Liquid detergent Covalent (but can be a mixture of covalent and ionic compounds) The covalent part is made of polar heads & nonpolar tails (in water forms polar clusters)
* Distilled water is water that has had dissolved ions removed. It is very close to pure H2O. See special feature on distilled water in Unit 2.
**Oils are usually mixes of different kinds of molecules
How Water Dissolves Other Substances
In our experiment in Activity 4, we found that water dissolves ionic salts and polar covalent compounds such as alcohol. We also saw that water is far less effective as a solvent for nonpolar covalent compounds such as oil. However, a list of substances in seawater suggests that water can dissolve small quantities of almost any substance. Water has dissolved elements such as iron (Fe), charged ions such as potassium (K+) as well as dissolved gases such as nitrogen (N2).
Grade Ocean Literacy Principle OLP
6-8 97% of all water on earth is ocean water, which has unique chemical and physical properties. 1.B
9-12 The properties of water (e.g. salinity, conductivity, freezing point, density, pH) affect the physical characteristics of the ocean and other bodies of water. 1.B
To understand how water dissolves substances, let us concentrate first on compounds that water dissolves easily – the ionic and polar covalent compounds. With these compounds it is the exceptionally strong polarity of water that gives it its dissolving power. The ionic salt sodium chloride (NaCl) is a good model of how this dissolving takes place.

Fig. 3-19: Water dissolving a salt crystal.

Clusters of water molecules are attracted to and surround sodium ions (Na+) and chloride ions (Cl-) on the surface of the salt crystal. Positive polar ends of water molecules are attracted to the chloride ions (Cl-), and their negative polar ends are attracted to the positive sodium ions (Na+). See Fig. 3-19. The bonding between the ions and water is strong, and shortly the ions are as strongly attracted to the water as to each other. As other water molecules collide with the ion-containing clusters, they knock them off, casting them into the solution. An ion surrounded by water is called a hydrated ion. A similar process occurs in the dissolving of polar covalent compounds except that the water is attracted to the poles of the dissolving polar compound. For example, sugar is a large polar molecule with negatively charged OH groups that help sugar easily dissolve in water.

Nonpolar Molecules in Water
Water is not attracted to everything. Because water molecules are polar, they are more attracted to molecules that are also polar or that have a charge (like an ion). Some kinds of molecules, like oils and fats, are nonpolar. These nonpolar molecules have no charge, and so water is not very attracted to them. 
Molecules of nonpolar compounds, such as oil and gasoline, even when mixed well into water, tend to separate from the water when the mixing stops. Water molecules tend to hold on to each other and squeeze out nonpolar oil and gasoline. Because of density differences between water and oil, this means that they form two separate liquid layers. For example, in oil-based salad dressings, the oil and water components separate into two layers and require mixing before being used. See Fig. 3-20.



Photo courtesy of Julie Falk

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Photo by Kanesa Seraphin

Fig. 3-20. Motor oil sheen on wet pavement (A) and separation of oil in salad dressing (B).
Although water will not dissolve oil, gasoline readily dissolves oil. This is because both gasoline and oil are nonpolar; they easily slide between each other when mixed. We can therefore make the generalization that like molecules dissolve like molecules.
Detergents and Water

Fig 3-21: Water, oil, and detergent molecules.

Detergents are an interesting class of compounds that permit large quantities of nonpolar compounds to dissolve in water. The molecules of detergents are long, with one polar end and one nonpolar end (see Fig. 3-21). Detergent molecules are so long, in fact, that their charged ends do not affect their nonpolar ends.  

When a detergent molecule contacts a nonpolar compound such as oil, it slides its nonpolar end between the nonpolar molecules of the oil (Fig. 3-21). While its nonpolar end is attracted to the oil, its charged end faces outward and attracts water molecules. When many detergent molecules attach to a nonpolar oil droplet, they surround it and make a detergent-surrounded oil droplet. These droplets are then easily carried into the water solution. The end result is clean dishes, clean cars, clean clothes, and clean people! 


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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.