What Dissolves in Water?
The Science of Water Solutions
Properties of water are related to the structure of the water molecule. Each water molecule consists of two hydrogen atoms and one oxygen atom. As with all molecules of this type, the oxygen atom and the hydrogen atoms share electrons. However, the electrons are not shared equally. They are pulled toward the oxygen side of the molecule, which ends up with a slight negative charge. Correspondingly, the hydrogen side of the molecule ends up with a slight positive charge. Each molecule in liquid water, therefore, has a positive end and a negative end.
This separation of positive and negative charges (polarity) makes each water molecule act like a tiny magnet, capable of clinging to other water molecules and to other particles. When salt is added to water, the force from the polarity of the water is able to pull the salt molecule (sodium chloride) apart into positively charged sodium ions and negatively charged chlorine ions. The negative oxygen ends of the water molecules will surround the positive sodium ions while the positive hydrogen ends of the water surround the chlorine ions. Other substances, such as sugar, have a positive and negative end (polar) and can be separated and surrounded by water molecules.
A uniform mixture that results when one substance (such as table sugar) is dissolved completely in another substance (such as water) is called a solution. There is a point at which a solution will not dissolve more of the introduced substance. At this point the solution is said to be a saturated. Many common items are solutions. Household vinegar, for example, is a solution of acetic acid in water.
A mixture containing fine particles dispersed throughout another substance can be an example of a colloid. In the case of colloids, particles larger than many molecules (from 1–1,000 nanometers) are dispersed through another continuous medium, such as water. An easy way to distinguish many colloids (such as with flour mixed with water) from true solutions is to shine a light through the mixture. The beam of light will pass through a solution without any without any visible effect. However, when light is shown though a colloid, the beam’s path will be illuminated clearly. There are many different types of colloids. A sol is a solid dispersed in a liquid. An aerosol is a solid or liquid in a gas (fog is an aerosol). An emulsion is small globules of one liquid in a second liquid and a foam is gas bubbles in a liquid or a solid.
The properties of water that students observed in this activity are:
- Liquid water is an excellent solvent of many substances. This makes water particularly valuable to living organisms. All of the thousands of chemical processes inside cells take place in water. Water also carries dissolved nutrients throughout the bodies of living organisms and transports wastes. Unfortunately, the same characteristics make liquid water easy to pollute, because so many different chemicals can be dissolved in it.
- Not all substances will dissolve in water. In the activity, students discovered that oil, flour and most of the coffee grounds do not dissolve in water.
- Moreno N., and B. Tharp. (2011). The Science of Water Teacher’s Guide. Third edition. Baylor College of Medicine. ISBN: 978-1-888997-61-3.
- National Science Foundation. (2005). The Chemistry of Water.
- Graphic by M.S. Young © Baylor College of Medicine.
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Funded by the following grant(s)
The Environment as a Context for Opportunities in Schools
Grant Numbers: 5R25ES010698, R25ES06932
Foundations for the Future: Capitalizing on Technology to Promote Equity, Access and Quality in Elementary Science Education