Solubility Review

A solution is a mixture of two or more substances. The solute is the substance being dissolved, and the solvent is the substance that the solute is dissolved in. The solvent is often water. The process of creating a solution is called solvation.

Not everything can dissolve in water – for example, oil will not dissolve in water. Whether or not a solute will dissolve in a particular solvent depends on the attraction between molecules. One rule to remember is “like dissolves like”, which means that ionic or polar solutes (which have a charge or slight charge) will dissolve in similarly polar solvents. Non-polar solutes dissolve in non-polar solvents. However, non-polar solutes, such as oil, will not dissolve in polar solvents, such as water, and vice-versa. As with many rules, there are exceptions in certain cases.

Review Question Set 1

  1. When you make hot chocolate, you add a hot chocolate packet to hot water and stir. In this example, which is the solute? Which is the solvent? Which is the solution?
  1. Knowing that water is polar, according to the “like dissolves like” rule, would you expect the hot chocolate mixture to be polar or non-polar?

Precipitation

Sometimes after a solution has been created, a solid can be formed out of the solution. If this happens in a liquid solution, the solid that forms is called the precipitate and the remaining liquid in the solution is called the supernate. Sometimes, a rotating machine called a centrifuge can be used to help collect all of the small particles that are still floating around in the liquid, called the suspension.

Application

One technique that scientists can use to concentrate DNA or RNA is called ethanol precipitation. In an aqueous (liquid) solution of DNA or RNA, salt and ethanol can be added to the solution and the nucleic acid precipitates out of the solution. The solution is then centrifuged and the nucleic acid forms a pellet separate from the solution. The pellet is then washed in ethanol and centrifuged again and then dries and is re-suspended in clean aqueous buffer.

We already know that water is polar, because of its slightly positive hydrogens and slightly negative oxygen, but what about nucleic acids like DNA and RNA? These molecules are also polar because of the negatively charged phosphate group (PO3-) along the sugar-phosophate backbone. Because of this, DNA and RNA can easily dissolve in water. So how do we get them to precipitate out of the water?

Adding salt neutralizes the charges on the sugar-phosphate backbone. The salt breaks down into positive and negative ions and the positive ion (often Na+) neutralizes the negative phosphate group (PO3-), which makes the molecule less polar and less soluble in water. Adding ethanol helps the Na+ and PO3- ions come together, because ions travel easier in ethanol than water. The supernate is removed and new ethanol is added in the second washing. This removes any residual salt that remained on the pelleted DNA.

Review Question Set 2

  1. Why does DNA dissolve in water?
  1. Why is salt added to solution of DNA and water?
  1. How does ethanol help the precipitate form?
  1. After a pellet is formed, is the DNA in the pellet or the supernate?
  1. What might happen to your sample of DNA if you removed the pellet instead of the supernate during the wash?
  1. What might happen to your sample of DNA if you didn’t perform the second washing?