ABSTRACT
Put some charged object or molecule into water. Now add salt. The salt will dissolve, dissociate, and swarm around the charged object, shielding it, often changing its properties. For example, DNA is a molecular chain of negative phosphate charges. Because its charges repel each other, DNA is elongated in pure water. When NaCl is added to an aqueous solution of DNA, the salt dissociates into positive ions (Na+) and negative ions (Cl−). The positive ions seek the DNA’s negative charges, surrounding and shielding them. This shielding weakens the intra-DNA charge repulsions, causing the DNA to become more compact. Added salts can precipitate colloids, aggregate proteins, speed up chemical reactions, or allow biological cells to fuse together.
