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Solutions are mixtures in a single phase. Universally, they are homogenous and are composed of a solute and a solvent, but can be liquid-liquid, solid-liquid, gaseous-gaseous, solid-solid, etc.

When you think of “solutions,” you probably think of solid-liquid or liquid-liquid solutions. Figure 1 depicts the former, in which sodium chloride (the solute) is dissolved in water (the solvent). When solid table salt is added to water, the positive hydrogen ends of the water molecules attract the negative Cl ions in the salt. Likewise, the negative oxygen ends of water attract the salt’s positive Na+ ions. The water molecules end up pulling apart the NaCl until it is completely ionized into Na+ and Cl. This process is called solvation.

Water is often dubbed the “universal solvent” because of the wide range of substances it can dissolve. Despite this, its effects are by far universal. Take, for example, the age old saying “oil and water do not mix.” Why is this so? The answer lies with polarity. Water is very much polar, while oil molecules are non-polar. Polarity is crucial to solvation, as the solvent must attach to an end of opposite charge to dissolves the solute. Because oil does not have any positive or negative ends, water cannot dissolve it. A good way to remember this is like dissolves like. Polar dissolves polar. Non-polar dissolves non-polar.

Gaseous solutions include the air we breathe, which is a mixture comprising principally nitrogen, oxygen, argon, and carbon dioxide. Figure 2 depicts a solution of N2 and O2 gas.

Solid-solid solutions include alloys. They have metallic properties and are composed of two or more elements homogenously mixed with one another. In steel (Fig. 3), for example, carbon atoms are wedged into gaps between iron atoms. Other types of alloys are brass, which has copper and zinc mixed together, and bronze, which is copper and tin. Alloys are formed to alter metallic properties, including enhancing durability. Because bronze is stronger than it’s constituents, it has consequently played an important role in the development of human civilization. Bronze weaponry was first used by Mesopotamians around 3000 B.C., during what’s known to us as the “Bronze Age.”

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