Chapter 7 Laboratory: Solubility and Solutions 121

Chemists routinely make up and use solutions. A solution is a homogeneous mixture of two

or more substances. In a solution, the solute is dissolved in another substance called the

solvent. For example, if you add a teaspoon of sugar to a cup of coffee, sugar is the solute

and coffee is the solvent.

The solute and solvent may be in any phase. One common form of solution is a solid solute

(such as salt or sugar) dissolved in a liquid solvent (such as water). A solution in which water

is the solvent is called an aqueous solution. Many other solvents are commonly used in

chemistry labs, including ethanol and other alcohols, acetone, benzene, and other organic

liquids. Some solutions are mixtures of one liquid dissolved in another liquid. For example,

the isopropyl rubbing alcohol found in drugstores contains about 30% water (the solute, in

this case) dissolved in about 70% isopropanol (the solvent). A solution may also be made

by dissolving a gas in a liquid. For example, aqueous ammonia (often incorrectly called

ammonium hydroxide) is a solution of ammonia gas dissolved in water, and hydrochloric acid

is a solution of gaseous hydrogen chloride dissolved in water.

Although the most common types of solutions used in chemistry labs are solid-liquid,

liquid-liquid, and gas-liquid solutions, there are other types of solutions. Mixtures of gases

can also be thought of as solutions. Our atmosphere can accurately be described as a

solution of about 21% oxygen gas in about 79% nitrogen gas, with minor amounts of other

gases present. Similarly, two or more solids can form a solution. For example, stainless steel

is a solution in which iron is the solvent and small amounts of chromium and other metals

are the solutes.

Laboratory: 7

Solubility and Solutions

SoNTLvE oR SoLUTE?

Conventionally, in a solid-liquid mixture, the liquid is always considered to be the solvent

and the solid the solute. Some solids, however, are so soluble in a given solvent that a

very concentrated solution may contain more of the solid by mass and by volume than

it contains of the liquid. In such cases, it’s not unreasonable to consider the solid as the

solvent and the liquid as the solute. In a liquid-liquid solution, the liquid present in the

greater amount is considered the solvent.