What is a solution? How does it differ from a colloid?

Imagine adding a pinch of sugar to your tea and stirring it until the sugar completely dissolves. This is an example of a homogeneous mixture, where the sugar and tea blend so well that it becomes impossible to distinguish them with the naked eye. Homogeneous mixtures have the same uniform appearance and composition throughout their mass.

The components of a homogeneous mixture are so finely mixed that they will not separate over time and cannot be easily separated by physical means. Solutions like your sweetened tea are common examples of homogeneous mixtures, where the sugar (solute) dissolves in the tea (solvent). Other examples include the air we breathe, which is a mixture of gases like nitrogen and oxygen, and alloys like brass, which is a solid solution of copper and zinc.

In contrast to homogeneous mixtures, a heterogeneous mixture contains substances that remain distinct. Take a bowl of cereal with milk, for instance. You can easily see and pick out individual cereal flakes and raisins. These types of mixtures do not have a uniform composition and can vary in makeup and phase from one part of the mixture to another.

Heterogeneous mixtures typically can be separated into their components by simple physical means, such as filtration or settling due to gravity. Colloids, like milk, are a type of heterogeneous mixture where tiny particles are evenly dispersed but are not dissolved within the surrounding substance. This results in a mixture that does not settle out over time, but the particles are still too big to create a true solution.

The Tyndall effect is named after the 19th-century scientist John Tyndall, who discovered it. It's a phenomenon that occurs when light passes through a colloid. Unlike a true solution, the dispersed particles of a colloid are big enough to scatter light. This scattering of light is the Tyndall effect. An excellent way to visualize it is by shining a flashlight through a foggy room: the light beam becomes visible due to the scattering by tiny water droplets in the air.

This effect is one way to distinguish between solutions and colloids: a solution does not scatter light (no Tyndall effect), while a colloid does. So, if you shine a light through a solution and cannot see the path of the light, it is because the particles are too small to affect the light passage.

In chemistry, a solution's composition involves two primary components: the solute and the solvent. The solute is the substance that gets dissolved, while the solvent is the substance that does the dissolving. In simple terms, if you're making a saltwater solution, the salt is the solute added to water, which acts as the solvent.

The solvent is typically present in greater quantity than the solute, and it determines the physical state of the solution. For example, if salt is dissolved in water, we get a liquid solution because the solvent (water) is liquid. It's also worth noting that in solutions, the interaction between solute and solvent molecules is a physical process, and no new chemical species is formed during the dissolution.