CHEMISTRY: Solutions, Colloids

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Solutions and mixtures

Types of Mixtures

Homogeneous mixtures are uniform throughout. The composition is the same in all directions in the substance. The types of particles observed in one direction are the same that are observed in all others. A solution of sugar in water is homogeneous.



Heterogeneous mixtures are not uniform. There are pockets of one substance surrounded by pockets of different substances. A mixture of soil in water to make "mud" is heterogeneous. Likewise a mixture of oil and water in salad dressing is heterogeneous.



Types of Homogeneous Mixtures

Solutions

Particle sizes distinguish one homogeneous mixture from another. Solutions are mixtures with particle sizes at the molecule or ion level. The particles have dimensions between 0.1 to 2 nanometers. Typically solutions are transparent. Light can usually pass through the solution. If the solute is able to absorb visible light then the solution will have a color. A blue liquid transmits blue light and absorbs the other colors of the spectrum.

A mixture of water H2O and ethanol CH3CH2OH is homogeneous. The particles are individual molecules of H2O and CH3CH2OH. The two molecules are spread uniformly throughout the solution.

A mixture of water and sodium chloride is homogeneous by chemistry standards. The particles in the mixture are molecules of H2O and hydrated sodium cations, Na+, and chloride anions, Cl1-.

Solutions are transparent. You can see through them. The mixture remains stable and does not separate after standing for any period of time. The particles are so small they cannot be separated by normal filtration.

A solution may have a "color" but it will still be transparent.

Colloids

Colloids are mixtures with particle sizes that consist of clumps of molecules. The particles have dimensions between 2 to 1000 nanometers.

The colloid looks homogeneous to the naked eye. Fog and milk are examples of colloids.

Colloids frequently appear "murky" or "opaque". The particles are large enough to scatter light. You have experience with the way fog interacts with the light from car headlights. Colloids generally do not separate on standing. They are not separated by filtration.


Suspensions

Suspensions are homogeneous mixtures with particles that have diameters greater than 1000 nm, 0.000001 meter. The size of the particles is great enough so they are visible to the naked eye. Blood and aerosol sprays are examples of suspensions.

Suspensions are "murky" or "opaque". They do not transmit light. Suspensions separate on standing. The mixture of particles can be separated by filtration.



Examples of solutions

Solutions are mechanical combinations of materials. The physical state for the materials in the solutions is surprising varied. The combinations of states of matter that form solutions are listed below.

States of matter in solution
Example

gas in gas
air ( N2, O2 , Ar, CO2 , other gases)

gas in liquid
soda pop (CO2 in water)

liquid in liquid
gasoline (a mixture of hydrocarbon compounds)

solid in liquid
sea water ( NaCl and other salts in water)

gas in solid
H2 in platinum or palladium

liquid in solid
dental amalgams ( mercury in silver)

solid in solid
alloys ( brass, (Cu/Zn), solder (Sn/Pb)

Online Introductory Chemistry

Dr. Walt Volland all rights reserved revised March 29, 2005