Is a solution a pure substance? A solution is quite common in scientific research as well as in our daily lives.
It can be found in various forms, including solid, liquid, and gaseous mixtures.
But, it raises the question, “Are solutions pure substances?”
Apparently, it seems they are. And that is mainly due to their uniform appearance. But, is it truly the case?
A solution is not a pure substance and is essentially a homogeneous mixture of multiple components with uniform properties.
Is a Solution a Pure Substance?
No, it is not. But, to understand it better, you need to learn a bit more about how a solution and a pure substance are defined.
Definition of a Solution
In chemistry, a solution is a homogeneous mixture of at least two substances.
Solutions are made up using solvents and solutes with the particles of the solvent being evenly distributed throughout the solvent making the mix homogeneous.
In solutions, the solute or solutes are so small that they do not settle or precipitate even over time.
The properties of the solution will be different from those of the solvent or the solute on its own.
Definition of a Pure Substance
A pure substance is made up of only one kind of atom or molecule.
There are only a few but for classification, they can be divided into two separate classes, such as:
- Elements
- Compounds
For example, salt, which is a compound, is also a pure substance because it only contains sodium chloride molecules.
Fact: Chemical elements are pure substances that can not be broken down either chemically or physically and have well-defined boiling and melting points.
What are the Components of a Solution?
Solutions take several forms depending on the state of matter of its components.
It could be:
- A liquid in a liquid
- A liquid in a solid
- A solid in a liquid
- A gas in a liquid
- A gas in a gas
- A gas in a solid
- A solid in solid
More about the Solvent
The solvent is the component into which a solute is dissolved.
Usually, the solvent is a liquid although air is a solvent for nitrogen. The solvent takes up most of the volume as in seawater.
The solvent for seawater is water. Its solute is salt. In a solution of seawater, there is more water than the solute, salt.
How much solvent is needed to dissolve a solute depends on its temperature and the other substance/s present.
More about a Solute
The components of a solution must be either its solvent or a solute.
The solute is the chemical or chemicals that are present in a lesser amount and they must be distributed uniformly throughout the solution.
In seawater, the salt is the solute because it can be dissolved into the water, the solvent, to form a chemically even, homogeneous, saline solution.
Fact: There can only be one solvent but it can contain one or more solutes.
Learn the Examples of Solutions
Examples of everyday solutions are:
- Liquid/liquid – mouthwash, antifreeze, vinegar, liquid soap, disinfectants.
- Solid/liquid – seawater, coffee or tea with sugar, syrup, sports drinks.
- Gas/liquid – sparkling water, fizzy soft drinks, pond and aquarium water.
- Gas/gas – air and natural gas, (a solution of ethane, propane and butane in methane gas).
- Solid/solid – sterling silver, bronze and pewter.
Regardless of each component’s state of matter, its chemicals will dissolve evenly and cannot be removed by filtering.
Characteristics of a Pure Substance
A pure substance contains only one kind of matter, an element or a compound.
Being single entities, pure substances have very well-defined characteristics. Every sample of the substance encountered will display the same characteristics.
No matter where it originates, every sample of the pure substance lead behaves in the same way.
Homogeneous Composition
A pure substance has consistent properties because it has a uniform composition.
This homogeneous makeup with the chemicals spread evenly throughout, meaning that the properties of the matter remain consistent all the way through the sample.
Therefore, a pure substance will always act predictably and exhibit the same characteristics when it is being manipulated.
This provides us with standardized boiling points, melting points and color change points that are useful in a wide variety of processes across many industries.
Fact: Unlike a homogeneous mixture which also has a uniform chemical spread, the composition of a pure substance never changes.
Fixed Physical and Chemical Properties
Therefore, we can define a pure chemical substance as any form of matter with a fixed physical composition and chemical properties.
We always know that oxygen, one of the few pure substances that exists on its own, will be a colorless, odorless gas at 250.
The properties and characteristics stay consistent and fixed because the chemical element or the chemicals that form the compound, have no other chemicals to react with.
Examples of Pure Substances
Most pure substances exist as various proportions of mixtures where they retain their own identity.
A few examples include:
- Gold
- Copper wire
- Diamond
- Hydrogen gas
- Sugar
- Salt
- Water
- Ammonia
Occasionally, the definition of a pure substance is expanded to include any homogeneous mixture or alloy.
It creates a gray area in the classification of matter that sterling silver, honey, air, vegetable oil, brass, stainless steel, tap water and rubbing alcohol fall into.
These substances do not have one type of atom or compound but are uniform in their structure.
Impact on the Classification of a Solution as a Pure Substance
Strictly, if a substance is pure, it exists as either one element or one compound.
However, as the confusion suggests, deciding if a solution is classified as a pure substance isn’t always straightforward.
To decide is a solution is a pure you could ask:
- Is the element written as a symbol as in Pb (lead)or the compound as a chemical formula, for example, NaCl (salt)?
- Is its consistency uniform?
- Is it one color throughout?
- Does it have the same properties as another pure solution already classified?
- Does it crystallize like sugar or honey?
Comparing Solutions and Pure Substances
When we consider a pure substance, we think of it as just one kind of element or compound, although many of the solutions contain a mix of matter, some of which retains its structure.
For example, in a fruit salad the fruit pieces aren’t transformed and can be lifted out.
Homogeneous vs. Heterogeneous Mixtures
The opposite of homogeneous is heterogeneous although both are used to describe kinds of mixtures like milk.
A heterogeneous mixture is made up of elements that are different. A fruit salad is a heterogeneous mixture. The parts of the mixture remain separate.
As homogeneous mixtures are uniform, a heterogeneous mixture is not.
Unlike a homogeneous mixture that will hold on to its components to create a new substance, a heterogeneous mixture gives components up easily.
Variable vs. Fixed Composition
A solution is pure because its composition is fixed. When a solution is one element there is one type of atom.
When the pure solution is a compound, it contains two or more elements with fixed composition so its atoms are fixed in proportion.
With fixed atoms, the substance has no atoms free to react and form new compounds.
The composition of a variable solution with no fixed atoms varies as different atoms react.
Separation Techniques
If a mixture has particles, it is not a pure solution.
When atoms are free to react, they create new insoluble compounds that can be physically forced out of the mixture separating the solid from the liquid.
Which technique to use depends on the type of mixture and the size of the particulates.
It could mean sieving, filtering, evaporating or distilling the solution or using chromatography to separate the components by binding affinities or charge.
Role of Temperature and Pressure in the Purity of Solutions
The role of temperature and pressure in the purity of a solution depends on the properties of the solvent and the solute, their interaction and their state of matter.
Liquids, solids and gases not only react with each other but react to temperature and pressure manipulation differently.
As a general rule, solids become more soluble as the temperature of the solution goes up and are less soluble as it comes down.
Similarly, solubility under pressure is governed by Henry’s law.
It describes the relationship between the partial pressure of the gas above a liquid and the gas dissolved in it.
Fact: Sugar takes longer to dissolve in cold water than it does hot, and its effect is utilized in the drug industry to sugarcoat tablets.
Takeaway
Is a solution a pure substance? Although it looks like it, it is not. Its uniform look is rather deceiving.
But, the truth is that a substance cannot be pure mainly because it contains two or more components mixed homogeneously.
Pure substances have uniform composition and properties, but that is not the case with solutions.
