Pure and Impure Substances Lesson: Definition & Concepts

Lesson Overview

• Pure Substances
• Impure Substances (Mixtures)
• Pure Substances vs. Mixtures
• Melting and Boiling Points: Indicators of Purity
• Separating Mixtures into Pure Substances

We often hear terms like "pure gold" or "pure water" in everyday life. In science, purity has a specific meaning: a pure substance is made of only one kind of material throughout. If there is more than one kind of substance mixed together, we call it an impure substance or a mixture. 

Understanding this difference is important because pure and impure substances have different properties and behaviors. Let's explore what pure substances are, the difference between elements and compounds, what mixtures are, how to tell them apart by their properties (like boiling point), and how we can separate mixtures into pure components.

Pure Substances

A pure substance consists of a single type of particle. This means if you zoom in to the smallest pieces of the material, they are all the same kind. Pure substances can be of two types: elements and compounds. Both elements and compounds are uniform (the same all the way through) and have fixed properties such as specific melting and boiling points. Let's look at each type:

Elements

An element is the simplest form of matter that is made of only one kind of atom. It cannot be broken down into anything simpler by ordinary chemical means. Each element is defined by the type of atoms it has. For example, the element oxygen is made entirely of oxygen atoms. A piece of pure gold (which is the element gold) contains only gold atoms. All the atoms in an element are the same kind, so an element is a pure substance.

Elements are listed in the Periodic Table of Elements (like hydrogen, oxygen, carbon, iron, etc.). They can exist in different forms: some are single atoms (like helium, He, in balloons) and some are molecules made of one type of atom bonded together (for instance, oxygen gas is O2, two oxygen atoms bonded, but it's still the element oxygen because both atoms are the same type). No matter the form, if only one element is present, we have a pure substance. 

Examples of elements 

• Oxygen gas in a tank, 
• A bar of gold, 
• Carbon in the form of diamond.

Each consists of one type of atom throughout

Compounds

A compound is a pure substance formed when two or more different elements combine chemically in a fixed ratio. The atoms in a compound are bonded together, making it behave as a single substance. Every molecule of a compound is identical and made of the same combination of elements. Even though compounds contain more than one type of element, they are pure substances because they consist of only one type of molecule.

For example, water (H2O) is a compound made of hydrogen and oxygen atoms bonded in a 2:1 ratio. Each molecule of water is the same. Similarly, carbon dioxide (CO2) is made of one carbon atom and two oxygen atoms, and all CO2 molecules are identical.

Compounds often have properties different from the elements that make them. For instance, hydrogen and oxygen are gases at room temperature, but when combined as water, they form a liquid with entirely different properties. Table salt (NaCl) is another compound made from sodium and chlorine, which individually are dangerous but together form a safe, edible substance.

Since compounds are chemically uniform, they have fixed properties like specific melting and boiling points. They can't be separated into their elemental parts by physical means. For example, breaking water into hydrogen and oxygen requires a chemical process like electrolysis. 

Impure Substances (Mixtures)

A mixture is an impure substance that contains two or more different substances physically combined, not chemically bonded. Each component in a mixture retains its own identity and properties. 

For example, mixing sand with water creates a mixture where sand remains sand and water remains water. Similarly, when you stir salt into water, you get salty water, but the salt and water are not chemically bonded.

Mixtures do not have a fixed composition; their components can vary in proportion. This means their properties, like melting and boiling points, can change depending on the amounts of each ingredient. In science, "impure" simply means more than one substance is present, even in substances like milk or air, which are mixtures.

Types of Mixtures: Homogeneous and Heterogeneous

• Homogeneous Mixtures: These are uniform throughout. A good example is salt dissolved in water, where every part of the solution is equally salty. Another example is air, a uniform mixture of gases like nitrogen and oxygen.
  
• Heterogeneous Mixtures: These are not uniform and have visibly different components. For example, oil and water form layers, and trail mix contains different types of nuts and raisins. If you took samples from different parts of a heterogeneous mixture, they could have different proportions of ingredients. Many rocks and soils are heterogeneous mixtures.
  

Take This Quiz:

Pure Substances And Mixtures Quiz

Pure Substances vs. Mixtures

Let's compare pure substances (elements or compounds) with mixtures to highlight their differences:

From the table above, you can see that a pure substance is the same throughout and has a fixed makeup, whereas a mixture has a blend of components and variable makeup.

Melting and Boiling Points: Indicators of Purity

A pure substance has a specific melting and boiling point. The melting point is when a solid turns to liquid, and the boiling point is when a liquid turns to gas. Pure substances melt and boil at consistent temperatures. 

For example, pure water boils at 100°C and freezes at 0°C. Pure iron melts at about 1538°C. This happens because the particles in a pure substance are the same, so they undergo phase changes at the same temperature.

Mixtures, however, do not have a clear melting or boiling point. When heated, mixtures boil over a range of temperatures. For instance, saltwater starts boiling at 100°C, but the boiling point increases as water evaporates and the solution becomes saltier. The added salt changes the boiling behavior.

Impurities usually raise the boiling point and lower the melting point. For example, adding anti-freeze to water in a car radiator raises the boiling point, preventing the coolant from boiling over. Similarly, spreading salt on icy roads lowers water's freezing point, helping ice melt at a colder temperature.

The key point is that pure substances have constant melting and boiling points, while mixtures do not. Scientists often use these points to check for purity. If a substance melts or boils at the exact known temperature for that material, it is likely pure. If not, it might be a mixture.

Separating Mixtures into Pure Substances

One advantage of mixtures (impure substances) is that we can separate them into their pure components using physical methods, because the components are not chemically bonded. There are many techniques to separate mixtures, and the best one depends on the differences in physical properties of the components:

• Filtration: Used to separate an insoluble solid from a liquid. If one part of the mixture is solid pieces and the other part is fluid, you can pour the mixture through a filter (like filter paper or a sieve). The solid particles (for example, sand) get caught on the filter, and the liquid (for example, water) passes through. This way you can separate sand from sandy water, or coffee grounds from liquid coffee, etc.
  
• Evaporation (Crystallization): Used to separate a dissolved solid from a liquid. If you have a solution (like saltwater or sugar water), you can let the liquid part evaporate (or heat it to speed it up) and the dissolved solid will be left behind as crystals. For instance, to get salt from saltwater, you can evaporate the water and collect the salt that remains. This is how sea salt is traditionally obtained from seawater.
  
• Distillation: Used to separate liquids with different boiling points, or to separate a liquid from dissolved solids while keeping the liquid. In distillation, you heat the mixture until one component boils and turns to vapor, then cool that vapor to condense it back into a liquid in a separate container. For example, to purify water from saltwater, you can boil the saltwater; water turns to steam at 100°C and leaves the salt behind. The steam is then cooled in a condenser to turn back into liquid water which is collected – now the water is separated from the salt. Distillation is also used to separate mixtures of liquids like alcohol and water, which have different boiling points.
  
• Magnetic separation: If one component of a mixture is magnetic (for example, iron filings in a mixture of sand and iron), you can use a magnet to attract the iron out of the mixture, leaving the non-magnetic sand behind. This is a simple physical separation based on magnetism.
  
• Sieving (Screening): If the components are solids of different sizes, you can use a sieve with holes of a certain size to separate larger particles from smaller ones. For example, a mixture of rice and powdered salt can be sieved to keep the rice grains on the sieve while the fine salt powder passes through.
  

In contrast, remember that in a pure compound the ingredients are chemically bonded, so physical methods won't separate the elements. To split a compound into its elements or simpler substances, a chemical change is needed. 

For example, water can be separated into hydrogen and oxygen gas, but only by passing an electric current through it (a process called electrolysis) – which is a chemical change. You cannot just filter or boil water to get hydrogen and oxygen – those methods only work for mixtures, not for breaking compounds.

Take This Quiz:

pure and impure substances