Welcome to Chemical Detectives: Purity and Chromatography!
Welcome, budding chemists! This chapter is all about becoming a chemical detective. When we analyze substances in the lab, or even when companies check the quality of food or medicine, the first question they ask is: Is this substance pure, or is it a mixture?
Understanding purity and how to separate mixtures using a technique called chromatography is fundamental to chemical analysis. Don't worry if this sounds complicated; we will break down this cool separation science into easy, manageable steps!
Section 1: Pure Substances vs. Impure Mixtures
In chemistry, the words "pure" and "impure" have very specific meanings.
What is a Pure Substance?
A pure substance is made up of only one type of element or one type of compound.
Example: Pure water (only H₂O molecules) or pure gold (only Au atoms).
Key Takeaway: If you have a scoop of sugar, and every tiny particle in that scoop is exactly the same (sucrose), it is chemically pure.
What is an Impure Substance (A Mixture)?
An impure substance is a mixture. It contains two or more different elements or compounds that are not chemically bonded together.
Example: Seawater (a mixture of water, salt, and other minerals) or air (a mixture of nitrogen, oxygen, etc.).
Testing for Purity: The Temperature Test
How can we prove something is pure without looking at the molecules? We use temperature! Pure substances have a "chemical fingerprint" – their melting and boiling points are fixed and specific.
1. Melting Point Analysis
A pure substance melts at one specific temperature.
- Pure Substance: Melts sharply at a single, fixed temperature (e.g., pure water melts at 0 °C).
- Impure Substance (Mixture): Impurities interfere with the structure, making it easier to break apart. Therefore, an impure substance will melt at a lower temperature and over a range of temperatures.
2. Boiling Point Analysis
A pure substance boils at one specific temperature.
- Pure Substance: Boils sharply at a single, fixed temperature (e.g., pure water boils at 100 °C at standard pressure).
- Impure Substance (Mixture): Impurities make it harder for the substance to escape into the gas phase. Therefore, an impure substance will boil at a higher temperature and over a range of temperatures.
Think of it this way: Impurities stop the substance from doing what it should.
- They Lower the Melting Point.
- They Raise the Boiling Point.
Section 2: An Introduction to Chromatography
Imagine you have a marker pen, and you want to know exactly how many different color dyes were mixed together to make that ink. You can't just look at it! This is where chromatography comes in.
What is Chromatography?
Chromatography is a powerful method used to separate substances in a mixture. It works especially well for separating dissolved solids, like colored dyes or amino acids.
The name comes from the Greek words chroma (colour) and graphy (writing or drawing), reflecting its early use in separating coloured pigments.
The Two Phases of Chromatography
All types of chromatography rely on two fundamental components:
1. The Stationary Phase (The Sitter)
Definition: This is the part that does not move. The mixture components stick or anchor to this phase.
In paper chromatography: This is the filter paper or chromatography paper itself.
2. The Mobile Phase (The Mover)
Definition: This is the part that moves, carrying the mixture along with it.
In paper chromatography: This is the solvent (often water or an ethanol solution).
Analogy: A Race Track
Imagine a race where the runners (the components of your mixture) start at the same line.
- The track is the stationary phase (the paper).
- The wind or current pushing them forward is the mobile phase (the solvent).
Section 3: Carrying out Paper Chromatography
Paper chromatography is the simplest and most common form of this technique you will perform. Here is the step-by-step process:
Step 1: Preparation
Draw a pencil line about 1-2 cm from the bottom of the chromatography paper. This is the baseline. Important: Always use pencil, because pencil marks (made of carbon) are insoluble and won't run up the paper; ink lines would separate and ruin the experiment!
Step 2: Spotting
Place a small, concentrated spot of the mixture (e.g., the ink) onto the pencil baseline.
Step 3: Setting Up the Container
Place the paper into a beaker or test tube containing a small amount of the solvent (the mobile phase).
Make sure the spot of mixture is above the level of the solvent. If the spot is submerged, the mixture will simply dissolve into the solvent pool instead of travelling up the paper.
Step 4: Running the Chromatogram
The solvent moves up the paper by capillary action (like water soaking up a sponge).
As the solvent moves, it dissolves the components of the mixture and carries them along.
- Components that are highly soluble in the solvent and less attracted to the paper travel the farthest.
- Components that are less soluble in the solvent and more attracted to the paper lag behind.
Step 5: Analysis
When the solvent reaches near the top (the solvent front), remove the paper immediately and draw a pencil line marking the solvent front before the solvent dries. This experiment shows:
- If the mixture separates into multiple spots, it is impure (it is a mixture).
- If the mixture only produces a single spot that travels up the paper, it is likely a pure substance (though further tests might be needed).
Section 4: Analyzing the Results – The Retention Factor (\(R_f\) Value)
How do we accurately identify the components once they are separated on the chromatogram? We use a calculated ratio called the Retention Factor, or \(R_f\) value.
What is the \(R_f\) Value?
The \(R_f\) value is a ratio that helps us identify a substance. It measures how far a component has traveled relative to how far the solvent has traveled.
The \(R_f\) value is unique for a specific substance when using a specific solvent and temperature. If you run an unknown substance and get an \(R_f\) of 0.7, and you know dye X has an \(R_f\) of 0.7 under the same conditions, you can identify the substance.
Calculating the \(R_f\) Value
The formula is:
\[R_f = \frac{\text{Distance traveled by spot}}{\text{Distance traveled by solvent front}}\]
Step-by-Step Calculation Guide:
1. Measure the distance (in mm or cm) from the baseline to the center of the spot (d_spot).
2. Measure the distance (in mm or cm) from the baseline to the solvent front (d_solvent).
3. Divide the two measurements.
Example: If the spot traveled 3.0 cm and the solvent front traveled 5.0 cm: \[R_f = \frac{3.0 \text{ cm}}{5.0 \text{ cm}} = 0.60\]
Did you know? The \(R_f\) value will always be a number between 0 and 1, because the spot can never travel farther than the solvent front!
Summary of Key Concepts
- Purity Test: Pure substances have fixed melting/boiling points. Impurities widen the range, lower the melting point, and raise the boiling point.
- Chromatography: Separates mixtures based on different solubilities and attractions.
- Phases: Stationary (paper) and Mobile (solvent).
- Result Interpretation: Multiple spots = Mixture. Single spot = Pure substance.
- Identification: The \(R_f\) value is a fixed ratio used to identify unknown components.
Great job! You now understand the essential methods used to test purity and separate mixtures—skills vital for all areas of chemistry and quality control!