Combined Science (9204) | Chemical analysis

Welcome to Chemical Analysis: Your Chemistry Detective Kit!

Hello future scientist! This chapter is all about becoming a chemistry detective. We will learn how to identify unknown substances, check if something is pure, and separate mixtures. This skill is vital in quality control, forensic science, and medicine.

Don't worry if some of the names sound complicated; we will break down the tests step-by-step. Let's get identifying!

Section 1: Testing for Purity

Why Do We Need to Know if Something is Pure?

In chemistry, a pure substance is made up of only one type of element or compound. A mixture contains two or more substances that are not chemically joined together.

For pharmaceuticals (medicines) or food production, knowing the purity is crucial for safety and effectiveness.

Testing Purity Using Melting and Boiling Points

A key property of a pure substance is that it has a very specific, sharp melting point (M.P.) and boiling point (B.P.).

How Impurities Affect Purity Tests

• Pure Substance: Melts and boils sharply at the known standard temperature (e.g., pure water boils at exactly 100°C at standard pressure).
• Impure Substance (Mixture): Impurities change these physical properties!
  • The Melting Point is lowered (gets colder before melting).
  • The Boiling Point is raised (needs more heat to boil).
  • The substance will melt or boil over a range of temperatures, not sharply at one specific temperature.

Analogy: Think of adding salt to ice. The salt (impurity) lowers the melting point, causing the ice to melt even when it's very cold!

Section 2: Identification of Ions (The Precipitation Method)

Ions are atoms or groups of atoms that have gained or lost electrons, giving them an electrical charge. We classify them as Cations (positively charged) or Anions (negatively charged).

We test for many ions using precipitation reactions. This means we mix two solutions, and if the ions react, they form an insoluble solid called a precipitate (ppt). The colour of this solid is our clue!

A) Testing for Cations (Metal Ions) using Sodium Hydroxide (NaOH)

Sodium hydroxide solution is the standard reagent for identifying certain common metal ions because it causes different coloured precipitates to form.

Step-by-Step Cation Test:

1. Add a few drops of sodium hydroxide solution (NaOH) to the unknown solution.
2. Observe the colour of the precipitate formed.

Memory Aid: Imagine traffic lights:
Red/Brown for Fe(III) (Stop!)
Green for Fe(II) (Go!)
(The blue copper doesn't fit the lights, but it's a common blue colour!)

B) Testing for Cations using Flame Tests

Some metal ions give off a characteristic colour when heated strongly in a flame. This is because the heat energy excites the electrons, and when they fall back down, they release energy as visible light.

Flame Test Procedure:

1. Clean a piece of wire (usually nichrome or platinum) by dipping it into concentrated hydrochloric acid (HCl) and heating it in a non-luminous flame until no colour is seen.
2. Dip the clean wire into the sample you want to test.
3. Place the wire into the edge of the blue (non-luminous) Bunsen burner flame and observe the colour produced.

Common Mistake Alert! Sodium is often present as an impurity in many compounds. If you get a very intense orange-yellow flash, it might just be a contaminant. Focus on the main, sustained colour.

C) Testing for Anions (Non-Metal Ions)

We use specific chemical tests to confirm the presence of negatively charged ions like carbonates, sulfates, and halides.

1. Test for Carbonate Ions (CO₃^2-)

Carbonates react with acid to produce carbon dioxide gas. This gas is then identified using limewater.

1. Add a small amount of dilute acid (e.g., HCl) to the sample.
2. If carbonates are present, the mixture will fizz (effervescence) as gas is released.
3. Bubble this gas through limewater (calcium hydroxide solution).
4. Positive Result: The limewater turns cloudy or milky.

Equation (conceptually):
$$ \text{Carbonate} + \text{Acid} \rightarrow \text{Salt} + \text{Water} + \text{Carbon Dioxide} $$

2. Test for Sulfate Ions (SO₄^2-)

Sulfate ions react with barium ions to form barium sulfate, which is a key white precipitate.

1. First, acidify the sample with a few drops of dilute hydrochloric acid (HCl). (This removes any interfering carbonate ions).
2. Add barium chloride solution (BaCl₂).
3. Positive Result: A thick white precipitate forms immediately.

Key Takeaway: Sulfate + Barium = Insoluble White Solid.

3. Test for Halide Ions (Chloride, Bromide, Iodide)

Halide ions (Group 17) are tested using silver nitrate solution (AgNO₃). They form insoluble silver halides, which produce distinct colours.

1. First, acidify the sample with a few drops of dilute nitric acid (HNO₃). (Again, this removes interfering ions).
2. Add silver nitrate solution (AgNO₃).

Did you know? These silver halides are sensitive to light, which is why they were essential for early photography!

Section 3: Separation and Identification using Chromatography

Chromatography is a technique used to separate and identify substances in a mixture (like dyes in ink, or amino acids in a protein sample).

How Chromatography Works

Chromatography relies on the fact that different substances have different solubilities (how well they dissolve) and different levels of attraction to the surface of the paper.

• Stationary Phase: The material that doesn't move. In paper chromatography, this is the chromatography paper.
• Mobile Phase: The solvent (often water or ethanol) that moves up the stationary phase, carrying the sample with it.

The Process of Paper Chromatography

Step-by-Step Procedure:

1. Draw a horizontal line near the bottom of the chromatography paper using a pencil. (Pencil is used because ink would run and interfere).
2. Place a small spot of the mixture (e.g., ink) on the pencil line. This is the baseline.
3. Place the paper into a beaker containing the solvent (mobile phase), ensuring the solvent level is below the pencil line.
4. The solvent moves up the paper, carrying the components of the mixture upwards.
5. When the solvent reaches near the top, remove the paper and mark the solvent front (the furthest point the solvent reached). This finished paper is called a chromatogram.

Interpreting the Results

• Pure Substance vs. Mixture: If the original spot separates into multiple spots, the substance was a mixture (like black ink usually is). If the substance shows only one spot, it is a pure substance.
• Identification: If the spot from the unknown mixture travels the same distance as a spot from a known reference substance, it suggests they are the same component.
• Don't worry if this seems tricky at first: The core idea is that the most soluble components that are least attracted to the paper travel the furthest!

Final Word of Encouragement

You now have the fundamental tools to identify unknown chemicals and test their purity. Chemical analysis is about careful observation and following procedures precisely. Practice visualizing those colour changes—they are your biggest clues! Keep up the great work!