How do chemists find the composition of unknown samples?

Twenty First Century Science - Chemistry B - J258 · Cambridge OCR GCSE (9-1) · 6 min read · Updated: 20 Sept 2026

Finding the Composition of Unknown Samples

Welcome to these study notes on chemical analysis! Have you ever wondered how scientists know exactly what is inside a mystery substance? Whether it's a doctor testing a blood sample, a forensic scientist at a crime scene, or an environmentalist checking water for toxins, they all use the techniques we are about to learn.

Important Note: This specific chapter (C5.2) is for Separate Science (Triple Chemistry) students only. If you are taking the Combined Science course, you might not need to know all of these specific tests, but they are fascinating nonetheless!

Don't worry if some of these names or colors seem tricky to remember at first—we have some great tricks to help you along the way!

1. Starting with a Good Sample

Before a chemist can start testing, they need a sample. But they can’t just grab any random bit of a substance. They need a representative sample.

What is representative sampling?
Imagine you have a giant vat of vegetable soup. If you only take a spoonful of the liquid from the top, you might miss the carrots and peas at the bottom. A representative sample means taking several small samples from different parts of the material and mixing them. This ensures the sample you test actually shows what the "bulk" (the whole thing) is like.

Real-world example: If a scientist is testing a field's soil, they will take samples from the corners, the middle, and different depths to get an accurate picture of the whole field.

Quick Review:
Representative sampling ensures that the small bit we test accurately represents the entire substance, catching any variations in the mix.

2. Identifying Metal Ions: The Flame Tests

Many metal atoms give off a very specific color when they are heated in a flame. This is a quick and beautiful way to identify cations (positive metal ions).

How to do a Flame Test:

1. Clean a nichrome wire loop by dipping it in concentrated hydrochloric acid and holding it in a blue Bunsen burner flame until it gives no color.
2. Dip the clean loop into the unknown sample (either a solid or a solution).
3. Hold the loop in the edge of a roaring blue flame and observe the color change.

The Colors You Need to Know:

• Lithium (\(\text{Li}^{+}\)): Crimson (a deep red)
• Sodium (\(\text{Na}^{+}\)): Yellow/Orange
• Potassium (\(\text{K}^{+}\)): Lilac (light purple)
• Calcium (\(\text{Ca}^{2+}\)): Orange-red (sometimes called brick red)
• Copper (\(\text{Cu}^{2+}\)): Green

Memory Aid:
• Lithium = Lovely Red
• Potassium = Purple (Lilac)
• Sodium = Sunshine Yellow

Key Takeaway: Flame tests use heat to make metal ions glow in unique colors. If you see a lilac flame, you know you've found Potassium!

3. Identifying Metal Ions: Precipitation Tests

Sometimes a flame test isn't enough, or the sample is in a solution. In these cases, we add dilute sodium hydroxide solution. This causes a chemical reaction that forms an insoluble solid called a precipitate.

What happens when you add Sodium Hydroxide (\(\text{NaOH}\))?

• Copper (\(\text{Cu}^{2+}\)): Forms a Blue precipitate.
• Iron(II) (\(\text{Fe}^{2+}\)): Forms a Green precipitate.
• Iron(III) (\(\text{Fe}^{3+}\)): Forms a Brown (orange-brown) precipitate.
• Zinc (\(\text{Zn}^{2+}\)): Forms a White precipitate, but it dissolves and becomes clear again if you add excess (extra) sodium hydroxide.
• Calcium (\(\text{Ca}^{2+}\)): Forms a White precipitate, but it does not dissolve in excess.

Writing Ionic Equations for Precipitation Reactions:

In precipitation reactions, aqueous ions combine to form an insoluble solid. Always include state symbols:
• Copper hydroxide: \(\text{Cu}^{2+}\text{(aq)} + 2\text{OH}^{-}\text{(aq)} \rightarrow \text{Cu(OH)}_{2}\text{(s)}\)
• Iron(II) hydroxide: \(\text{Fe}^{2+}\text{(aq)} + 2\text{OH}^{-}\text{(aq)} \rightarrow \text{Fe(OH)}_{2}\text{(s)}\)
• Iron(III) hydroxide: \(\text{Fe}^{3+}\text{(aq)} + 3\text{OH}^{-}\text{(aq)} \rightarrow \text{Fe(OH)}_{3}\text{(s)}\)

Common Mistake: Many students get Zinc and Calcium mixed up because they both start white. Remember: Zinc is "shy"—it disappears (dissolves) when there is too much sodium hydroxide!

Quick Review Box:
• Blue = Copper
• Green = Iron(II)
• Brown = Iron(III)
• White (dissolves) = Zinc
• White (stays solid) = Calcium

4. Identifying Negative Ions (Anions)

Now that we can find the metals (positive ions), we need to find the non-metals (negative ions). Chemists use different reagents for these.

Testing for Carbonates (\(\text{CO}_{3}^{2-}\))

Add dilute acid (like hydrochloric acid) to the sample. If it fizzes (effervescence), it is likely a carbonate. To be 100% sure, bubble the gas through limewater—it will turn cloudy due to carbon dioxide (\(\text{CO}_{2}\)).

Testing for Halides (Chloride, Bromide, Iodide)

First, add dilute nitric acid (to remove any carbonate impurities), then add silver nitrate solution. You will see a precipitate color:
• Chloride (\(\text{Cl}^{-}\)): White precipitate (\(\text{Ag}^{+}\text{(aq)} + \text{Cl}^{-}\text{(aq)} \rightarrow \text{AgCl}\text{(s)}\))
• Bromide (\(\text{Br}^{-}\)): Cream precipitate (\(\text{Ag}^{+}\text{(aq)} + \text{Br}^{-}\text{(aq)} \rightarrow \text{AgBr}\text{(s)}\))
• Iodide (\(\text{I}^{-}\)): Yellow precipitate (\(\text{Ag}^{+}\text{(aq)} + \text{I}^{-}\text{(aq)} \rightarrow \text{AgI}\text{(s)}\))

Mnemonic for Halides:
Chloride, Bromide, Iodide = Milk, Cream, Butter (White, Cream, Yellow)!

Testing for Sulfates (\(\text{SO}_{4}^{2-}\))

Add dilute hydrochloric acid, then add barium chloride solution (or barium nitrate). If a white precipitate forms, a sulfate is present:
\(\text{Ba}^{2+}\text{(aq)} + \text{SO}_{4}^{2-}\text{(aq)} \rightarrow \text{BaSO}_{4}\text{(s)}\)

Key Takeaway: Carbonates fizz with acid; Halides react with silver nitrate (White/Cream/Yellow); Sulfates react with barium chloride to make a white solid.

5. Instrumental Methods

Everything we have talked about so far is "manual" lab work. However, modern chemists often use big machines for Instrumental Analysis.

Emission Spectroscopy

A sample is placed in a hot flame or electric arc. The light emitted is passed through a spectroscope. This machine produces a "spectrum" (a pattern of lines). Each element has its own unique pattern of lines, almost like a barcode or a fingerprint.

Application: Emission spectroscopy is how we know what stars are made of! Since we can't travel to a star to collect a sample, we analyze the light emitted using these line patterns. It is also widely used to test the composition of steel alloys.

Gas Chromatography

Gas chromatography is an instrumental technique used to separate and identify components in complex mixtures (such as identifying pollutants in water or drugs in blood). The mixture is carried by a gas through a column, separating the components based on how quickly they travel.

Advantages and Disadvantages of Instrumental Methods:

Advantages:
1. Sensitivity: They can detect tiny amounts of a substance.
2. Accuracy: They are much less likely to make a mistake than the human eye.
3. Speed: They can test samples very quickly and automatically process data.

Disadvantages / Common Mistake: Don't assume machines are always better. They are very expensive, require specialised training to operate, and must be calibrated, whereas basic bench tests are cheap and easy to do in any school laboratory!

Final Key Takeaway: Instrumental methods like emission spectroscopy and gas chromatography are fast, sensitive, and accurate compared to traditional chemical tests.

References

OCR GCSE (9-1) Chemistry B (Twenty First Century Science) Specification — OCR
OCR 21st Century GCSE Chemistry B Revision Notes — Doc Brown's Chemistry

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is representative sampling and why is it important in chemical analysis?

Representative sampling involves taking multiple small samples from different parts and depths of a bulk material to ensure the tested sample accurately reflects the entire substance.

How is a flame test carried out to identify positive metal ions?

A clean nichrome wire loop is dipped in concentrated hydrochloric acid, placed in a flame to clean, dipped into the sample, and then held in the edge of a roaring blue flame to observe the color change.

What flame colors are produced by lithium, sodium, potassium, calcium, and copper ions?

Lithium (\(\text{Li}^{+}\)) gives crimson, sodium (\(\text{Na}^{+}\)) gives yellow/orange, potassium (\(\text{K}^{+}\)) gives lilac, calcium (\(\text{Ca}^{2+}\)) gives orange-red, and copper (\(\text{Cu}^{2+}\)) gives green.

What precipitates are formed when sodium hydroxide solution is added to copper(II), iron(II), and iron(III) ions?

Copper(II) forms a blue precipitate (\(\text{Cu(OH)}_{2}\)), iron(II) forms a green precipitate (\(\text{Fe(OH)}_{2}\)), and iron(III) forms a brown precipitate (\(\text{Fe(OH)}_{3}\)).

How can you distinguish between zinc (\(\text{Zn}^{2+}\)) and calcium (\(\text{Ca}^{2+}\)) ions using sodium hydroxide solution?

Both initially form a white precipitate, but the zinc hydroxide precipitate redissolves in excess sodium hydroxide to form a clear solution, while the calcium hydroxide precipitate remains insoluble.

What is the chemical test and positive result for carbonate ions (\(\text{CO}_{3}^{2-}\))?

Add dilute acid to produce effervescence (fizzing), then bubble the resulting carbon dioxide gas through limewater to turn it cloudy.

How do you test for halide ions (chloride, bromide, iodide) and what are the expected results?

Add dilute nitric acid followed by silver nitrate solution: chloride gives a white precipitate (\(\text{AgCl}\)), bromide gives a cream precipitate (\(\text{AgBr}\)), and iodide gives a yellow precipitate (\(\text{AgI}\)).

How do you test for sulfate ions (\(\text{SO}_{4}^{2-}\))?

Add dilute hydrochloric acid followed by barium chloride solution; the formation of a white precipitate (\(\text{BaSO}_{4}\)) indicates the presence of sulfate ions.

How does emission spectroscopy identify elements in a sample?

A sample is heated in a flame or electric arc, and the emitted light is passed through a spectroscope to produce a unique line spectrum that acts like a barcode for that element.

What is gas chromatography used for?

Gas chromatography is an instrumental technique used to separate and identify individual components in complex mixtures, such as pollutants in water or drugs in blood.

What are the main advantages and disadvantages of instrumental methods compared to traditional bench tests?

Advantages include higher sensitivity, greater accuracy, and faster automated analysis, while disadvantages include high equipment costs, required specialist training, and the need for calibration.

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