Nature of Matter: Elements, Compounds, and Mixtures · Lesson 4 of 6
Iron and Sulfur: Mixture or Compound?
“Follow one investigation that contrasts a physical mixture with the compound made by heating it.”
• Explain how an unheated iron–sulfur mixture differs from heated iron sulfide. • Compare appearance, magnet response, and acid-reaction observations as evidence. • Write word reactions connecting the starting substances and products. • Distinguish separating a mixture from decomposing or reacting a compound. • Use several observations together instead of judging purity by uniform appearance.
One Investigation, Two Samples
Definitions become more useful when we can test them against the same starting materials. The teacher demonstration uses 5.6 g of iron filings and 3.2 g of sulfur powder. Before heating, the iron looks dark grey and sulfur looks yellow. Mixing them makes Sample A. Half of that mixture is heated to form a black solid, then cooled and ground to give Sample B.
The purpose is to decide whether Sample B is merely a more finely mixed version of Sample A or a different substance. Grinding alone would only reduce size. Heating can permit the elements to react chemically. We therefore compare several properties rather than treating the black appearance as sufficient proof.
Heating, acids, and gases are handled only in an appropriate teacher demonstration. For the acid tests, use recorded observations or a suitable demonstration video. Hydrogen sulfide is toxic: students should never smell, waft, make, or ignite it.
Sample A: The Unheated Mixture
In Sample A, iron and sulfur remain distinct substances. Small dark and yellow grains can still be identified, so it is a non-uniform solid–solid mixture. The iron retains its attraction to a magnet. Moving a magnet close to the mixture removes iron filings while leaving sulfur, providing a physical way to separate the components.
Notice what the magnet actually does. It pulls out iron that was already present in the mixture. It does not turn sulfur into iron or break a chemical compound into elements. This makes the magnet test strong evidence that the individual properties have been retained.
Problem
A magnet lifts some dark grains from Sample A. What can you infer, and what can you not infer?
- 1.The lifted grains show that iron remains present with its familiar magnetic response.
- 2.Since sulfur is left behind, the components can be physically separated.
- 3.The result supports classifying Sample A as a mixture. It does not show that every dark substance in every possible material is iron.
Sample B: A New Compound
Heating part of Sample A produces iron sulfide, a compound of iron and sulfur. In the completed classroom reaction, the material is black and looks similar throughout. Unlike the iron filings, the product does not show their strong attraction to the magnet. Its elements cannot be recovered simply by moving a magnet over the ground solid.
Iron + Sulfur → Iron sulfide Heating allows the reaction to take place. Iron sulfide is a new substance, not a new name for unreacted iron and sulfur lying side by side.
The new substance has different properties because the elements are chemically combined. The absence of freely separable iron matters more than black colour alone. If the reaction were incomplete, leftover iron could still respond to a magnet, so experimental observations must be interpreted with the condition of the sample in mind.
Problem
Sample B is ground into powder after cooling. Would grinding let a magnet separate iron from it?
- 1.Grinding changes the size of pieces of the iron sulfide.
- 2.It does not undo the chemical combination of iron and sulfur.
- 3.The expected compound therefore remains iron sulfide; physical grinding does not restore its elements.
The Acid Tests Give More Evidence
The chapter next compares how small portions respond to dilute hydrochloric acid. Sample A contains ordinary iron, which reacts with the acid to form iron chloride and hydrogen gas. The sulfur remains as yellow solid in this test. Hydrogen is colourless and has no smell; the teacher’s controlled test identifies it by a pop when ignited.
Iron + Dilute hydrochloric acid → Iron chloride + Hydrogen Sulfur in the mixture remains behind in this comparison.
Sample B reacts differently. Iron sulfide and dilute hydrochloric acid give iron chloride and hydrogen sulfide gas. This gas is colourless and is associated with a rotten-egg odour, but students must treat that as supplied information, not a test to perform. Its toxicity makes smelling or wafting inappropriate. It is also flammable, so a flame is not a safe or uniquely identifying comparison test for this gas.
Iron sulfide + Dilute hydrochloric acid → Iron chloride + Hydrogen sulfide Hydrogen and hydrogen sulfide are different substances; similar names do not make their properties identical.
| Comparison | Sample A: iron and sulfur mixture | Sample B: iron sulfide compound |
|---|---|---|
| Appearance | Distinct dark iron and yellow sulfur grains | Black product, similar throughout |
| Texture or distribution | Separate grains of two materials | Ground pieces of one reacted product |
| Magnet | Attracts and separates the iron | Does not show the iron filings’ strong response after complete reaction |
| Physical separation | Iron can be removed with a magnet | A magnet does not recover its elements |
| Gas with dilute hydrochloric acid | Hydrogen; sulfur remains | Hydrogen sulfide |
| Reported odour | Hydrogen has no smell | Hydrogen sulfide has a rotten-egg association; do not smell it |
| Burning information | Hydrogen is identified by a teacher’s pop test | Hydrogen sulfide is flammable; no student ignition test |
Problem
Why does making a different gas with the same acid support the claim that Sample B is a new substance?
- 1.The same reagent, dilute hydrochloric acid, is used in both comparisons.
- 2.Sample A gives hydrogen through its uncombined iron; Sample B gives hydrogen sulfide through the reacted iron sulfide.
- 3.Different chemical behaviour, alongside the magnet and appearance results, supports a change in substance.
What the Whole Investigation Shows
Iron and sulfur separately are elements. Their unheated physical mixture is Sample A. The heated, reacted product iron sulfide is a compound, Sample B. Each classification follows from composition and behaviour. The evidence is strongest when several tests agree: retained iron attraction and physical separation for the mixture, changed properties and a new chemical reaction for the compound.
A uniform-looking black sample is not automatically a pure compound. Uniform mixtures exist too. Nor can every compound be classified just by whether a magnet attracts it. In this investigation, the comparison concerns the specific starting iron filings and their reacted product.
Problem
If extra sulfur is added to unheated iron filings, is the material still a mixture?
- 1.Changing the proportion does not by itself force the materials to form a new substance.
- 2.Before chemical reaction, the iron and sulfur keep their identities and can still be considered separate components.
- 3.Mixture composition can vary. A particular pure compound has a fixed elemental composition; leftover excess material would make the reacted sample a mixture again.
Quiz
Which observation most directly shows physical separation of Sample A?
What new substance is formed when iron and sulfur react on heating?
What happens when iron sulfide is merely ground into smaller pieces?
Which gas is produced by iron in dilute hydrochloric acid?
Why are appearance and magnet tests considered together?
How should students use the hydrogen sulfide odour information?
Practice Problems
- Describe the starting materials and how Sample A and Sample B are obtained in the teacher demonstration.
- Compare the expected appearance and magnetic response of the two samples. Explain what each observation means.
- Write the word reaction for iron and sulfur forming iron sulfide.
- Explain why grinding iron sulfide cannot separate its elements, whereas a magnet can separate the unheated mixture.
- Write the two word reactions with dilute hydrochloric acid and name the different gases produced.
- A heated sample still contains grains attracted by a magnet. Give one reason why the reaction result needs closer inspection.
- Explain why “black and uniform-looking” is not enough to prove that a sample is a pure compound.
- Classify iron, sulfur, Sample A, iron sulfide, hydrogen, and hydrogen sulfide as elements, compounds, or mixtures.
Key Takeaways
• The unheated iron–sulfur sample is a mixture with physically separable components. • Heating can chemically combine iron and sulfur to form iron sulfide. • The compound has properties different from those of the separate elements. • Iron and iron sulfide produce different gases when reacting with dilute hydrochloric acid. • Use several observations to support classification, and treat hazardous gas results as supplied data.