The World of Metals and Non-metals · Lesson 4 of 6
How Metals React with Oxygen
“Follow magnesium from a bright-burning metal to a basic product, and compare it with reactive sodium.”
• Describe what happens when magnesium burns in air. • Distinguish magnesium from the magnesium oxide it forms. • Interpret red and blue litmus tests of the water-treated product. • Explain the general pattern that metal oxides are basic. • Explain why sodium is kept away from air and water in kerosene.
A metal can form a new substance
Hammering iron changes its shape, and drawing copper makes a wire. In these changes the material is still the same metal. Burning magnesium shows a different kind of behaviour: the shiny metal ribbon becomes a white powder with properties different from those of the original metal.
To understand the change, follow the stages carefully. First identify the starting material. Next observe what happens during burning and what remains. Finally, test the product rather than assuming it must behave like the original metal. This sequence links an observation to an explanation.
Observing burning magnesium
Magnesium is a metal often provided as a thin ribbon. The demonstration uses a short piece, about three to four centimetres long. Rubbing it with sandpaper cleans away a surface coating so the underlying magnesium is exposed. The teacher holds it with tongs and heats one end until it burns.
Burning magnesium produces an intensely bright light and hot material. A teacher should perform the demonstration with appropriate eye protection, suitable equipment, and students at a safe distance. Do not stare directly at the flame or attempt to burn the ribbon yourself.
The ribbon burns with a dazzling white flame. White powder remains, which the teacher collects safely after it cools. The powder is magnesium oxide. Oxygen from the air has combined with magnesium to form this new substance; the air is part of the change, not merely empty space around the flame.
A substance formed when a metal combines with oxygen.
In words, the relationship is magnesium plus oxygen produces magnesium oxide. Oxygen is a non-metal, but the product is named a metal oxide because it is the oxide of magnesium. Do not confuse the category of the starting element with the properties of the substance that forms.
Problem
A student says the white powder is simply magnesium dust. What evidence suggests otherwise?
- 1.The starting ribbon is a metal, while the product is a white powder produced during burning in air.
- 2.Magnesium combines with oxygen during the change. This forms magnesium oxide.
- 3.The product can therefore have different properties from magnesium. The changed appearance is a clue, and the reaction explanation identifies the new substance.
Using indicators to investigate the product
Appearance does not tell us whether a substance is acidic, basic, or neutral. We can use indicators to investigate that question. The teacher adds a few drops of warm water to the collected powder, stirs it, and tests the water-treated mixture with red and blue litmus.
Red litmus changes to blue, while blue litmus remains blue. This is the familiar indicator pattern for a basic sample. Magnesium oxide does not need to dissolve completely into a clear liquid for the water-treated mixture to show basic nature. The indicator is responding to the sample in contact with water.
| Indicator | Before testing | After testing the water-treated magnesium oxide | Interpretation |
|---|---|---|---|
| Red litmus | Red | Blue | Evidence of basic nature |
| Blue litmus | Blue | Blue | Consistent with basic nature; no colour change |
The indicator behaviour of a base, including a change of red litmus to blue.
The chapter uses this investigation to illustrate a general pattern: metal oxides are usually basic. A pattern helps us organise observations, but it should be written as “generally” rather than “every metal oxide always”. At this level, magnesium oxide is the example that establishes the connection between a metal, oxygen, its oxide, and an indicator result.
Problem
The water-treated white powder turns red litmus blue and leaves blue litmus blue. What should you conclude?
- 1.A red-to-blue change is the characteristic evidence of basic nature.
- 2.Unchanged blue litmus agrees with that interpretation; a basic sample does not need to change both colours.
- 3.Conclude that the water-treated magnesium oxide is basic. Do not call it acidic just because one indicator stayed unchanged.
The litmus conclusion comes from the product treated with water. It is not a test of the original dry metal ribbon, and one unchanged litmus colour by itself is not enough to identify acidity or neutrality.
Not all metals react at the same pace
Iron rusts over days in the bottle investigation, while magnesium reacts conspicuously when heated to burning. Sodium provides an even more reactive example: it reacts vigorously with oxygen and water, releasing much heat. A general category called metals does not mean that every metal behaves at the same rate.
Sodium is stored under kerosene to keep it away from air and moisture. Kerosene provides a separating layer suitable for this particular metal. This is a storage precaution based on the metal’s reactions, not evidence that sodium is a poor conductor or that it is not a metal.
Sodium can react violently with water and may ignite. Its storage and handling require trained supervision. Use the explanation to reason about the material; do not reproduce the reaction.
Problem
The chapter asks you to predict the nature of a sodium oxide using the metal-oxide pattern. How should you reason?
- 1.Sodium is a metal, so its oxide belongs to the metal-oxide group.
- 2.The pattern studied here is that metal oxides are generally basic. This supports a prediction of basic nature.
- 3.The prediction is based on the pattern; sodium should not be handled or tested by students to verify it.
Compare two reasons for keeping materials away from their surroundings. A coat on iron can reduce gradual damage from rusting. Kerosene around sodium prevents much more vigorous contact with air and water. Both choices respond to chemical behaviour, but their immediate purposes and handling requirements are different.
Quiz
What white product forms when magnesium burns in air?
Which substance from the air combines with magnesium during burning?
What litmus pattern is expected for the water-treated magnesium oxide?
Why is sodium stored under kerosene?
Which statement is the best scientific generalisation from this lesson?
Practice Problems
- Describe the starting material, flame, and product in the magnesium demonstration.
- Explain why oxygen is included in the description of burning magnesium.
- Write the magnesium reaction as a relationship in words and explain what a metal oxide is.
- Predict both litmus results for water-treated magnesium oxide and explain the conclusion.
- Explain why cleaning the ribbon and using tongs are separate parts of the demonstration.
- Compare the pace and conditions of iron rusting, magnesium burning, and sodium reacting with air or water.
- Explain the reason for storing sodium in kerosene without proposing a student experiment.
No. Magnesium has combined with oxygen to form magnesium oxide, a different substance.
Key Takeaways
• Magnesium burns with a bright white flame and combines with oxygen to form white magnesium oxide. • A metal oxide is a substance formed from a metal and oxygen. • Water-treated magnesium oxide changes red litmus to blue and is basic. • Metal oxides are generally basic; this is a pattern rather than an unrestricted rule. • Sodium reacts vigorously with air and water and is stored under kerosene to exclude them.