NCERT Solutions for Class 10 Science Chapter 3 Activity 3.9: Reaction of Metals with Oxygen
Understanding the Question 🧐
Activity 3.9 from the NCERT textbook asks us to observe and analyze what happens when different metal samples are burnt in the air over a flame. This involves noting their ease of burning, flame color, surface appearance after burning, and the properties of the product formed.
Our Expert’s Approach
To answer this, our experts first identify the core scientific principle at play: the reaction of metals with oxygen (oxidation), which is governed by their position in the reactivity series. We will systematically address each question posed in the activity by explaining the expected observations for different metals and the chemical reasons behind them. This step-by-step method ensures clarity and accuracy, reflecting deep subject matter expertise in chemical reactions.
Part 1: Which metals burn easily? 📝
The ease with which a metal burns depends on its reactivity. Highly reactive metals burn more easily and vigorously.
- Extremely Reactive Metals: Potassium (&&K&&) and Sodium (&&Na&&) are so reactive that they catch fire when exposed to air. They don’t need external heating to burn.
- Highly Reactive Metals: Magnesium (&&Mg&&) burns readily when heated, producing a dazzling white flame.
- Moderately Reactive Metals: Metals like Aluminium (&&Al&&), Zinc (&&Zn&&), and Iron (&&Fe&&) do not burn easily. When heated strongly, they don’t produce a flame, but their surface will glow and get coated with an oxide layer. For example, iron filings will sparkle when sprinkled on a flame, but an iron nail will just glow red hot.
- Less Reactive Metals: Copper (&&Cu&&) does not burn but forms a black oxide coating when heated for a long time.
- Least Reactive Metals: Silver (&&Ag&&) and Gold (&&Au&&) do not react with oxygen even at high temperatures.
Part 2: What flame colour did you observe when the metal burnt? 📝
Many metals, especially those from the alkali and alkaline earth groups, impart a characteristic color to the flame upon burning.
- Sodium (&&Na&&): Burns with an intense golden-yellow flame.
- Potassium (&&K&&): Burns with a lilac (pale violet) flame.
- Magnesium (&&Mg&&): Burns with a dazzling, brilliant white light.
Other metals like zinc, iron, and lead do not produce a distinct colored flame when heated in this manner.
Part 3: How does the metal surface appear after burning? 📝
When a metal burns, it reacts with oxygen to form a metal oxide. This process is called oxidation. The new layer of metal oxide is typically dull and non-lustrous compared to the original shiny metal surface.
Example 1: Magnesium
When magnesium ribbon is burnt, it forms a white, powdery substance called magnesium oxide.
Chemical Equation: &&2Mg(s) + O_2(g) \rightarrow 2MgO(s)&&
Example 2: Copper
When copper is heated in the air, its reddish-brown surface gets coated with a black substance, which is copper(II) oxide.
Chemical Equation: &&2Cu(s) + O_2(g) \rightarrow 2CuO(s)&&
Part 4: Arrange the metals in the decreasing order of their reactivity towards oxygen. 📝
Based on the vigor of the reaction, we can arrange the metals in a reactivity series. The metal that reacts most violently is the most reactive, and the one that doesn’t react is the least reactive.
The decreasing order of reactivity towards oxygen is:
Potassium (&&K&&) > Sodium (&&Na&&) > Magnesium (&&Mg&&) > Aluminium (&&Al&&) > Zinc (&&Zn&&) > Iron (&&Fe&&) > Lead (&&Pb&&) > Copper (&&Cu&&)
Part 5: Are the products (metal oxides) soluble in water? 📝
The solubility of the resulting metal oxides in water varies.
- Soluble Oxides: Oxides of highly reactive metals like sodium and potassium are soluble in water. They dissolve to form alkalis (bases).
Equation: &&Na_2O(s) + H_2O(l) \rightarrow 2NaOH(aq)&& (Sodium Hydroxide)
Equation: &&K_2O(s) + H_2O(l) \rightarrow 2KOH(aq)&& (Potassium Hydroxide) - Insoluble Oxides: Most other metal oxides, such as magnesium oxide, copper oxide, and iron oxide, are insoluble in water.
Conclusion and Key Takeaways ✅
This activity demonstrates a fundamental property of metals: their reaction with oxygen. The key takeaway is that metals show varying degrees of reactivity. This reactivity determines how easily they burn, the appearance of the product, and the properties of the resulting metal oxides. This concept is crucial for understanding processes like corrosion (rusting) and for organizing metals in the reactivity series, a foundational tool in chemistry.
💡 Trick to Remember
Remember the reactivity series with a mnemonic like: “Please Stop Calling Me A Zebra, I Like Her Call Smart Goat” for Potassium, Sodium, Calcium, Magnesium, Aluminium, Zinc, Iron, Lead, Hydrogen, Copper, Silver, Gold.
📌 Points to Remember
- Reaction of a metal with oxygen produces a metal oxide.
- Metal oxides are generally basic in nature.
- Some metal oxides like &&Al_2O_3&& and &&ZnO&& are amphoteric (show both acidic and basic behavior).
- Reactivity decreases as you go down the series.
Frequently Asked Questions (FAQ)
What is a metal oxide?
A metal oxide is a chemical compound that contains at least one metal atom and one oxygen atom. They are typically formed when metals react with oxygen in the air, a process known as oxidation. Most metal oxides are basic in nature.
Why do some metals like sodium and potassium need to be stored in kerosene?
Sodium and potassium are extremely reactive metals. They react vigorously with both oxygen and moisture present in the air, and can even catch fire. To prevent this accidental fire and reaction, they are stored immersed in kerosene oil, which cuts off their contact with air.
What are amphoteric oxides? Give an example.
Amphoteric oxides are metal oxides that exhibit both acidic and basic properties. They react with both acids and bases to form salt and water. Common examples include Aluminium oxide (&&Al_2O_3&&) and Zinc oxide (&&ZnO&&).
Why doesn’t gold or silver corrode easily?
Gold (&&Au&&) and Silver (&&Ag&&) are at the bottom of the reactivity series. They are very unreactive metals, often called noble metals. They do not react with oxygen, water, or common acids under normal conditions, which is why they do not corrode and are used to make jewelry.
Further Reading
For more detailed information, you can refer to the official NCERT textbook. Visit: https://ncert.nic.in/
This completes the ncert solutions for this activity.
