Question
Which of the following metals CANNOT be cut with a knife?
This question was previously asked in
RRB NTPC 2024 Undergraduate CBT 1 Question Paper (29-Aug-2025) (Shift 1)
The correct answer is
Copper
Metals and Knife Cutability Explained
This question asks us to identify which metal from the given options cannot be cut using a standard knife. Metals have varying physical properties, including hardness. Hardness refers to a material's resistance to scratching or indentation. Some metals are very soft and can be easily cut, while others are much harder and require specialized tools.
Properties of Metals in the Question
Let's examine the properties of each metal listed:
- Copper (Cu): Copper is a transition metal. While it is known for being malleable (can be hammered into thin sheets) and ductile (can be drawn into wires), it is significantly harder than alkali metals. A typical knife edge will not easily cut through solid copper. It requires tools like saws or cutters designed for metal.
- Sodium (Na): Sodium is an alkali metal. Alkali metals are known for their characteristic softness and high reactivity. Sodium is soft enough that it can be easily sliced with a knife, often appearing waxy. It must be stored under oil to prevent reaction with air and moisture. Its chemical symbol is \(Na\).
- Lithium (Li): Lithium is also an alkali metal, located at the top of its group in the periodic table. It is the least dense metal and is quite soft, although slightly harder than sodium. A knife can readily cut through lithium metal. Its chemical symbol is \(Li\).
- Potassium (K): Potassium is another alkali metal. It is even softer than sodium and lithium and possesses a very low density. It can be easily cut with a knife, often described as being softer than butter at room temperature. Like sodium, it is highly reactive and must be handled carefully. Its chemical symbol is \(K\).
Conclusion on Knife Cutability
Based on the physical properties, the alkali metals listed – Sodium (Na), Lithium (Li), and Potassium (K) – are all soft enough to be easily cut with a knife. Copper (Cu), being a transition metal, is considerably harder and cannot be cut with a simple knife.
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