Which soft metal can be easily cut with a knife at room temperature because of its low hardness?

Introduction / Context:
Metals differ greatly in their hardness and mechanical properties. Some are so hard that they are used to make cutting tools, while others are so soft that they can be cut with an ordinary knife. General science and chemistry examinations often ask about these differences to test knowledge of alkali metals and their characteristic properties. This question specifically looks at which metal is soft enough at room temperature to be cut easily with a knife.

Given Data / Assumptions:

• The options are copper, gold, sodium, and aluminium.
• The question mentions that the metal can be easily cut with a knife, indicating very low hardness.
• We assume normal room temperature conditions and pure metal samples.

Concept / Approach:
Alkali metals such as lithium, sodium, and potassium are known for being very soft compared with most other metals. They have only one valence electron, low density, and relatively weak metallic bonding, which makes them easy to deform and cut. In chemistry laboratories, fresh sodium is often sliced with a knife and stored under kerosene because it reacts quickly with air and moisture. Copper and aluminium are harder structural metals and cannot be easily cut with a knife. Gold is softer than many metals but still not as soft as sodium; jewellery is usually an alloy to make it harder.

Step-by-Step Solution:
1. Recall that sodium is an alkali metal that is very soft and can be cut with a knife to show a shiny fresh surface.2. Remember that copper is used for electrical wires and plumbing because it is relatively strong and tough, not knife soft.3. Aluminium, used for utensils and structural parts, has more hardness and cannot be cleanly sliced with an ordinary knife.4. Gold is malleable and ductile, but pure gold is rarely handled in large blocks and still is not as knife soft as sodium.5. Since sodium matches the textbook demonstration of a metal cut by a knife, it is the correct answer.

Verification / Alternative check:
Chemistry demonstrations frequently show a piece of sodium being cut with a knife to reveal a bright metallic luster that quickly dulls as it reacts with oxygen. This is a standard example used to illustrate the softness and reactivity of alkali metals. Safety guidelines also mention that sodium must be handled carefully because cutting exposes a fresh surface that reacts vigorously with water. In contrast, copper, aluminium, and gold jewelry do not show such behaviour and must be shaped using more forceful methods like rolling, forging, or machining.

Why Other Options Are Wrong:
Option A, copper, is too hard to be cleanly cut with a simple knife and is normally cut with saws or shears. Option B, gold, is soft and malleable but not so soft that it is a textbook example of being sliced with a knife in school science. Option D, aluminium, is also relatively hard and is used for structurally strong but lightweight components. None of these three metals fits the classic description of a knife soft metal as well as sodium does.

Common Pitfalls:
Students sometimes assume that gold must be the softest metal because it is described as soft and malleable in jewellery contexts. However, softness in this everyday sense is different from the extreme softness of alkali metals. Another pitfall is to think of aluminum foil as soft because it bends easily, forgetting that the bulk metal is still too hard to slice like soap. Remembering that sodium is an alkali metal that can literally be cut with a knife in demonstrations will help you select the correct option.

Final Answer:
The metal that can be easily cut with a knife at room temperature is sodium.