An element A is soft and can be cut with a knife. This is very reactive to their and cannot be kept open in air. It reacts vigorously with water. Identify the element from the following:

Options

A. Mg
B. Na
C. P
D. Ca

Answer

To identify the element described in the question, we can break down the characteristics provided and analyze them step by step.

### Step 1: Analyze the Characteristics
The question states that the element A is:
- Soft and can be cut with a knife.
- Very reactive and cannot be kept open in air.
- Reacts vigorously with water.

### Step 2: Identify the Softness
The first characteristic indicates that the element is soft. In the periodic table, alkali metals (Group 1 elements) are known for their softness. For example, lithium, sodium, and potassium can all be cut with a knife.

**Hint:** Look for elements in Group 1 of the periodic table, as they are typically soft.

### Step 3: Assess Reactivity
The second characteristic mentions that the element is very reactive and cannot be kept open in air. Alkali metals are highly reactive, especially with moisture in the air, and they readily oxidize. This means they must be stored in oil (like kerosene) to prevent them from reacting with air.

**Hint:** Consider the storage requirements of the element. If it reacts with air, it is likely an alkali metal.

### Step 4: Examine Reaction with Water
The third characteristic states that the element reacts vigorously with water. When alkali metals react with water, they produce hydroxides and hydrogen gas. For example, sodium reacts with water to form sodium hydroxide (NaOH) and hydrogen gas (H2), often resulting in an explosive reaction due to the heat released.

**Hint:** Recall the reactions of alkali metals with water. Which one is known for a vigorous reaction?

### Step 5: Identify the Element
Based on the analysis:
- Sodium (Na) fits all the criteria: it is soft, highly reactive, cannot be exposed to air, and reacts vigorously with water.

Thus, the element A is **Sodium (Na)**.

### Final Answer
The element is Sodium (Na).

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