Formalin is:

Options

A. Liquid formaldehyde
B. `40%` solution of formaldehyde in water
C. `40%` solution of formaldehyde in acetone
D. `40%` solution of formaldehyde in acetaldehyde

Answer

To determine what formalin is, we can break down the information provided in the question and the video transcript step by step.

### Step 1: Understanding Formalin
Formalin is commonly known as a solution that contains formaldehyde. It is important to identify the concentration and the solvent used in this solution.

### Step 2: Analyzing the Options
The options given are:
1. Liquid formaldehyde
2. 40% solution of formaldehyde in water
3. 40% solution of formaldehyde in stone
4. 40% solution of formaldehyde in acetaldehyde

### Step 3: Identifying the Correct Option
From the video transcript, it is stated that formalin is specifically a 40% solution of formaldehyde in water. This is the most widely accepted definition of formalin in the context of chemistry and its applications.

### Step 4: Confirming the Composition
The transcript mentions that formalin contains approximately 40% formaldehyde, 52% water, and 8% methanol. This further confirms that the primary solvent in formalin is water, and the concentration of formaldehyde is indeed around 40%.

### Step 5: Conclusion
Based on the analysis of the options and the information provided, the correct answer is:
**Formalin is a 40% solution of formaldehyde in water.**

### Final Answer
The correct option is **B: 40% solution of formaldehyde in water.**

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