Which of the following changes increases the over all density of the object?

Options

A. A block of iron is formed into a hollow shape.
B. A submarine fills its ballast tanks with water.
C. A submarine fills its ballast tanks with air.
D. A fish increases the amount of gas in its swim bladder.

Explanation

To determine which change increases the overall density of an object, we need to understand the relationship between mass, volume, and density. Density is defined as the mass of an object divided by its volume (Density = Mass/Volume).

Let's analyze the options provided in the question step by step:

### Step 1: Understand Density
- Density is calculated using the formula:
\[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \]
- To increase the overall density of an object, we can either increase the mass while keeping the volume constant or decrease the volume while keeping the mass constant.

### Step 2: Analyze Option 1
- **Option 1**: A block of iron is formed into a hollow sphere.
- In this case, the mass of the iron remains the same, but the volume increases because a hollow sphere has a larger volume than a solid block of iron.
- Since density is mass divided by volume, increasing the volume while keeping mass constant will decrease the overall density.

**Conclusion for Option 1**: This change does not increase density.

### Step 3: Analyze Option 2
- **Option 2**: A submarine fills its blast tank with water.
- Initially, the blast tank is filled with air (which has a low density). When water (which has a higher density) is added, the overall density of the submarine increases.
- This is because we are replacing a low-density substance (air) with a high-density substance (water).

**Conclusion for Option 2**: This change increases density.

### Step 4: Analyze Option 3
- **Option 3**: A submarine fills its blast tank with air.
- If the submarine fills its blast tank with air, it is not changing the substance inside the tank. The density remains the same as it is still filled with air.

**Conclusion for Option 3**: This change does not increase density.

### Step 5: Analyze Option 4
- **Option 4**: A fish increases the amount of gas in its swimming bladder.
- Increasing the amount of gas in the swimming bladder increases the volume of the bladder but does not change the mass of the fish. Since density is mass divided by volume, increasing the volume while keeping the mass constant will decrease the overall density.

**Conclusion for Option 4**: This change does not increase density.

### Final Conclusion
The only option that increases the overall density of the object is **Option 2**: A submarine fills its blast tank with water.

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