Specific gravity is a key property used to compare the density of a substance to the density of a reference substance, usually water. It’s an important concept in fields such as material science, fluid mechanics, geology, and chemistry. The Specific Gravity (SG) is a dimensionless number that indicates how dense a substance is relative to water, which has a specific gravity of 1 at 4°C (its maximum density).

In this guide, we’ll explain specific gravity, how to calculate it, and provide a Specific Gravity Calculator to make these calculations easier.

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What is Specific Gravity?

Specific gravity (SG) is the ratio of the density of a substance to the density of a reference substance, typically water (at 4°C), which has a density of 1000 kg/m³ or 1 g/cm³.

Mathematically, the formula for specific gravity is:

SG = (Density of the substance) / (Density of water)

Since water’s density is approximately 1 g/cm³ at 4°C, specific gravity can also be simplified to:

SG = (Density of the substance) / 1 g/cm³

Because specific gravity is a ratio, it has no units. If the SG of a substance is greater than 1, it means the substance is denser than water and will sink. If the SG is less than 1, the substance is less dense than water and will float.

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Formula for Specific Gravity

The general formula for specific gravity (SG) is:

SG = (Density of the substance) / (Density of water)

Where:

• Density of the substance: The mass per unit volume of the substance (in kg/m³ or g/cm³).
• Density of water: The mass per unit volume of water at 4°C, typically 1 g/cm³ (or 1000 kg/m³).

If you have the mass and volume of the substance, you can also calculate the density as:

Density = Mass / Volume

Then, use the density value to calculate the specific gravity.

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How to Calculate Specific Gravity

Step 1: Measure or obtain the density of the substance (in g/cm³ or kg/m³).

• If you have the mass and volume, calculate the density: Density = Mass / Volume.

Step 2: Use the density of water as the reference.

• For most common cases, use 1 g/cm³ for water at 4°C.

Step 3: Apply the formula.

• Use the specific gravity formula:
  SG = (Density of the substance) / (Density of water).

Example Calculation 1: Specific Gravity of Gold

Suppose you have a piece of gold with a mass of 500 grams and a volume of 25 cm³. You want to calculate its specific gravity.

1. Density of Gold:
   • Density = Mass / Volume
   • Density of Gold = 500 g / 25 cm³ = 20 g/cm³.
2. Density of water at 4°C is 1 g/cm³.
3. Specific Gravity (SG):
   • SG = 20 g/cm³ / 1 g/cm³ = 20.

So, the specific gravity of gold is 20.

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Example Calculation 2: Specific Gravity of Air

Now, let’s calculate the specific gravity of air, knowing that the density of air is approximately 1.225 kg/m³ at 0°C.

1. Density of Air = 1.225 kg/m³.
2. Density of water = 1000 kg/m³ (at 4°C).
3. Specific Gravity (SG):
   • SG = 1.225 kg/m³ / 1000 kg/m³ = 0.001225.

So, the specific gravity of air is 0.001225, which means air is much less dense than water and will float on it.

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Specific Gravity Calculator

Now that you understand the formula, let’s make it easier to calculate the specific gravity using the calculator below. You just need to input the density of the substance and the density of water (if not using the standard value of 1 g/cm³ or 1000 kg/m³).

Input Values:

• Density of the substance (in g/cm³ or kg/m³).
• Density of water (in g/cm³ or kg/m³).

Example:

Parameter	Value	Unit
Density of Substance	20	g/cm³
Density of Water	1	g/cm³
Specific Gravity (SG)	20	–

You can use this same method to calculate SG for any material, such as metals, liquids, or gases.

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Practical Applications of Specific Gravity

1. Mining and Metallurgy: Specific gravity is used to identify minerals and metals. For instance, gold has a very high specific gravity, making it easy to separate from other materials using gravity-based methods like panning.
2. Hydrometers: A hydrometer uses specific gravity to measure the density of liquids, which is helpful in industries like brewing, winemaking, and chemical processing.
3. Buoyancy: Specific gravity is essential for understanding why objects float or sink in a fluid. Objects with a specific gravity less than 1 will float in water, while those with a specific gravity greater than 1 will sink.
4. Material Identification: Specific gravity can be used to identify substances, especially when dealing with liquids and solids. For example, oils and fuels have specific gravities less than 1, while metals and rocks have specific gravities greater than 1.
5. Engineering and Design: In material selection for engineering projects, specific gravity helps engineers choose appropriate materials based on their density and weight relative to water.

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Frequently Asked Questions (FAQs)

1. Why is specific gravity important?

Specific gravity helps in determining whether a substance will float or sink in water, aids in material selection, and plays a crucial role in processes like mining, fluid mechanics, and chemical engineering.

2. What is the specific gravity of water?

The specific gravity of water is 1 at 4°C, as its density is considered the reference value. At other temperatures, the density of water may change slightly, but its specific gravity remains close to 1.

3. Can specific gravity be negative?

No, specific gravity cannot be negative because density is always a positive quantity, and the specific gravity is a ratio of densities. A negative value would imply that the substance has negative density, which is not possible.

4. How do I measure the density of a substance?

To calculate the density, divide the mass of the substance by its volume. If you have the mass and volume in known units (e.g., grams and cubic centimeters), you can directly compute the density in g/cm³.

5. What substances have high and low specific gravity?

• High Specific Gravity: Metals like gold (SG ≈ 19.3), lead (SG ≈ 11.3), and platinum (SG ≈ 21.4) have high specific gravity, meaning they are much denser than water.
• Low Specific Gravity: Substances like air (SG ≈ 0.0012), helium (SG ≈ 0.00018), and light oils have low specific gravity, meaning they are less dense than water.

6. Can specific gravity change with temperature?

Yes, specific gravity can change with temperature because the density of the substance and the density of water vary with temperature. However, for most practical purposes, water’s density is considered constant at 1 g/cm³ at 4°C.

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Conclusion

Specific gravity is a powerful tool for comparing the densities of substances, identifying materials, and understanding buoyancy in various fluids. Whether you’re a student, scientist, engineer, or professional working in mining, chemistry, or material science, knowing how to calculate and apply specific gravity is crucial for solving many practical problems.

By using the Specific Gravity Calculator and understanding the formula, you can easily calculate specific gravity for different substances, helping you make informed decisions in various applications from material selection to fluid mechanics.