Difference Between Density and Specific Gravity with Examples
Specific gravity is an important concept in Physics that compares how dense a substance is relative to a reference material. For most solids and liquids, this reference is water at 4°C. For gases, the standard is dry air at standard conditions. Understanding specific gravity is useful in various Physics and real-world applications, especially when analyzing fluids and materials.
What is Specific Gravity?
Specific gravity is defined as the ratio of the density of a substance to the density of a reference substance. For solids and liquids, water at 4°C (with a density of 1.0 kg per liter) serves as the standard. For gases, dry air (density 1.29 grams per liter) is typically used. This means that specific gravity allows you to compare how heavy or light a material is compared to water or air, without worrying about the exact units.
Formula for Specific Gravity
The formula to calculate specific gravity is simple and straightforward. Let ρsubstance represent the density of the substance, and ρreference represent the density of the reference material (water for liquids/solids, or air for gases).
| Formula | Description | Unit |
|---|---|---|
| Specific Gravity (SG) = ρsubstance / ρreference | Ratio of the substance's density to that of the reference | Dimensionless (No unit) |
Since both densities are measured in the same units, their ratio has no units. Specific gravity is thus a dimensionless number, often simply written as "SG".
How to Calculate Specific Gravity: Step-by-Step Approach
- Find the density of your substance (in kg/L, g/cm³, etc.).
- Identify the reference density. For most cases, water at 4°C (1.0 kg/L) is used.
- Divide the density of the substance by the density of the reference.
- Interpret the result: SG > 1 means the substance is denser than the reference; SG < 1 means it is less dense.
| Step | What to Do |
|---|---|
| 1 | Measure the density of the substance. |
| 2 | Select the reference density (e.g., water at 4°C). |
| 3 | Apply the SG formula: SG = ρsubstance / ρreference. |
| 4 | Analyze the result. |
Example Calculation
Suppose a liquid has a density of 0.8 kg/liter. The reference density (water at 4°C) is 1.0 kg/liter.
- Specific Gravity = 0.8 kg/liter ÷ 1.0 kg/liter = 0.8
A specific gravity of 0.8 means the liquid is less dense than water and would float if poured into water.
Comparing Solids, Liquids, and Gases
For solids and liquids, specific gravity tells us whether an object will float or sink in water. Substances with specific gravity less than 1 float; those with specific gravity greater than 1 sink. For gases, the reference point is air. For example, a gas with a specific gravity less than 1 compared to air will rise.
Key Applications of Specific Gravity
- Quality checking in industries (e.g., determining if a liquid is pure).
- Geology and mineral identification by comparing minerals’ densities to water.
- Checking if materials will float or sink in fluids.
- Medical analysis, such as determining whether substances like urine are concentrated or diluted.
Important Data Table: Water and Air as Reference Standards
| Reference Substance | Condition | Density |
|---|---|---|
| Water | 4°C | 1.0 kg/L |
| Dry Air | Standard conditions | 1.29 g/L |
Using Vedantu Resources for Practice
To deepen your understanding, you can explore more about relative density on Vedantu. Practice questions and explanations on these topics will help reinforce your concepts and problem-solving abilities.
Next Steps for Mastery
- Review worked examples on the difference between density and specific gravity.
- Learn how the density of water changes with temperature for more accurate calculations.
- Attempt practice problems and daily quizzes to test your concept clarity.
Understanding specific gravity helps in predicting whether substances will float, identifying materials, and verifying purity in labs and industries. For more in-depth learning and stepwise guides, continue exploring Vedantu’s comprehensive Physics resources.
FAQs on Specific Gravity in Physics: Meaning, Formula, Units & Uses
1. What is specific gravity?
Specific gravity is the ratio of the density of a substance to the density of water at 4°C. It is a dimensionless quantity, often used to compare how heavy or light a substance is relative to water. This concept is fundamental in Physics, Chemistry, and Medicine, and is also called relative density.
2. What is the formula for specific gravity?
The specific gravity (SG) of a substance is calculated using the formula:
SG = (Density of substance) / (Density of water at 4°C)
Since it's a ratio, specific gravity has no units and is dimensionless.
3. How is specific gravity different from density?
Density is the mass per unit volume of a substance and has units (kg/m³), while specific gravity is a unitless ratio of the density of a substance to that of water. Specific gravity is a relative measure, while density is an absolute physical property.
4. What is the unit of specific gravity?
Specific gravity has no unit (it is dimensionless) because it is a ratio of two densities having identical units which cancel out.
5. What is the specific gravity of water at 4°C?
The specific gravity of pure water at 4°C is exactly 1, because it is compared to itself.
6. Why is specific gravity important in Physics and daily life?
Specific gravity is important because:
- It helps identify substances quickly by comparing their densities to water.
- It is critical in fluid mechanics, engineering, geology, and medical fields.
- Used for quality analysis of liquids like oil, urine, and battery acid.
7. How to find specific gravity using mass and volume?
To find specific gravity using mass and volume:
- Calculate the density of the substance as mass/volume.
- Divide this value by the density of water at 4°C (1000 kg/m³).
- SG = (Mass/Volume) / 1000 kg/m³
8. What does a specific gravity of 1.030 mean in urine analysis?
A urine specific gravity of 1.030 means the urine is more concentrated than water, which may indicate dehydration or a high solute presence. The normal range for urine specific gravity is typically 1.005–1.030.
9. What are common values for specific gravity of oil and other substances?
- Oil: Around 0.8–0.9 (less than water, so it floats)
- Mercury: About 13.6 (much denser than water)
- Iron: Approximately 7.8
- Alcohol: Around 0.79
10. How can you interpret if a substance sinks or floats using specific gravity?
- If specific gravity > 1: The substance is denser than water and will generally sink.
- If specific gravity < 1: The substance is less dense than water and will generally float.
11. Is specific gravity the same as relative density?
Yes, specific gravity and relative density mean the same thing—both are the ratio of the density of a substance to the density of water (for solids and liquids).
12. How is specific gravity measured in laboratories?
- Using devices like a hydrometer or pycnometer for liquids.
- By measuring the mass and volume of a known sample, then applying the specific gravity formula.
