Short Answer

Specific gravity is the ratio of the density of a substance to the density of water. It shows how heavy or light a substance is compared to water. Since it is a ratio, specific gravity has no unit. A substance with specific gravity more than 1 is heavier than water, while a value less than 1 means it is lighter.

For example, the specific gravity of ice is less than 1, so ice floats on water. Metals like iron have specific gravity greater than 1, which is why they sink. Specific gravity is useful for comparing materials easily without complex calculations.

Detailed Explanation :

Specific Gravity

Specific gravity is a simple but very important concept in science, especially in chemistry and physics. It tells us how heavy a substance is compared to water. More clearly, specific gravity is defined as the ratio of the density of a substance to the density of water. Since it is a comparison of two densities, it does not have any unit.

Water is used as the reference substance because its density is well-known and easy to measure. At 4°C, water has a density of 1 g/cm³, which makes calculations easier. Specific gravity helps in identifying substances, checking purity, and understanding whether objects will float or sink in water.

Because specific gravity is a ratio, it remains the same regardless of the size of the object. A small piece of iron and a large block of iron will have the same specific gravity, even though their masses are different.

Understanding Specific Gravity

1. Relation Between Density and Specific Gravity

Specific gravity depends directly on density.
The formula is:

Specific Gravity = Density of Substance ÷ Density of Water

If the density of a substance is:

• More than water, specific gravity > 1
• Equal to water, specific gravity = 1
• Less than water, specific gravity < 1

This simple relationship makes specific gravity very useful in comparing substances.

2. Why Specific Gravity Has No Unit

Density has units (g/cm³, kg/m³), but when we divide one density by another, the units cancel out.
This means specific gravity is just a number without units.

Because of this, specific gravity is also called relative density.

3. Floating and Sinking Based on Specific Gravity

Specific gravity explains why some objects float and others sink.

• If specific gravity < 1 → floats
  Example: Ice has a specific gravity of about 0.92, so it floats on water.
• If specific gravity > 1 → sinks
  Example: Iron has a specific gravity of around 7.8, so it sinks in water.
• If specific gravity = 1 → stays suspended
  Example: A substance with equal density will neither sink nor rise completely.

This principle is used in ship designing, swimming, and many scientific tests.

4. Specific Gravity and Different Substances

Different materials have different specific gravities due to variations in their particle arrangement and density.

Examples:

• Wood: < 1
• Oil: < 1
• Plastic: varies but usually < 1
• Sand: > 1
• Metals: mostly > 1

These differences play a role in separating mixtures, recycling materials, and identifying objects.

5. Why Water Is Used as Standard

Water is used to define specific gravity because:

• It is easily available
• Its density is constant and well-known
• It is stable at normal temperatures
• It makes calculations simple

Using water as the reference makes specific gravity easy to measure in laboratories and industries.

6. Uses of Specific Gravity

Specific gravity is widely used in science and daily life.

1. Identifying Substances

Many materials have their own specific gravity values. Scientists compare unknown samples with known values to identify them.

1. Testing Purity

Impure substances have different densities, so their specific gravity changes.
Example: Pure milk has higher specific gravity than water, and adding water lowers its value.

1. Construction and Engineering

Engineers check the specific gravity of sand, cement, and stones before using them in construction.

1. Medical Uses

Urine samples are tested for specific gravity to understand hydration levels and kidney function.

1. Industries

Specific gravity is used in:

• Gold and mineral separation
• Petroleum industry
• Chemical production
• Battery acid testing

1. Everyday Uses

• Oil floats on water due to lower specific gravity
• Icebergs float in oceans
• Wooden logs float in rivers

7. Specific Gravity and Temperature

The specific gravity of a substance can change slightly with temperature because density changes when temperature changes. Liquids and gases expand or contract with temperature, affecting their density.
Since water density also changes slightly with temperature, specific gravity measurements are sometimes corrected based on temperature.

Conclusion

Specific gravity is the ratio of the density of a substance to the density of water. It tells us whether a substance is heavier or lighter than water and helps predict if something will float or sink. Because it has no unit and is easy to measure, specific gravity is widely used in science, engineering, medical tests, and daily life. Understanding this concept helps compare materials and understand their behavior in different situations.