Understanding Relative Density

The question asks about the definition of relative density. Relative density is a fundamental property used to compare how dense a substance is compared to a reference substance. The most common reference substance is water.

Let's analyze the given options:

• Option 1: Ratio of its density to the density of water. This means dividing the density of the substance by the density of water. This aligns with the standard definition of relative density.
• Option 2: Ratio of the density of water to its density. This is the inverse of the correct definition.
• Option 3: Product of the density of water to its density. This operation does not define relative density.
• Option 4: Ratio of the volume of water to its volume. This relates to volumes, not densities.

Based on the definition, relative density is indeed the ratio of the density of a substance to the density of a reference substance, which is typically water, especially for liquids and solids.

Definition of Relative Density

Relative density, also known as specific gravity, is a dimensionless quantity. It is defined mathematically as:

\(\text{Relative Density} = \frac{\text{Density of Substance}}{\text{Density of Reference Substance}}\)

For solids and liquids, the reference substance is usually water at a specific temperature (commonly \(4^\circ\text{C}\) because water has its maximum density at this temperature). The density of water at \(4^\circ\text{C}\) is approximately \(1000 \text{ kg/m}^3\) or \(1 \text{ g/cm}^3\).

So, the relative density of a substance with respect to water is:

\(\text{Relative Density} = \frac{\text{Density of Substance}}{\text{Density of Water}}\)

Why Option 1 is Correct

Option 1 states that relative density is the "Ratio of its density to the density of water". This perfectly matches the mathematical definition provided above. It correctly identifies the substance's density as the numerator and the density of water as the denominator in the ratio.

Why Other Options are Incorrect

• Option 2 defines the inverse ratio. This would be \( \frac{\text{Density of Water}}{\text{Density of Substance}} \), which is not the definition of relative density.
• Option 3 suggests multiplying the densities. This product does not give the relative density.
• Option 4 compares volumes. While related to density (density = mass/volume), the ratio of volumes of water to the substance for the same mass would give the inverse of the relative density, and a simple volume ratio without considering mass or density is not relative density.

Therefore, the correct definition of relative density is the ratio of the density of the substance to the density of water.

Revision Table: Key Concepts in Relative Density

Additional Information on Relative Density and Specific Gravity

Relative density is often used interchangeably with specific gravity, especially in fields like chemistry and engineering. Both terms refer to the ratio of a substance's density to a reference density. However, sometimes specific gravity might imply a comparison to water at \(4^\circ\text{C}\), while relative density can be used with any specified reference substance and temperature.

Since relative density is a ratio of two densities, its value is a pure number with no units. For example, if a substance has a density of \(2000 \text{ kg/m}^3\) and water has a density of \(1000 \text{ kg/m}^3\), its relative density is \(2000/1000 = 2\).

Understanding relative density helps us determine if an object will sink or float in a liquid. If the relative density of the object is greater than the relative density of the liquid, it will sink. If it's less, it will float. If it's equal, it will remain suspended.