The specific gravity of pure water is 1.00000 at 20°C. While the term "water solution" can imply water with dissolved substances, the specific gravity typically refers to pure water as the benchmark, especially when its properties as a solvent are considered. The density of water, and thus its specific gravity, is influenced by temperature.
Understanding Specific Gravity
Specific gravity is a dimensionless quantity that represents the ratio of the density of a substance to the density of a reference substance. For liquids and solids, the reference substance is typically water at a specified temperature (often 4°C, where its density is nearly 1 g/mL, or 20°C as provided in the reference).
Formula for Specific Gravity (SG):
SG = (Density of Substance) / (Density of Reference Substance)
Since the density of water at 4°C is approximately 1 g/mL (or 1000 kg/m³), the specific gravity of a substance is numerically very close to its density in g/mL.
Specific Gravity of Pure Water
As per the provided reference, the specific gravity of pure water is 1.00000 at 20°C. This value serves as a crucial reference point in many scientific and industrial applications.
- Temperature Dependency: It's vital to note that the density of water changes with temperature.
- From 0°C to 4°C, the density of water increases, reaching its maximum density at approximately 4°C (nearly 1 g/mL or 1000 kg/m³).
- From 4°C to higher temperatures (like 20°C and beyond), the density decreases. This is why the specific gravity is explicitly stated for 20°C.
- Reference Standard: Due to its consistent and well-understood properties, water is the standard against which the densities of most liquids and solids are compared to determine their specific gravity.
Specific Gravity of "Water Solutions" (with Solutes)
When the term "water solution" refers to water with dissolved substances (solutes), the specific gravity will differ from that of pure water. The specific gravity of a solution depends on:
- Nature of the Solute: Different substances have different molecular weights and packing characteristics.
- Concentration of the Solute: Generally, the higher the concentration of a dissolved substance, the higher the density and thus the specific gravity of the solution.
Examples of How Solutes Affect Specific Gravity:
- Saltwater (Brine): Dissolving salt (e.g., sodium chloride) in water increases the solution's density, making its specific gravity greater than 1.000. This is why objects float more easily in saltwater.
- Sugar Solution: Similarly, dissolving sugar in water will increase its specific gravity.
- Alcohol Solution: If alcohol (e.g., ethanol) is dissolved in water, the specific gravity of the solution might be less than, equal to, or greater than pure water, depending on the concentration and the specific alcohol, as ethanol itself has a specific gravity less than 1.0.
Table: Illustrative Specific Gravity Values (Approximate)
| Substance/Solution Type | Specific Gravity (Approximate at 20°C) | Notes |
|---|---|---|
| Pure Water | 1.00000 | The standard reference. |
| Saltwater (3.5% NaCl) | 1.025 – 1.030 | Typical specific gravity of ocean water, which is a solution of various salts in water. |
| Sugar Solution (10%) | ~1.040 | A 10% by weight solution of sugar. Increases with higher sugar concentration. |
| Ethanol (95% in water) | ~0.800 – 0.900 | Pure ethanol has a specific gravity of about 0.789. Solutions will have specific gravities between ethanol and water, depending on the ratio. |
| Antifreeze (Glycol) | ~1.050 – 1.150 | Common antifreeze solutions (e.g., ethylene glycol/propylene glycol in water) are denser than water. |
| Urine | 1.003 – 1.030 | A biological water solution containing various dissolved substances. Varies based on hydration and kidney function. |
In summary, while the specific gravity of pure water is 1.00000 at 20°C, the specific gravity of a "water solution" with dissolved substances will vary based on the nature and concentration of those substances.
