Specific Gravity: Why Some Things Float and Others Sink

What Is Specific Gravity?

Have you ever wondered why a massive wooden log floats effortlessly on water while a tiny pebble sinks immediately to the bottom? The answer lies in a fundamental property💡 Definition:An asset is anything of value owned by an individual or entity, crucial for building wealth and financial security. called specific gravity (SG), one of the most useful measurements in science and industry.

Specific gravity is the ratio of a substance's density to the density of a reference substance—almost always water for liquids and solids. Unlike density, which requires units like grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³), specific gravity is a dimensionless number. This makes it incredibly practical for quick comparisons and calculations.

The Water Baseline: 1.0

Water at 4°C (39.2°F) has a density of exactly 1.0 g/cm³, which is why it serves as the universal reference point. This temperature is significant because water reaches its maximum density at 4°C. At this point:

• Pure water has a specific gravity of exactly 1.0
• Any substance with SG < 1.0 will float in water
• Any substance with SG > 1.0 will sink in water

This simple rule💡 Definition:Regulation ensures fair practices in finance, protecting consumers and maintaining market stability. explains countless everyday phenomena, from ice cubes floating in your drink to oil slicks remaining on the ocean surface.

Things That Float: Specific Gravity Below 1.0

Materials with a specific gravity less than 1.0 are less dense than water and will float. Here are some common examples:

Wood (SG: 0.3–0.9)

Most woods float because they contain air pockets within their cellular structure. However, specific gravity varies significantly between species:

• Balsa wood: 0.12–0.20 (extremely light, used in model aircraft)
• Pine: 0.35–0.60 (common construction lumber)
• Oak: 0.60–0.90 (hardwood, barely floats)
• Ebony: 1.1–1.3 (actually sinks—one of few woods that does)

This explains why some antique furniture woods sink while most construction lumber floats.

Oils and Fats (SG: 0.8–0.95)

Oils are famously immiscible with water and float on its surface:

• Olive oil: 0.91–0.92
• Vegetable oil: 0.91–0.93
• Gasoline: 0.71–0.77
• Crude oil: 0.87–0.95

This property is crucial for oil spill containment—booms and skimmers take advantage of oil's tendency to float.

Ice (SG: 0.917)

Perhaps the most important floating substance on Earth, ice has a specific gravity of about 0.917. This anomaly—water expanding when it freezes—is essential for life on our planet. If ice sank:

• Lakes and oceans would freeze from the bottom up
• Aquatic life couldn't survive winters
• Earth's climate would be dramatically different

The roughly 9% of an iceberg visible above water (the proverbial "💡 Definition:A voluntary payment given to service workers in addition to the bill amount, typically based on quality of service.tip💡 Definition:A voluntary payment to service workers, typically a percentage of the bill, given as thanks for good service. of the iceberg") directly reflects this specific gravity ratio.

Other Common Floaters

• Cork: 0.12–0.24 (why it works for wine bottles and fishing floats)
• Human body: ~0.95–1.05 (we float in salt water more easily than fresh)
• Apples: ~0.80 (hence bobbing for apples works)
• Most plastics: 0.90–0.97 (explaining plastic pollution floating in oceans)

Things That Sink: Specific Gravity Above 1.0

Materials denser than water have specific gravities greater than 1.0 and sink:

Metals

Metals are typically much denser than water:

• Aluminum: 2.70
• Iron/Steel: 7.85
• Copper: 8.96
• Lead: 11.35
• Gold: 19.32
• Platinum: 21.45

Yet steel ships float! This is because ship design creates hollow, air-filled structures with an overall specific gravity less than 1.0.

Rocks and Minerals

Most geological materials sink:

• Granite: 2.65–2.75
• Limestone: 2.30–2.70
• Quartz: 2.65
• Diamond: 3.51

However, pumice (volcanic rock with many air pockets) has an SG of 0.25–0.90 and floats—sometimes forming massive rafts after volcanic eruptions.

Glass and Concrete

• Glass: 2.40–2.80
• Concrete: 2.30–2.50

Practical Applications of Specific Gravity

Battery Testing

Car batteries contain sulfuric acid solutions whose specific gravity indicates charge level:

• Fully charged: SG 1.265–1.299
• 50% charged: SG 1.190–1.210
• Discharged: SG 1.120–1.140

Mechanics use hydrometers to test battery health quickly and non-destructively.

Brewing and Winemaking

Brewers rely heavily on specific gravity measurements:

• Original gravity (OG): The SG of wort before fermentation (typically 1.040–1.090)
• Final gravity (FG): The SG after fermentation (typically 1.006–1.020)
• Alcohol content: Calculated from the difference between OG and FG

A refractometer or hydrometer reading tells brewers exactly when fermentation is complete.

Gemology

Specific gravity helps identify gemstones without damaging them:

• Amber: 1.05–1.10 (distinguishes real from plastic imitations)
• Pearl: 2.60–2.85
• Ruby: 3.97–4.05
• Sapphire: 3.95–4.03
• Diamond: 3.50–3.53
• Cubic zirconia: 5.50–5.95 (much denser than diamond)

Gemologists use this property to authenticate gems and detect simulants.

Urine Analysis

Medical professionals use urine specific gravity (USG) to assess hydration and kidney function:

• Normal range: 1.005–1.030
• Dilute (overhydrated): < 1.005
• Concentrated (dehydrated): > 1.030

Industrial Quality Control

Many industries use specific gravity to verify product quality:

• Petroleum: Confirms fuel grades and detects contamination
• Dairy: Measures milk fat content
• Sugar production: Monitors syrup concentration
• Chemical manufacturing: Verifies solution concentrations

How to Measure Specific Gravity: The Hydrometer

A hydrometer is the classic instrument for measuring specific gravity of liquids. It works on Archimedes' principle—a floating object displaces its own weight in fluid.

Using a Hydrometer

1. Fill a tall, clear cylinder with your liquid sample
2. Lower the hydrometer gently into the liquid
3. Let it settle and stop bobbing
4. Read the scale at the liquid's surface (at eye level, reading the bottom of the meniscus)

Types of Hydrometers

Different applications require different scales:

• Brewing hydrometers: Scaled 0.990–1.170
• Battery hydrometers: Scaled 1.100–1.300
• Antifreeze testers: Indicate freeze protection temperature
• Brix hydrometers: Measure sugar content (used in winemaking)

Digital Alternatives

Modern digital density meters offer higher accuracy and require smaller sample sizes. They work by measuring the oscillation frequency of a vibrating U-tube containing the sample.

Specific Gravity vs. Density: Understanding the Difference

While often used interchangeably in casual conversation, these terms have distinct meanings:

Converting Between Them

For water-referenced specific gravity:

• Specific gravity = Density (g/cm³) ÷ 1.0 g/cm³
• At other reference conditions, adjustments apply

Since water's density is 1.0 g/cm³, specific gravity numerically equals density in g/cm³ under standard conditions.

Common Materials: Specific Gravity Reference Table

Here is a quick reference for everyday materials:

Why Specific Gravity Matters in Everyday Life

Understanding specific gravity explains numerous everyday observations:

• Why oil and vinegar separate: Different specific gravities cause layering
• Why cream rises to the top of milk: Lower SG fat floats above higher SG water
• Why icebergs are dangerous to ships: Only 10% is visible above water
• Why the Dead Sea makes floating easy: High salt content gives SG of ~1.24
• Why helium balloons rise: Helium's SG relative to air is very low

Conclusion

Specific gravity is a deceptively simple concept with profound implications across science, industry, and daily life. By expressing density as a ratio to water, this dimensionless number provides an intuitive way to predict whether substances will float or sink, verify product quality, authenticate gemstones, and monitor countless industrial processes.

Whether you are a brewer tracking fermentation, a gemologist authenticating stones, or simply curious about why some things float while others sink, understanding specific gravity opens up a deeper appreciation💡 Definition:The increase in an asset's value over time, whether it's real estate, stocks, or other investments. of the physical world around us.

Ready to explore density conversions? Try our Specific Gravity to kg/m³ Converter or browse our Density Unit Converter for quick calculations.