Water Density and Properties

Overview

This lecture covers the concept of water density, its dependence on temperature and dissolved substances, differences between ice and liquid water, "heavy ice," and methods for measuring density.

What is Density?

• Density is defined as the mass (weight) per unit volume of a substance.
• The typical unit for water density is grams per milliliter (g/ml) or grams per cubic centimeter (g/cm³).
• The density of pure water is about 1 g/ml (1 g/cm³), but at 4°C (39.2°F) it is exactly 0.9998395 g/ml.

How Water Density Changes

• Water's density decreases as temperature increases.
• A gallon of water at 70°F weighs 8.329 pounds; at boiling (212°F), it weighs 7.996 pounds.
• Table data shows water density and weight per cubic foot at various temperatures; highest density occurs around 4°C.

Ice vs. Water: Why Does Ice Float?

• Ice is less dense than liquid water, so it floats; its density decreases by about 9% upon freezing.
• Ice's lower density results from its lattice structure, making molecules more spread out than in liquid.
• About 10% of an iceberg or ice cube is above the water line, while 90% is submerged.
• Water is densest at 4°C, allowing lakes to freeze from the top down, not the bottom up.

Effects of Dissolved Substances

• Water in nature often contains dissolved minerals, gases, salts, and other materials.
• Dissolved substances increase water's density; ocean water is denser than freshwater.

Heavy Water and "Heavy Ice"

• "Heavy ice" made from heavy water (D₂O) is denser than normal water and sinks in it.
• Heavy water contains deuterium instead of regular hydrogen atoms.

Measuring Water Density

• A hydrometer, a floating instrument with a marked scale, is used to measure liquid density.
• Hydrometers float higher in dense liquids and lower in less dense ones; water's standard mark is "1.000."
• Hydrometers are used in science, dairy, and brewing to assess properties like salinity, fat content, and sugar level.

Key Terms & Definitions

• Density — Mass per unit volume of a substance.
• Hydrometer — Device that measures the density of liquids by floating in them.
• Heavy Water (D₂O) — Water with deuterium atoms, heavier than normal H₂O.
• Specific Gravity — Ratio of the density of a substance to the density of water.

Action Items / Next Steps

• Review the table showing water density at different temperatures.
• Try making a simple hydrometer as a science project.
• Take the interactive water-properties quiz to test your knowledge.