Properties of Matter

Two Kinds of Properties

In Lesson 1, you learned that all matter is made of atoms. But how do scientists tell one type of matter from another? They measure physical properties, which are characteristics of a substance that you can observe or measure without changing the substance into something else. When you measure the mass of a rock, you still have the same rock afterward. When you measure the boiling point of water, the water is still water. No new substance is created.

Physical properties fall into two important categories: intensive properties and extensive properties. Understanding the difference between these two categories is one of the most important ideas in this lesson.

Intensive Properties: The Substance's Fingerprint

Intensive properties are physical properties that do not depend on how much of the substance you have. They stay the same whether you have a tiny sample or a huge amount.

Think about pure water. A single drop of pure water and an entire ocean of pure water share the same density (1.0 g/mL), the same boiling point (100°C), the same freezing point (0°C), and the same color (colorless). The amount of water is wildly different, but these properties are identical. That is what makes them intensive: they are built into the substance itself.

Common intensive properties include:

- Density (how much mass per unit of volume)
• Melting point (the temperature at which a solid becomes a liquid)
• Boiling point (the temperature at which a liquid becomes a gas)
• Solubility (how much of a substance dissolves in a given amount of solvent)
• Color, luster (shininess), and hardness

Because intensive properties do not change with amount, scientists use them like a fingerprint to identify what a substance is. If you find a shiny, yellow metal with a density of 19.3 g/cm³ and a melting point of 1,064°C, you can be confident it is gold, no matter how large or small the sample.

Extensive Properties: Depends on How Much You Have

Extensive properties are physical properties that do change depending on the amount of substance present.

- Mass: A gallon of water has more mass than a cup of water.
• Volume: A swimming pool of water takes up more space than a bathtub of water.
• Weight: A kilogram of iron weighs more than a gram of iron.
• Length: A longer piece of rope has a greater length.

Extensive properties tell you about the amount of matter, but they cannot tell you what the matter is. Knowing that something has a mass of 50 grams does not help you figure out whether it is water, iron, or sugar. You need intensive properties for identification.

Side-by-Side Comparison

Density: The Key Identifier

Density is one of the most useful intensive properties in science. Density describes how much mass is packed into a given volume. A substance with high density has a lot of mass squeezed into a small space. A substance with low density has less mass spread over the same space.

The Density Formula

Density is calculated using a simple formula:

$$d = \frac{m}{V}$$

where:
• d = density
• m = mass (usually measured in grams, g)
• V = volume (usually measured in cubic centimeters, cm³, or milliliters, mL)

The units for density are typically g/cm³ for solids or g/mL for liquids. An important fact: 1 cm³ is exactly equal to 1 mL, so these units are interchangeable.

Worked Example

A student finds a rock with a mass of 50 g and measures its volume to be 20 cm³. What is the rock's density?

Step 1: Write the formula: d = m/V

Step 2: Plug in the values: d = 50 g / 20 cm³

Step 3: Calculate: d = 2.5 g/cm³

The rock has a density of 2.5 g/cm³. The student could then compare this value to a table of known densities to help identify the type of rock.

Density and Floating

Density explains why some objects float and others sink. The rule is straightforward:

- If an object's density is less than the density of the fluid it is placed in, it floats.
• If an object's density is greater than the fluid's density, it sinks.

Water has a density of 1.0 g/mL. Any object with a density below 1.0 g/mL will float in water. Any object with a density above 1.0 g/mL will sink.

Why Does Ice Float?

Here is one of nature's most important quirks: ice is less dense than liquid water. Most solids are denser than their liquid form, but water is an exception. Ice has a density of about 0.92 g/cm³, while liquid water is 1.0 g/mL. Because 0.92 is less than 1.0, ice floats.

This is not just a fun fact; it is essential for life on Earth. When lakes and ponds freeze in winter, the ice forms a floating layer on top, insulating the liquid water below and allowing fish and other aquatic life to survive. If ice sank, bodies of water would freeze from the bottom up, killing most aquatic organisms.

Common Densities

Melting Point, Boiling Point, and Solubility

Density is not the only intensive property scientists use to identify substances. Three other intensive properties are extremely useful: melting point, boiling point, and solubility.

Melting Point

The melting point of a substance is the temperature at which it changes from a solid to a liquid. For example, ice (solid water) melts at 0°C (32°F). At that exact temperature, the solid begins to break down into liquid form.

Every pure substance has its own specific melting point. Iron melts at 1,538°C, which is why iron stays solid in everyday life but can be melted in a very hot furnace. Gold melts at 1,064°C. These values never change for a given substance, no matter how much of it you have, which is exactly what makes them intensive properties.

Boiling Point

The boiling point is the temperature at which a substance changes from a liquid to a gas. Water boils at 100°C (212°F) at standard atmospheric pressure. When water reaches this temperature, it rapidly turns into water vapor (steam).

Like melting point, every pure substance has a unique boiling point. Ethanol (the alcohol found in beverages) boils at 78°C, well below water's boiling point. Mercury, a liquid metal, boils at 357°C. These differences help scientists tell substances apart.

Data Table: Melting and Boiling Points of Common Substances

Solubility

Solubility is a measure of how much of one substance (the solute) can dissolve in a given amount of another substance (the solvent) at a specific temperature.

For example, sugar is highly soluble in water: you can dissolve a large amount of sugar in a glass of water before it stops dissolving. Sand, on the other hand, is essentially insoluble in water: no matter how much you stir, sand will not dissolve.

Temperature affects solubility. In general, more solid solute dissolves in a hot solvent than in a cold one. That is why it is easier to dissolve sugar in hot tea than in iced tea. This relationship is another important property that helps identify substances.

The Big Picture

Density, melting point, boiling point, and solubility are all intensive properties. They do not change based on how much of the substance you have, and they serve as reliable identifiers. If you know a substance's density is 19.3 g/cm³, its melting point is 1,064°C, and its boiling point is 2,856°C, you can say with confidence: that substance is gold.

In contrast, knowing a substance's mass is 500 g or its volume is 200 mL tells you how much you have, but not what it is. Those are extensive properties: useful for measurement, but not for identification.