Matter, Classification, and Measurement

What Is Chemistry?

Chemistry is the study of matter — what it is made of, how it behaves, and how it changes. Every object you can touch, every gas you breathe, every liquid you drink is matter. Even things you cannot see, like the nitrogen and oxygen molecules in the air around you, are matter. Chemistry gives us the tools to understand what is happening at the atomic and molecular level when ice melts, when iron rusts, or when a battery powers your phone.

Before we can study how matter changes, we need two foundational skills: how to classify matter and how to measure it precisely.

Classifying Matter

All matter can be sorted into a hierarchy based on composition:

Pure Substances

A pure substance has a fixed, definite composition. There are two kinds:

• Elements: Cannot be broken down into simpler substances by chemical means. Gold (Au), oxygen (O), and carbon (C) are elements. There are 118 known elements, organized on the periodic table.
• Compounds: Two or more elements chemically bonded in fixed ratios. Water (H₂O) is always two hydrogen atoms for every one oxygen atom. Table salt (NaCl) is always one sodium for every one chlorine.

Mixtures

A mixture contains two or more substances physically combined — not chemically bonded. The components retain their individual properties and can be separated by physical means.

• Homogeneous mixtures (solutions): Uniform composition throughout. Saltwater, air, and brass are homogeneous — you cannot see the individual components.
• Heterogeneous mixtures: Non-uniform composition. Trail mix, oil and vinegar dressing, and granite have visibly distinct parts.

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Properties of Matter

Chemists describe matter using two categories of properties:

Physical properties can be observed or measured without changing the substance's chemical identity: color, density, melting point, boiling point, solubility, and conductivity. Crushing ice into smaller pieces is a physical change — it is still H₂O.

Chemical properties describe a substance's ability to undergo chemical change and form new substances: flammability, reactivity with acids, tendency to corrode, and ability to decompose. When iron rusts, it combines with oxygen to form iron oxide (Fe₂O₃) — a completely different substance.

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SI Units: The Language of Measurement

Science requires a universal measurement system. The International System of Units (SI) provides seven base units from which all other units are derived.

The five SI base units most important in chemistry:

SI Prefixes

Scientists handle very large and very small numbers using prefixes:

A single human hair is about 80 μm (micrometers) wide. A water molecule is about 0.275 nm (nanometers) across. Chemistry frequently works at scales far smaller than what we can see.

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Significant Figures

Every measurement has some degree of uncertainty. When you read a thermometer that shows 22.3°C, you are certain about the 22 but estimating the .3. Significant figures (sig figs) communicate how precise a measurement actually is.

Rules for Counting Significant Figures

1. All nonzero digits are significant. 247 has 3 sig figs.
2. Zeros between nonzero digits are significant. 1.007 has 4 sig figs.
3. Leading zeros are NOT significant. 0.0042 has 2 sig figs (the 4 and the 2).
4. Trailing zeros after a decimal point ARE significant. 2.500 has 4 sig figs.
5. Trailing zeros in a whole number without a decimal point are ambiguous. 1500 might have 2, 3, or 4 sig figs — use scientific notation (1.50 × 10³) to clarify.

Sig Fig Rules in Calculations

• Multiplication and division: The answer has the same number of sig figs as the measurement with the fewest sig figs.
• Addition and subtraction: The answer has the same number of decimal places as the measurement with the fewest decimal places.

Example: 4.56 × 1.4 = 6.384, but 1.4 has only 2 sig figs, so the answer rounds to 6.4.

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Concept Check

How many significant figures are in each: 0.00340, 1.200 × 10⁴, and 5020? Then calculate 12.11 + 0.3 + 1.456 using correct sig fig rules.

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0.00340 has 3 sig figs (the 3, 4, and trailing 0). 1.200 × 10⁴ has 4 sig figs. 5020 is ambiguous — likely 3 sig figs. For addition: 12.11 + 0.3 + 1.456 = 13.866, but 0.3 has only one decimal place, so round to 13.9.

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Dimensional Analysis

Dimensional analysis (the factor-label method) is a problem-solving technique that uses conversion factors to cancel units, guiding you to the correct answer. If the units work out, the math is almost certainly correct.

A conversion factor is a fraction equal to 1, such as 100 cm / 1 m or 1 kg / 1000 g.

Worked Example: Single-Step Conversion

Convert 5.85 km to meters.

5.85 km × (1000 m / 1 km) = 5850 m = 5.85 × 10³ m

The "km" units cancel, leaving meters.

Worked Example: Multi-Step Conversion

Convert 72.0 km/h to m/s.

72.0 km/h × (1000 m / 1 km) × (1 h / 3600 s) = 72,000 / 3600 m/s = 20.0 m/s

Notice how the unwanted units (km, h) cancel step by step, leaving only m/s.

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Accuracy vs. Precision

These terms are often confused but mean different things:

• Accuracy is how close a measurement is to the true or accepted value.
• Precision is how close repeated measurements are to each other.

You can be precise without being accurate (consistently wrong) or accurate without being precise (scattered but centered on the right value). Good science demands both.

Percent error quantifies accuracy: % error = |experimental value − accepted value| / accepted value × 100%

If the accepted density of aluminum is 2.70 g/mL and a student measures 2.85 g/mL, the percent error is |2.85 − 2.70| / 2.70 × 100% = 5.56%.

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Summary

• Matter is classified as elements, compounds, or mixtures (homogeneous or heterogeneous)
• Physical properties can be observed without changing identity; chemical properties involve forming new substances
• The SI system provides standardized units; prefixes handle extreme scales
• Significant figures communicate measurement precision and govern how calculated results are reported
• Dimensional analysis uses conversion factors to systematically convert units and solve problems
• Accuracy measures closeness to truth; precision measures consistency of repeated measurements