1.2 Phases and Classification of Matter | Chemistry: Atoms First 2e

Learning Objectives

Describe the basic properties of each physical state of matter: solid, liquid, and gas
Distinguish between mass and weight
Apply the law of conservation of matter
Classify matter as an element, compound, homogeneous mixture, or heterogeneous mixture with regard to its physical state and composition
Define and give examples of atoms and molecules

Core Concepts & Principles (De-Verbosed)

States of Matter

Matter exists in four primary physical states, characterized by how their shape and volume respond to external conditions:

Solid: Rigid with a definite shape and volume.
Liquid: Flows and takes the shape of its container, forming a distinct upper surface, with a nearly constant volume.
Gas: Takes both the shape and volume of its container, expanding or compressing to fill it completely.
Plasma: A high-temperature gaseous state containing electrically charged particles, naturally occurring in stars and lightning.

Core Mental Model: States of Matter

Solids maintain fixed shape and volume.
Liquids maintain fixed volume but adapt their shape.
Gases adapt both shape and volume to completely fill their container.
Plasma is an ionized gas with unique electrical conductivity properties.

Mass vs. Weight

While often confused in everyday language, mass and weight represent distinct physical concepts:

Mass is the absolute measure of the amount of matter in an object. It remains constant regardless of location.
Weight is the gravitational force exerted on an object. Because gravity varies by location, an object's weight changes depending on where it is (e.g., an astronaut weighs one-sixth on the Moon compared to Earth, but their mass is identical).

Law of Conservation of Matter

Matter is neither created nor destroyed during physical phase transitions or chemical reactions. The total quantity of matter remains strictly constant.

Law of Conservation of Matter

In any physical or chemical change, total mass is conserved: $m_{\text{reactants}} = m_{\text{products}}$ .

Classification of Matter

Matter is categorized based on its composition into pure substances and mixtures:

Pure Substances: Possess a constant composition and identical properties throughout every sample.
- Elements: Pure substances that cannot be broken down into simpler substances by chemical changes (e.g., gold, oxygen).
- Compounds: Pure substances composed of two or more elements chemically combined in fixed ratios (e.g., water ( $H_2O$ ), table salt ( $NaCl$ )).
Mixtures: Composed of two or more types of matter present in varying amounts that can be separated by physical changes.
- Heterogeneous Mixtures: Composition varies from point to point throughout the sample (e.g., Italian dressing, granite).
- Homogeneous Mixtures (Solutions): Exhibit a uniform composition and appear visually identical throughout (e.g., sports drinks, air).

Atoms and Molecules

Atoms: The fundamental, smallest particles of an element that retain that element's chemical properties.
Molecules: Units consisting of two or more atoms bound together by strong chemical bonds (can be identical atoms like $O_2$ or different atoms like $H_2O$ ).

Problem-Solving Routines & Methods

How to Classify Matter

1
Step 1: Determine if the sample has a fixed, constant composition. If no, it is a mixture. If yes, it is a pure substance.
2
Step 2 (If Mixture): Is the composition uniform throughout? If yes, classify as a homogeneous mixture (solution). If no, classify as a heterogeneous mixture.
3
Step 3 (If Pure Substance): Can it be broken down into simpler substances by chemical changes? If no, it is an element. If yes, it is a compound.

Pro-Tip: Always check whether proportions can vary (mixture) or are fixed by chemical formulas (compound).

Law of Conservation of Mass

m_{\text{reactants}} = m_{\text{products}}

Total mass remains constant in closed systems during chemical and physical changes.

Variables: m = mass

Practice & Concept Checks

Concept Check

An astronaut travels from Earth to the Moon. How do her mass and weight change?

Concept Check

Classify table salt (NaCl) as a pure substance or a mixture, and further categorize it as an element, compound, homogeneous mixture, or heterogeneous mixture.

Key Terms & Vocabulary

MatterGeneral

Anything that occupies space and has mass.

Example: Water, rocks, and air.

solidStates of Matter

State of matter characterized by rigidity and a definite shape and volume.

Example: An ice cube or diamond.

liquidStates of Matter

State of matter that flows, takes the shape of its container, and has a fixed volume.

Example: Water or liquid mercury.

gasStates of Matter

State of matter that takes both the shape and volume of its container.

Example: Oxygen or helium.

plasmaStates of Matter

Gaseous state of matter containing an appreciable number of electrically charged particles.

Example: Stars and lightning.

massProperties

A measure of the amount of matter in an object.

Example: Measured in grams or kilograms.

WeightProperties

The force that gravity exerts on an object, directly proportional to its mass.

Example: An object weighs less on the Moon.

law of conservation of matterFundamental Laws

States that there is no detectable change in total mass during physical or chemical changes.

Example: Total mass of reactants equals total mass of products.

pure substanceClassification

Matter that has a constant composition and identical properties throughout.

Example: Pure water or table sugar.

elementsClassification

Pure substances that cannot be broken down into simpler substances by chemical changes.

Example: Gold, silver, and oxygen.

compoundsClassification

Pure substances comprised of two or more elements chemically combined in fixed ratios.

Example: Water (H₂O) or sodium chloride.

mixtureClassification

Matter composed of two or more types of matter present in varying amounts and separable physically.

Example: Trail mix or salt water.

heterogeneous mixtureClassification

A mixture with a composition that varies from point to point.

Example: Italian dressing or granite.

homogeneous mixtureClassification

A mixture that exhibits a uniform composition and appears visually the same throughout.

Example: Sports drink or air.

solutionClassification

Another name for a homogeneous mixture, uniform at the molecular level.

Example: Salt dissolved in water.

atomAtomic Theory

The smallest particle of an element that has the properties of that element and enters into chemical combinations.

Example: A single gold atom.

DaltonHistory

John Dalton, the British scientist who established modern quantitative atomic theory.

Example: Proposed atomic theory in the early 19th century.

moleculeAtomic Theory

Consists of two or more atoms joined by strong chemical bonds acting as a single unit.

Example: Water molecule (H₂O) or oxygen gas (O₂).