Properties and States of Matter
Matter comes in three states: solid, liquid, and gas. Each state has distinct properties based on how its particles are arranged and how much they move. Understanding these states is the foundation for learning how substances change and interact.
Matter can also be classified by composition: elements, compounds, or mixtures. These categories tell you what a substance is made of and how its parts are held together.
Characteristics of Matter States
The difference between solids, liquids, and gases comes down to particle arrangement and energy.
- Solids maintain a fixed shape and fixed volume. Their particles are closely packed and vibrate in place but don't flow past each other. Examples: ice, metal, wood.
- Liquids have a fixed volume but take the shape of their container. Particles are close together but can slide past one another, which is why liquids flow. Examples: water, milk, oil.
- Gases have no fixed shape or volume and expand to fill whatever container they're in. Their particles have high kinetic energy and move randomly with large spaces between them. Examples: air, helium, carbon dioxide.
The key pattern: as you go from solid to liquid to gas, particles gain energy and move more freely.
Mass vs. Weight Distinctions
These two terms get mixed up constantly, but they measure different things.
- Mass is the amount of matter in an object, measured in kilograms (kg) or grams (g). Your mass stays the same whether you're on Earth, the Moon, or floating in space.
- Weight is the gravitational force acting on that mass, measured in newtons (N). Because gravity differs by location, your weight on the Moon would be about 1/6 of your weight on Earth, even though your mass hasn't changed.
In everyday life, people use "kilograms" or "pounds" for weight, but technically those are units of mass (kg) or force (lbs). For this course, just remember: mass is constant, weight depends on gravity.
Conservation of Matter
The law of conservation of matter states that matter cannot be created or destroyed, only rearranged. This has two major applications:
- In chemical reactions, the total mass of reactants equals the total mass of products. Atoms get rearranged into new substances, but none appear or disappear. For example, in , you start with 4 hydrogen atoms and 2 oxygen atoms, and you end with exactly 4 hydrogen atoms and 2 oxygen atoms, just bonded differently.
- In physical changes, mass also stays constant. When ice melts into water, or sugar dissolves in water, no new substances form. The substance changes appearance or state, but the total amount of matter remains the same.
Phase Transitions and Critical Points
When matter changes from one state to another, that's a phase transition. Each transition has a specific name:
- Melting: solid → liquid (occurs at the melting point)
- Freezing: liquid → solid
- Boiling/Evaporation: liquid → gas (occurs at the boiling point)
- Condensation: gas → liquid
- Sublimation: solid → gas directly, skipping the liquid phase (dry ice is a classic example)
- Deposition: gas → solid directly
A phase diagram is a graph that plots temperature vs. pressure and shows which state a substance will be in under different conditions. You don't need to memorize specific diagrams for now, but know that both temperature and pressure determine a substance's state.
Classification of Matter
Substance Types
All matter falls into one of three categories based on its composition:
Elements are pure substances made of only one type of atom. They cannot be broken down into simpler substances by chemical means. Each element is represented by a symbol on the periodic table: hydrogen (H), oxygen (O), gold (Au).
Compounds are pure substances made of two or more elements chemically bonded in a fixed ratio. Unlike elements, compounds can be broken down into simpler substances through chemical reactions. Water () always has a 2:1 ratio of hydrogen to oxygen. Table salt () always has a 1:1 ratio of sodium to chlorine. That fixed ratio is what makes a compound different from a mixture.
Mixtures are combinations of two or more substances that are physically blended, not chemically bonded. The components keep their individual properties and can be separated by physical means (filtering, evaporating, etc.). Mixtures come in two types:
- Homogeneous mixtures (also called solutions) have a uniform composition throughout. You can't see the individual components. Examples: salt water, air, metal alloys like bronze.
- Heterogeneous mixtures have a non-uniform composition where you can distinguish different parts. Examples: sand mixed with water, oil and vinegar salad dressing.
Atoms and Molecules
- An atom is the smallest unit of an element that still retains that element's chemical properties. Atoms consist of protons, neutrons, and electrons. Every carbon atom in a diamond is a carbon atom; every iron atom in steel is an iron atom.
- A molecule is two or more atoms chemically bonded together. Molecules can contain atoms of the same element (, the oxygen gas you breathe) or different elements (, water). The chemical formula tells you exactly which atoms are present and how many of each.