# Welcome Back: Here's What You Missed

If you were absent for Lessons 1, 2, or 3, this remix is for you. We're going to move quickly through the three biggest ideas so you're ready to keep moving with the class.

Here's the roadmap for this review:

1. The Building Blocks - What matter is made of (atoms, subatomic particles, elements) 2. How Matter Behaves - Physical properties, density, and how we identify substances 3. Particles in Motion - The three states of matter and what happens during phase changes

By the end, you'll have the vocabulary and core concepts you need. Let's go.

# Part 1: The Building Blocks of Matter

Matter is anything that has mass and takes up space. Every solid, liquid, and gas around you is matter. Your desk, the air you're breathing, the water in your bottle: all matter.

## Atoms: The Tiniest Pieces

All matter is made of incredibly tiny particles called atoms. A single grain of sand contains about 50 quintillion atoms, that's 50,000,000,000,000,000,000. You cannot see them with the naked eye or even with a regular microscope.

## Inside Every Atom

Atoms are not solid spheres. They have an internal structure built from three types of subatomic particles:

| Subatomic Particle | Location | Charge | Relative Mass | |---|---|---|---| | Proton | Inside the nucleus | Positive (+1) | 1 (baseline) | | Neutron | Inside the nucleus | None (neutral) | ~1 | | Electron | Electron cloud (around nucleus) | Negative (−1) | ~1/1836 |

- The nucleus is the tiny, dense center of the atom where protons and neutrons are packed together. Almost all of the atom's mass is here. - The electron cloud is the region outside the nucleus where electrons move rapidly. - In a normal atom, the number of protons equals the number of electrons, so the atom is electrically neutral (charges cancel out).

## Elements and Atomic Number

An element is a pure substance made of only one type of atom. What makes one element different from another? The number of protons, called the atomic number.

- Every atom with 6 protons is carbon. - Every atom with 8 protons is oxygen. - Every atom with 79 protons is gold.

Change the number of protons, and you change the element. All atoms of the same element are identical to each other but different from atoms of any other element. Scientists have identified about 118 elements, all organized on the Periodic Table of the Elements by their atomic number.

# Part 2: Properties of Matter - How We Describe and Identify Substances

Once you know matter is made of atoms, the next question is: how do scientists tell different substances apart? The answer is physical properties, characteristics you can observe or measure without changing the substance into something else.

Physical properties fall into two categories:

## Intensive Properties: The Substance's Fingerprint

Intensive properties do not depend on how much of the substance you have. They stay the same whether you have one drop or a whole ocean.

Common intensive properties: - Density - how much mass is packed into a 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

Because these properties don't change with amount, scientists use them to identify substances. A material with a density of 19.3 g/cm³ and a melting point of 1,064°C is gold, no matter how big or small the sample.

## Extensive Properties: Depends on Amount

Extensive properties do change when you change the amount of substance. Examples: mass, volume, weight. These tell you how much matter is present, but not what the matter is.

## Density: The Formula You Need

Density is the most-used intensive property. It's calculated with:

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

where d is density (g/cm³ or g/mL), m is mass (grams), and V is volume (cm³ or mL).

Example: A rock has a mass of 50 g and a volume of 20 cm³. Its density = 50 ÷ 20 = 2.5 g/cm³.

Float or sink? If an object's density is less than water's density (1.0 g/mL), it floats. If greater, it sinks. Ice (0.92 g/cm³) floats on water, and that's why frozen lakes don't kill all the fish.

# Part 3: States of Matter and Phase Changes

## How Particles Behave in Each State

All matter is made of particles in constant motion. This is the kinetic molecular theory. Temperature measures the average kinetic energy (energy of motion) of those particles. The more energy the particles have, the faster they move and the higher the temperature.

The three common states of matter differ in how their particles are arranged and how much they move:

| State | Particle Arrangement | Particle Motion | Shape | Volume | |---|---|---|---|---| | Solid | Fixed, organized grid | Vibrate in place | Definite | Definite | | Liquid | Close but disordered | Slide past each other | Takes shape of container | Definite | | Gas | Far apart, random | Move rapidly in all directions | Takes shape of container | Fills entire container |

Solids have the least energy; gases have the most.

## Phase Changes: Energy In, Energy Out

A phase change is when matter transforms from one state to another. Every phase change is driven by adding or removing thermal energy (heat).

Adding energy causes: - Melting - solid → liquid (ice cube melting) - Evaporation/Boiling - liquid → gas (puddle drying up; pot boiling) - Sublimation - solid → gas directly (dry ice turning to fog)

Removing energy causes: - Freezing - liquid → solid (water becoming ice in a freezer) - Condensation - gas → liquid (dew on morning grass; mirror fogging) - Deposition - gas → solid directly (frost forming on a cold window)

## The Key Surprise: Temperature Stays Constant During Phase Changes

When matter is changing phase, adding more energy does not raise the temperature. Instead, all the energy goes into breaking (or forming) the bonds between particles. This creates flat plateaus on a heating curve. Ice melts at exactly 0°C, and the temperature holds there until every bit of ice has turned to water. Water boils at exactly 100°C, and the temperature holds there until every bit of liquid has turned to steam.

# Quick Reference: Combined Vocabulary

Here are the most important terms from Lessons 1, 2, and 3 in one place.

| Term | Definition | From Lesson | |---|---|---| | Matter | Anything with mass and volume | 1 | | Atom | Smallest unit of an element; made of protons, neutrons, electrons | 1 | | Nucleus | Dense center of an atom; contains protons and neutrons | 1 | | Atomic number | Number of protons in the nucleus; defines the element | 1 | | Element | Pure substance made of only one type of atom; ~118 known | 1 | | Intensive property | Physical property that does NOT change with amount (density, melting point, boiling point, solubility) | 2 | | Extensive property | Physical property that DOES change with amount (mass, volume, weight) | 2 | | Density | Mass per unit volume; d = m/V; units: g/cm³ or g/mL | 2 | | Melting point | Temperature at which a solid becomes a liquid | 2 & 3 | | Boiling point | Temperature at which a liquid becomes a gas | 2 & 3 | | Solubility | How much solute dissolves in a given amount of solvent | 2 | | Kinetic molecular theory | All matter is made of particles in constant motion; temperature = average kinetic energy | 3 | | Solid | Fixed arrangement; particles vibrate; definite shape and volume | 3 | | Liquid | Particles slide past each other; definite volume, no fixed shape | 3 | | Gas | Particles far apart, moving rapidly; no fixed shape or volume | 3 | | Phase change | Transformation between states of matter caused by adding/removing thermal energy | 3 | | Sublimation | Solid → gas directly (adds energy) | 3 | | Deposition | Gas → solid directly (removes energy) | 3 |