Grade 7
The Human Knowledge Project
Unit 6 — Matter, Atoms, Elements, Compounds, and Chemical Change
Welcome to Grade 7 Science!
Everything around us is made of matter.
The air we breathe, the water we drink, the food we eat, and even our own bodies are composed of tiny particles.
Understanding matter helps scientists explain the physical world and forms the foundation of chemistry.
Today you will investigate atoms, elements, compounds, mixtures, and chemical change.
What Is Matter?
Matter is anything that has:
- mass
- volume
Matter exists in many forms.
Examples include:
- rocks
- water
- air
- plants
- animals
- metals
- gases
Everything you can touch—and many things you cannot see—is made of matter.
States of Matter
Matter commonly exists in four states.
Solid
- definite shape
- definite volume
Liquid
- definite volume
- changes shape to fit its container
Gas
- no definite shape
- no definite volume
Plasma
- extremely hot, electrically charged matter
- found in stars, lightning, and some laboratory devices
Heating or cooling matter can change it from one state to another.
Atoms
An atom is the basic building block of matter.
Atoms are extremely small.
Every atom contains:
- protons
- neutrons
- electrons
Protons carry a positive electric charge.
Electrons carry a negative electric charge.
Neutrons have no electric charge.
Different combinations of these particles create different elements.
Elements
An element is a pure substance made of only one kind of atom.
Examples include:
- hydrogen
- oxygen
- carbon
- iron
- gold
- copper
Scientists have identified more than one hundred elements.
Each has unique physical and chemical properties.
The Periodic Table
The Periodic Table of Elements organizes elements according to their properties.
Elements in the same column often have similar chemical behavior.
The periodic table helps scientists:
- identify elements
- predict properties
- understand chemical reactions
It is one of the most important tools in chemistry.
Compounds
A compound forms when two or more different elements chemically combine.
Examples include:
- water (H₂O)
- carbon dioxide (CO₂)
- table salt (NaCl)
Compounds often have properties very different from the elements that compose them.
Mixtures
A mixture contains substances that are physically combined but not chemically bonded.
Examples include:
- trail mix
- air
- soil
- saltwater
The substances in a mixture can often be separated by physical methods.
Physical and Chemical Changes
A physical change alters the appearance or state of matter without changing its chemical identity.
Examples include:
- melting ice
- breaking glass
- dissolving sugar in water
A chemical change produces one or more new substances.
Examples include:
- rusting iron
- burning wood
- baking bread
- digesting food
Recognizing the difference between physical and chemical changes is an important scientific skill.
Activity
Complete the following:
- identify twenty examples of matter
- classify materials as solids, liquids, gases, or plasma
- identify ten common elements
- identify five compounds
- identify five mixtures
- classify ten changes as physical or chemical
Explain your reasoning for each classification.
Practice
Answer these questions.
What is matter?
How do solids differ from liquids and gases?
What particles make up an atom?
What is an element?
How is a compound different from a mixture?
What is the difference between a physical change and a chemical change?
Review
Today you learned:
- matter has mass and occupies space
- matter exists as solids, liquids, gases, and plasma
- atoms are the building blocks of matter
- elements contain one kind of atom
- compounds form through chemical bonding
- mixtures combine substances physically
- physical and chemical changes are different processes
Understanding matter provides the foundation for chemistry, physics, biology, engineering, and many other scientific fields.
Looking Ahead
Next we will explore energy, forces, motion, and simple machines, learning how the laws of physics help explain movement and the behavior of objects throughout the universe.