ChemistrySame Stuff or Brand-New Stuff? Two Kinds of Change
Atlas the friendly guide stands at a bright safe lab bench, holding a melting ice cube in one mittened hand and pointing at a rusty orange nail glowing under a warm desk lamp, with safety goggles on and a curious smile.
- Define a physical change as one where a substance keeps its identity even as its form or shape shifts
- Define a chemical change as one where atoms bond in new ways to form a brand-new substance with a different identity
- Classify everyday examples as physical changes or chemical changes using observable clues
- Identify clues that a brand-new substance has formed, such as a new color, new smell, bubbles, or light
- Explain why scientists observe unknown substances carefully with their eyes instead of tasting or touching them
Key terms
- Physical change
- A change where the stuff keeps its identity.
- Chemical change
- A change that builds a brand-new substance.
- Identity
- What a substance really is.
- Iron oxide
- Rust, made when iron joins with oxygen.
Same Stuff, New Form
In a physical change, a substance keeps its identity even when its form or shape shifts. Melt an ice cube and it becomes liquid water, then freeze it back into ice. The water molecules never stopped being water. You can tear paper, squish clay, or stir sugar into water. The sugar molecules spread out through the water but stay sugar. Physical changes alter how something looks or where it sits, but they never change what the substance truly is inside.
Brand-New Substance
In a chemical change, atoms break their old connections and join in new ways to build a brand-new substance with a different identity. When an iron nail rusts, iron atoms join with oxygen from the air to make rust, which is not iron anymore. When you bake batter, new substances form that can never be un-baked. Clues that a new substance formed include a new color, bubbles, a new smell, heat, or light. Use a few clues together, not just one.
Worked examples
Why is rusting a chemical change?
- A shiny iron nail sits outside for a long time.
- Iron atoms join with oxygen from the air and form rust.
- Rust is a brand-new substance, so the iron changed into something new.
Answer: Rusting is a chemical change, because a new substance, rust, forms.
Activity
Sort each card into Physical Change or Chemical Change — one card is tricky, so think carefully!
Practice
Is melting an ice cube a physical or chemical change?
Name two clues that a brand-new substance has formed.
Common mistakes to avoid
- A color change always means a physical change.A color change can be a clue that a new substance may have formed.
- Dissolving sugar turns it into water.The sugar molecules spread out but stay sugar, so it is physical.
Check your understanding
An ice cube melts into a puddle of water. What kind of change is this?
A shiny iron nail sits outside for weeks and slowly turns orange and crumbly. Why is rusting a chemical change?
A friend says, 'Any change in color is always just a physical change.' What is wrong with that idea?
Atlas finds an unknown fizzing substance on the lab bench. What is the safest next step?
Recap
A physical change keeps a substance the same even as its form shifts, like melting ice or dissolving sugar. A chemical change makes a brand-new substance, like rusting iron, shown by clues such as new color, bubbles, or smell.
Reflect
How would you decide if a change made new stuff?