Bubbles, Color, and Heat Are Clues a Reaction Happened
Atlas stands at a cluttered kitchen science table, eyes wide with excitement, holding a fizzing cup of baking soda and vinegar that is bubbling over the rim while a thermometer and color-changing cabbage juice sit nearby
- Identify at least three observable signs that a chemical reaction has occurred
- Explain why bubbling gas, a color change, heat release, and a new smell each signal that a new substance was made
- Compare a chemical change to a physical change using everyday examples
- Predict whether a described change is likely a chemical reaction based on observable evidence
Key terms
- Chemical reaction
- When substances mix and make something brand new.
- Clue
- A sign that helps you spot a reaction.
- Carbon dioxide
- A gas made when baking soda and vinegar mix.
- Physical change
- A change in shape, but the same substance.
Clues to Look For
When a chemical reaction happens, a brand-new substance forms, and it leaves behind clues. Bubbles of gas that keep forming, like when baking soda meets vinegar, are one clue. A sudden color change, like purple cabbage juice turning green, is another. Heat or cold showing up without a stove or ice is a third clue. A new smell that was not there before is a fourth. Each clue is a hint that something new was made in the cup.
Use Clues Together
One clue alone is not always enough, so good scientists look for several clues together before deciding. Remember that not every change is a chemical reaction. Tearing paper, melting ice, or crushing a can are physical changes. The paper, water, and aluminum are still the same substances, just in a different shape. In a real chemical reaction, the starting substances are gone and a brand-new substance is there instead, leaving clues for you to find.
Worked examples
Two liquids mix, turn red, feel warm.
- Notice clue one, the color changed to bright red.
- Notice clue two, the cup feels warm without any stove.
- Two clues together point to a brand-new substance forming.
Answer: A chemical reaction happened, because of the color change and the heat.
Activity
Sort each observation into the correct reaction-clue bucket or mark it as not a reaction clue
Practice
Name two clues that a chemical reaction has happened.
Is melting an ice cube a chemical reaction or a physical change?
Common mistakes to avoid
- Any shape change is a reaction.Tearing or crushing only changes shape, so the substance stays the same.
- One clue always proves a reaction.Scientists look for several clues together before they decide for sure.
Check your understanding
You mix two clear liquids and the mixture suddenly turns bright red and feels warm. What is the best conclusion?
Marcus tears his sandwich wrap in half. His friend says a chemical reaction happened because the shape changed. Is his friend correct?
Which of the following is the BEST sign that a chemical reaction made a new substance?
Recap
A chemical reaction makes a brand-new substance and leaves clues like bubbling gas, a color change, heat or cold, and a new smell. One clue is not enough, so scientists look for several clues together.
Reflect
Which reaction clue would be the most fun to spot?