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Learn why a sudden change in color is a major clue that chemistry is at work.
Have you ever wondered why a bright green banana turns yellow, or why a shiny silver bicycle turns crusty orange if left in the rain? These aren't just surface changes—they are 'secret messages' from atoms telling you that something brand new has been created!
When you mix yellow and blue paint to get green, that is a physical change. You still have paint; it just looks different. But in chemistry, an unexpected color change happens when two things meet and their atoms rearrange to build a new substance. Imagine mixing two clear liquids and suddenly seeing bright blue! Because the new substance reflects light differently than the old ones, the color 'flips' like a switch. This is a major clue that a chemical reaction has occurred. The original ingredients are gone, and something entirely different has taken their place.
Observe how a simple color change reveals hidden starch: 1. Take a slice of a white potato (contains starch). 2. Add a drop of yellowish-brown Iodine solution. 3. Watch as the contact point turns dark purple or black. 4. This happens because the Iodine reacts with the starch to create a brand new substance that reflects purple light.
Quick Check
If you mix red water and blue water to get purple water, is that a chemical change?
Answer
No, that is a physical change (mixing) because no new substance was created; it is just a blend of the original colors.
In the world around us, color changes act like a 'status bar.' When an apple turns from green to red, it isn't just getting older. Inside the fruit, complex chemical reactions are breaking down acids and creating sugars. Another famous example is oxidation. When iron () meets oxygen () and water, they create iron oxide (), which we know as crumbly, orange-brown rust. The change from silver to orange tells us the metal is no longer pure iron; it has become a new compound.
The Statue of Liberty is made of shiny brown copper (), like a penny. Over many years, it changed color: 1. Copper reacted with Oxygen () and Carbon Dioxide () in the air. 2. A new substance called 'patina' formed on the surface. 3. This chemical reaction transformed the brown exterior into the iconic sea-foam green we see today!
Quick Check
Why is rust a different color than the original iron nail?
Answer
Because rust is a completely new substance (iron oxide) formed by a chemical reaction, and new substances have their own unique colors.
Scientists use special chemicals called indicators to test for other substances. An indicator is like a 'chemical sensor' that changes color when it touches an acid or a base. For example, red cabbage juice is a natural indicator. If you add it to something acidic (like lemon juice), it turns bright red. If you add it to something basic (like baking soda), it turns blue or green. The color change is the evidence that the molecules in the cabbage juice have changed their structure to react to the environment!
You can use chemistry to hide and reveal messages: 1. Write a message using lemon juice (an acid) on white paper. 2. Let it dry until it is invisible (a physical change of evaporation). 3. Heat the paper carefully over a lightbulb. 4. The heat triggers a chemical change where the carbon in the juice reacts with oxygen. 5. This creates a brown, 'burnt' substance, making your secret message appear!
Which of these is an example of a chemical color change?
Why does a new color appear during a chemical reaction?
When a banana turns from green to yellow, it is undergoing a chemical change.
Review Tomorrow
In 24 hours, try to explain to a friend why the Statue of Liberty is green even though it is made of brown copper.
Practice Activity
Kitchen Detective: Look for 'unexpected' color changes in your kitchen. Can you find a piece of fruit, a piece of bread, or a metal pan that has changed color? Ask yourself: Is this a new substance?