The Phenomenon: The Mystery of the Green Penny
Here's the mystery: the green coating on the penny is not just dirty copper. Scientists have tested it and found that the green material is actually a completely different substance — one that wasn't there when the penny was brand new. Something happened to the copper to change it into something new.
What Scientists Know: Chemical Reactions and New Substances
When you mix substances together — or expose them to heat, light, or air — one of two things can happen. Sometimes the substances just mix together without changing. You can still separate them, and each substance keeps its own properties. This is called a physical change. Think of mixing sand and salt: you still have sand and you still have salt, just sitting next to each other.
But sometimes something more dramatic happens. The starting substances actually react with each other, and entirely new substances form that have different properties than what you started with. This is called a chemical reaction. The original substances are gone — you can't just pull them apart again. The green coating on a penny is a perfect example: copper reacted with gases in the air and moisture to create a completely new substance called copper carbonate, which has different properties than pure copper.
New Properties = New Substance
Evidence You Can Observe
Not All Changes Are Chemical
Weight Is Conserved
Let's Investigate: Testing for New Substances
What scientists do: Constructing Explanations from Evidence
Scientists don't just guess whether a new substance has formed — they gather evidence by carefully observing what happens before, during, and after mixing substances. They record changes in color, temperature, gas production, and other properties. Then they use that evidence to construct an explanation about whether a chemical reaction occurred.
Investigation question: When you combine baking soda and vinegar, does a new substance form? How do you know?
Materials:
- Baking soda (white powder)
- Vinegar (clear liquid with a sour smell)
- A clear cup or jar
- A thermometer (optional)
- A kitchen scale
Procedure:
- Observe the baking soda and vinegar separately. Record their color, smell, texture, and temperature.
- Weigh the cup with baking soda and the vinegar separately. Record the total starting weight.
- Pour the vinegar into the cup of baking soda and immediately observe what happens.
- Record all changes you observe: bubbles, temperature, color, smell, sound.
- After the reaction stops, observe the remaining substance. Has anything changed?
- Weigh the cup and its contents again. Compare to the starting weight.
What you would observe: Immediate fizzing and bubbling (gas production!), a temperature drop (the mixture gets colder), and after the reaction, a clear liquid remains that tastes salty — very different from sour vinegar. The gas that escaped is carbon dioxide, a substance that wasn't there before.
What We Discovered: Using Evidence to Build Explanations
Let's go back to our baking soda and vinegar investigation. Before mixing, we had two substances with known properties: baking soda is a white, odorless solid; vinegar is a clear liquid with a strong sour smell. After mixing, several pieces of evidence told us that new substances formed.
First, we saw vigorous bubbling. Those bubbles were filled with carbon dioxide gas — a substance that didn't exist in either the baking soda or the vinegar by itself. Second, the temperature dropped, which tells us energy was absorbed during the reaction. Third, after the fizzing stopped, the liquid remaining in the cup tasted salty, not sour — it had different properties than the vinegar we started with. The remaining liquid was actually a new substance: sodium acetate dissolved in water.
Now here's the critical scientific thinking: not every change means a new substance formed. If you dissolve salt in water, the water looks clear and different — but that's a physical change. You can boil the water away and get your salt crystals back, exactly the same as before. The properties of the salt didn't change. With our baking soda and vinegar reaction, though, you cannot get the baking soda and vinegar back from the products. The change is irreversible because new substances with new properties were created.
| Property | Before Reaction (Starting Substances) | After Reaction (New Substances) | Evidence of New Substance? |
|---|---|---|---|
| Appearance | White powder + clear liquid | Clear liquid only (solid dissolved) | ✓ Different |
| Smell | No smell (baking soda) + sour (vinegar) | Very mild, not sour | ✓ Different |
| Taste | Bitter (baking soda) + sour (vinegar) | Salty | ✓ Different |
| Gas produced? | No gas present | Carbon dioxide gas (bubbles) | ✓ New substance |
| Temperature | Room temperature (about 22°C) | Cooler (about 17°C) | ✓ Energy change |
Each row of this data table is a piece of evidence. Individually, one change might not prove a new substance formed. But when we look at multiple pieces of evidence together — a new color, new smell, new taste, gas production, and temperature change — the case becomes very strong. Scientists always look for multiple lines of evidence before concluding that a chemical reaction took place.
Patterns and Connections: Cause and Effect
The crosscutting concept in this lesson is Cause and Effect. In science, events have causes that generate observable patterns. When two substances interact in a chemical reaction, the cause is the interaction between the substances, and the effect is the formation of new substances with new properties. Scientists design tests to identify these cause-and-effect relationships — they don't just assume a reaction happened; they look for the evidence.
This same pattern — cause and effect — shows up across all areas of science. Let's look at how cause-and-effect thinking helps scientists in different fields.
| Science Area | Cause | Effect (Observable Evidence) | How Scientists Know |
|---|---|---|---|
| Physical Science | Iron is exposed to oxygen and water | Reddish-brown rust forms on the surface | New color, new texture, crumbles easily — different properties than iron |
| Life Science | A plant receives no sunlight | Leaves turn yellow, plant stops growing | Observable changes in color and growth pattern — evidence that the plant's food-making process stopped |
| Earth Science | Acid rain falls on limestone rock | Rock surface fizzes and wears away | Gas production (bubbles) and erosion of rock — a chemical reaction between acid and limestone |
| Everyday Life | Bread dough is baked in an oven | Dough turns into bread with a brown crust | New color, new texture, new smell — the dough has chemically changed |
Real-World Connections: Chemical Reactions All Around You
Chemical reactions aren't just something that happens in science labs. They're happening all around you, every single day — and now you have the tools to spot the evidence!
🍳 In the Kitchen
🏗️ In Construction
🔋 In Technology
🛡️ In Protecting Things
Key Vocabulary Review
- Chemical reaction — A process in which one or more substances interact and are changed into new substances with different properties.
- Physical change — A change in the form or appearance of a substance that does not create a new substance. The original substance can usually be recovered.
- Properties — Characteristics of a substance that can be observed or measured, such as color, smell, texture, melting point, and whether it dissolves in water.
- Evidence — Observations, data, or information that support or help explain a claim. Scientists use evidence to determine whether a chemical reaction occurred.
- Gas production — The formation of bubbles or fizzing during a reaction, which indicates that a new gaseous substance has been created.
- Precipitate — A solid that forms when two liquids are mixed together, indicating that a new substance has been created.
- Conservation of matter — The principle that the total weight of substances before a reaction equals the total weight of substances after a reaction. Matter is not created or destroyed — it rearranges.