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Summarize everything you have learned about chemical changes and energy.
Why does a campfire feel hot while a chemical cold pack feels freezing? The secret lies in how atoms 'shake hands' and trade energy!
In chemistry, we look at how matter changes. A physical change is like a costume change; the substance looks different, but it is still the same stuff inside. Think of an ice cube melting into water (). It changed from solid to liquid, but it's still water! A chemical change, however, is like a transformation. The atoms rearrange to create a brand-new substance with different properties. When you burn wood, it turns into ash and smoke—you can't turn that ash back into a log! In a chemical change, the 'identity' of the matter actually changes.
Step-by-step analysis of a rusty nail: 1. An iron nail is left outside in the rain. 2. The iron () reacts with oxygen () in the air. 3. A new orange, crumbly substance called iron oxide () forms. 4. Because a new substance was created, this is a chemical change.
Quick Check
If you rip a piece of paper into tiny scraps, is it a physical or chemical change?
Answer
It is a physical change because the scraps are still paper; no new substance was created.
Chemical reactions always involve energy, usually in the form of heat. We categorize them into two groups. An exothermic reaction releases energy into the surroundings. Think of 'Exo' as 'Exit.' These reactions feel hot to the touch because heat is leaving the reaction. An endothermic reaction absorbs energy from the surroundings. Think of 'Endo' as 'Enter.' These reactions feel cold because they are 'stealing' heat from your hand or the air to make the reaction happen!
How a sports cold pack works: 1. You squeeze the pack to break a seal inside. 2. Two chemicals mix and begin to react. 3. The reaction needs energy to break bonds, so it pulls heat from the environment. 4. The pack feels freezing cold. This is an endothermic reaction.
Quick Check
A candle flame feels hot. Is the burning wax an exothermic or endothermic reaction?
Answer
Exothermic, because it is releasing heat energy into the air.
How do you know for sure if a chemical change happened? Scientists look for five 'clues' or indicators: 1. Color Change: Like a silver spoon turning black (tarnish). 2. Gas Production: Seeing bubbles or smelling an odor. 3. Precipitate: When two liquids mix and a solid 'gunk' forms at the bottom. 4. Temperature Change: The mixture gets significantly hotter or colder. 5. Light: Like the glow of a firefly or a firework.
Analyzing Vinegar and Baking Soda: 1. Mix liquid vinegar with solid baking soda. 2. Observation A: Intense bubbling occurs (Gas production). 3. Observation B: The beaker feels slightly colder (Temperature change). 4. Conclusion: This is a chemical change that is endothermic.
Which of the following is a sign of a chemical change?
If a reaction releases energy and the container feels hot, it is:
Boiling water is a chemical change because bubbles are produced.
Review Tomorrow
Tomorrow morning, try to name the five indicators of a chemical change while you eat breakfast!
Practice Activity
Go into your kitchen and find one physical change (like cutting fruit) and one chemical change (like toasted bread or a bubbling soda).