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Apply your knowledge to the chemical changes that happen when we cook food.
Have you ever wondered how a runny, clear egg turns into a solid white breakfast, or how a flat scoop of dough 'magically' grows into a giant, fluffy loaf of bread?
In the kitchen, we see two types of changes. A physical change is like melting butter; it changes shape or state, but it is still butter. However, cooking usually involves a chemical change. This happens when the molecules in your food are rearranged to create something entirely new! You can tell a chemical change is happening if you see a color change, smell a new aroma, or see bubbles forming. Most importantly, chemical changes are usually irreversible—you can't 'un-fry' an egg or turn a cake back into liquid batter.
1. You start with a clear, liquid egg white containing folded proteins. 2. You add heat energy from the stove. 3. The heat causes the proteins to uncoil and stick together. 4. The result is a solid, white substance that is chemically different from the raw egg.
Quick Check
If you freeze juice into a popsicle, is that a physical or chemical change?
Answer
It is a physical change because it is still juice, just in a solid state, and can be melted back.
Why isn't a muffin as flat as a cracker? The secret is a chemical reaction involving leavening agents like baking powder. Baking powder contains a dry acid and a base. When you add liquid and heat, these two ingredients react to create carbon dioxide gas, written as . These tiny bubbles of gas get trapped in the stretchy dough. As the heat increases, the bubbles expand, pushing the dough upward and making it light and airy. This is why you see tiny holes in a slice of bread!
1. Mix baking powder into wet cake batter. 2. The water allows the acid and base to meet. 3. Place the tray in a oven. 4. The heat speeds up the reaction, releasing gas rapidly. 5. The cake 'rises' as the gas expands before the heat sets the structure solid.
Quick Check
What specific gas is responsible for making cake and bread fluffy?
Answer
Carbon dioxide (or ).
Have you ever smelled toast or noticed the brown crust on a loaf of bread? This is caused by the Maillard Reaction. When food reaches high temperatures, the sugars and proteins on the surface react with each other. This doesn't just change the color to brown; it creates hundreds of new flavor and smell molecules. This is a complex chemical change that only happens when enough thermal energy (heat) is applied to the surface of the food.
1. Take a slice of white bread (mostly starch/sugars and some protein). 2. Apply intense heat in a toaster. 3. Observe the color change from white to brown. 4. Note the new 'toasty' aroma—this is evidence that new chemical substances have been created that weren't there before!
Which of the following is a sign that a chemical change has occurred while cooking?
What role does heat play in the chemical reactions of cooking?
Baking powder makes bread rise by releasing oxygen gas.
Review Tomorrow
In 24 hours, try to explain to a friend why a pancake has tiny holes in it using the term 'Carbon Dioxide'.
Practice Activity
Next time you are in the kitchen, observe a piece of fruit ripening or bread toasting. Identify one sign that a chemical change is happening.