What is a chemical reaction, and how do we define it? At some point in your life, you may have mixed baking soda and vinegar together to witness a bubbling chemical reaction. In the same scope of kitchen concoctions, if you threw a banana, strawberries, and yogurt into a blender and blended them into a smoothie, would you have conducted a chemical reaction? No! Instead, what has taken place is a physical change.
What defines a physical change versus a chemical change?
In chemical changes, new substances are formed, whereas, in physical changes, the identity of the substances remains the same.
List three ways in which you know that a chemical change has occurred.
Often to distinguish between chemical and physical changes, you can look towards these tell-tale signs that a chemical reaction is occurring: temperature change, color change, formation of precipitate, bubbles (formation of gas), presence of an odor (formation of gas), and change in acidity (pH change).
Beware! Recall that changes in states of matter, that is changes between solids, liquids, and gases, are all forms of physical change which occur due to temperature changes. Be careful not to interpret these changes as chemical ones! A good way to distinguish these changes in states of matter to chemical changes is to think whether or not temperature change caused the change in matter or is a byproduct of the change. If it's the former, it's physical, but if it's the latter, it's chemical.
Recall that mixtures are also forms of physical change. Mixtures come in two "flavors": homogeneous and heterogeneous.
What defines each type of mixture?
Since mixtures are physical changes, the identity of the substances don't change. What are some ways we can separate mixtures?
Describe whether a chemical or physical change is taking place for each of the below scenarios. If it is a phase change or mixture, classify which type.
1. Lettuce, croutons, chicken, tomatoes, onions, and salad dressing are mixed together in a large bowl.
2. Equal amounts of equally concentrated hydrochloric acid and sodium hydroxide solutions are mixed together in a big bowl. It is discovered that the temperature of the mixture has increased by 5 degrees celsius, and the pH of the mixture is 7, whereas the pH values of the original solutions were 1 and 13, respectively.
3. Liquid nitrogen is poured into a cup, and you notice the presence of a gas surrounding the cup. Within a few minutes, the cup is empty. You also notice that the room has become slightly colder.
4. Black coffee is mixed with milk and is stirred until the drink is a light brown.
Highlight below for answers
1. heterogeneous mixture (physical change)
2. chemical change
3. sublimation (physical change)
4. homogeneous mixture (physical change)
Now, try to come up with some of your own examples for physical and chemical changes for practice.