Start your day with a solid breakfast! Episode 1 reviews, compares and contrasts physical and chemical changes and how to recognize them as well as the states of matter.
Start your day with a solid breakfast! Episode 1 reviews, compares and contrasts physical and chemical changes and how to recognize them as well as the states of matter. During a physical change a substance retains its identity (1:48) - examples are changes in state: solids, liquids and gas and signs are the change of shape or size (3:24). During chemical changes the identity and properties of substances changes (4:00), which you can observe as color change, heat being released, or bubbles being formed (4:20).
Is dissolving salt in water a physical or chemical change? (5:05)
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Hi and welcome to the APsolute Recap: Chemistry Edition. Our very first episode! We are excited to be partnering with Dr. Sarah Rayder to bring you the content and skills you need to be successful on the AP Chemistry Exam. Our first few episodes will review some introductory chemistry content that is essential knowledge for the AP Course. Make sure to check for episode updates, available study guides and a release schedule on our website - theapsoluterecap.com. Today’s episode will recap physical and chemical changes
When studying chemistry we will be observing two types of changes: physical changes and chemical changes. An example of physical are changes of state, whereas during chemical changes a substance will be transformed into a new substance. So to start off our journey, this episode will recap the difference between physical and chemical changes, talk about states of matter and the signs that help you to determine if a change is physical or chemical.
Let’s zoom in!
It's time to make some breakfast - To start your day off right, you are boiling water for your tea and making yourself some toast. You are enjoying the quiet morning, the birds are chirping, the sun is shining, when all of a sudden it starts to smell like something is burning - oh no! Your little sister put the toaster on high and your toast is now roast. Despite your somewhat ruined breakfast, (put some cream cheese on it, it fixes most cooking blunders), this is a great example of some chemical and physical changes.
During a physical change a substance retains its identity - it still is the same substance. In our example: we are boiling water for the tea. It is water, no matter if liquid from our faucet or in the form of water vapor when boiling. The molecular make-up, where 2 hydrogen atoms are connected to one oxygen atom, stays the same. What does change is the arrangement of the water molecules.
In solid form, the water molecules experience strong forces between each other. These forces are intermolecular forces - “inter” is the latin word for “between” - so these are literally forces between molecules. Due to these intermolecular forces the water molecules are held in place in a regular pattern with only very very little space between the molecules. Therefore, ice is incompressible and doesn’t assume the shape of the container. Your ice cube just sits on the bottom of an empty glass in the form you froze them.
When heating ice, the energy is used to overcome some of the intermolecular forces between the water molecules. The molecules are free to move around each other, the regular pattern of ice is loosened and the water, now in its liquid form, takes on the shape of the glass. Liquids are barely compressible, because there is still only very little space between the molecules. If you continue to add heat, the intermolecular forces that are in liquid water and are holding the molecules in the glass, are no longer in place: you now have a gas! The water molecules are free to move about at high speed. If you don’t have a lid, they even leave the glass! Due to the relatively large spaces between the molecules, gases can be compressed.
As you’ve seen, in all three forms, ice, water and vapor, the chemical make-up stays the same: They are always water molecules. Changing the physical state of a substance doesn’t change the identity. Signs of physical changes other than changes in state are changes in shape, for example when you try to build a paper airplane out of a piece of paper, or changes in size: when you get frustrated that the plane only flew two feet and you rip it into tiny pieces.
Back to your breakfast: Your toast definitely has undergone a chemical change: Its color changed from a white - or light brown if you pick a whole grain toast - to charred black. Additionally, the lovely smell of bread which makes your mouth water, is now replaced by the acrid smell of burning. And don’t even get me started on the taste! You definitely have a different substance with different and new properties.
This is the definition of a chemical change! Chemical changes break chemical bonds and form new bonds and therefore new substances. A few other signs of chemical changes that you can observe are the formation of precipitates - meaning the formation of solids, temperature changes, when something gets hot or cold as with hand warmers or cooling packs, or the formation of bubbles.
To recap……
During chemical changes a substance undergoes a transformation into a different substance with new properties. Signs of chemical changes may be changes in color, releasing of heat, or the formation of a precipitate. If a substance does not change its identity and therefore its properties, it is a physical change. To identify it as a physical change, look for changes in size, texture, shape or for phase changes.
Coming up next on the Apsolute RecAP Chemistry Edition: Atomic Structure
Today’s Question of the day is about physical and chemical changes.
Question: Is dissolving salt in water a physical or chemical change? (as choice on Instagram)