Physical Changes Are Reversible, Chemical Changes Are Not… Not!

My Mistake

I’ve been teaching for twenty four years, and somewhere along the way, years ago, I was teaching a lesson from the textbook on physical and chemical changes. The textbook lesson stated that physical changes are reversible, and chemical changes are not. So that’s how I taught the lesson…

When I got to the lesson on photosynthesis (I was teaching fifth grade at the time, where we teach Earth, Life, and Physical science in one year), I thought something was off a bit.

Plants

Plants need sunlight, water, and carbon dioxide to survive. They use the energy of the Sun during photosynthesis to change molecules of water and carbon dioxide into molecules of glucose. The plant uses the glucose for energy, to build new plant parts, and to store in fruiting plants.

Animals

Animals need food, water, and oxygen to survive. Animals eat plants to obtain glucose. The animals body uses for energy, to build new body parts, and stores energy for use later (fat).

The Carbon Cycle

Plants use need carbon dioxide, and give off oxygen. Animals need the oxygen produced by plants, and give of carbon dioxide. The Carbon Cycle.

A simple reversal of two chemical changes, which you are performing right notw. Chemical changes are reversible.

Physical and Chemical Changes

Physical changes can be reversible: when water evaporates from a liquid into a gas and then condense back into a liquid, that physical change has been easily reversed.

Fold a piece of paper in half. That was a physical change. Unfold it, and you have reversed the change (ignore the crease in the middle of the paper?!?).

Now tear the paper in half. That was a physical change. Try reversing that… Drop a glass on the floor and shatter it. That’s a physical change, but it can’t be reversed (but can’t you melt the glass and make a new one? Yup. Solid to liquid to solid: physical change).

I teach sixth grade Earth/Space Science now, and I hear all the time from students that “physical changes are reversible and chemical changes aren’t.”

Imagine their pain when they hit Life Science later on, and have to deal with the Carbon Cycle…

Student Misconceptions in Science

I don’t know why the textbook I was using phrased it that way… maybe it was a simple “hey, here’s a quick way to remember the difference,” but it makes it almost impossible to overcome that misconception with students that have been taught that idea. It’s the same problem with everything they do from day one of kindergarten being an “experiment,” and a hypothesis being an “educated guess,”
but those are topics for their own blog posts.

It’s hard enough teaching students who lack a lot of the background experiences that we grew up with… today, much of their experience comes from social media. Whether it’s the student’s use of social media, or parent and caregivers use of social media is a great topic for study. The problem was highlighted for me personally during Hurricane Milton recently, when too many of my own friends and family were getting their information from social media. I had to use my own social media to give them the website of my local news channel (WINK News, if you’re interested…). When we introduce misconceptions to students, they are extremely hard to break. I certainly never intended to confuse them when I taught them that lesson, and I should probably apologize to their eight grade Physical Science teacher for that mistake. Luckily, he’s a good friend of mine, and the next time I see him, I’m going say I’m sorry!

I’d be interested to hear your experiences encountering student misconceptions in the comments below.