Educational Resources Lets Talk Science Challenge participants

Holiday meal with roast turkey and cookies

Holiday meal with roast turkey and cookies (GMVozd, iStockphoto)

STEM in Context

Physical and Chemical Changes in the Kitchen

Moushumi Nath
Format
Video Text Images
Readability
4.77
Subjects

Summary

Many physical and chemical changes happen when food is prepared. Chemistry never tasted so good!

How do you go from flour, sugar, and eggs to cookies? Or from fruit juice to popsicles? Making food is all about changes! These can be physical changes or chemical changes.

Physical Changes

A physical change is a change in the state of matter. There are three main states of matter - solid, liquid and gas.

If you freeze fruit juice to make a popsicle, this is a change in state. The juice changes from a liquid to a solid. Ice cream melting on a hot day is also a change in state. The ice cream changes from a solid to a liquid. The juice and ice cream are still juice and ice cream, even when they change from one state of matter to another.

Misconception Alert

Sometimes when you cook a liquid it becomes a solid, like when a raw egg becomes a scrambled egg. This is not a change in state. Changes in state can usually be reversed. You can’t change scrambled eggs back into raw eggs!

If you break something large into smaller pieces, this is also a physical change. When you grate a piece of cheese for your pizza, you are changing the block of cheese. The cheese is still cheese, even though the little shreds don’t look like the block of cheese you started with.

Frozen fruit treats, melting ice cream and grated cheese
Left to right: Frozen fruit treats, melting ice cream and grated cheese (Sources: LauriPatterson via iStockphoto, Collin BH via iStockphoto and (sergio_kumer via iStockphoto).

 

Sometimes physical changes are hard to see. If you put some sugar into water and stir, soon you wouldn’t be able to see the sugar. It hasn’t disappeared! The sugar has just been broken down into pieces that are too small to see. 

It’s like if you mixed a few red candies into a big bag of blue candies. The red candies might be hard to see, but they’re still there. 

Chemical Changes

chemical change is a change in what something is made of. We can tell that a chemical change has happened by looking for certain clues. 

One clue is gas. Gas is produced during a chemical change. The next time you bake a cake or flip a pancake, look for the bubbles. Inside each bubble is a little bit of a gas. This gas is made when the ingredients change each other. 

Bubbles help make the food you bake fluffy. If you cut into a cake, you can see the spaces where there were bubbles of gas in the wet batter.

Misconception Alert

Not all bubbles come from chemical changes. When you mix batter with a whisk, you might see bubbles. These are just air that gets trapped in the batter during whisking.

Speaking of gas… Smell is the second clue that a chemical change is happening. Sour milk, rotten bananas and mouldy cheese all make smells. These smells warn us not to eat this food. Food that’s rotten or mouldy could make us sick. 

But there are many smelly foods that are perfectly safe to eat! Blue cheese and Durian fruit have very strong smells. They will not harm you unless you are allergic to them. Everyone has different tastes. Lots of people love foods with unique odours. 

Did you know?

Some people say Durian fruit smells like paint thinner and onions, with a hint of stinky gym socks!

Can you think of nice smells that might come from the kitchen? The smell of baking bread or roasting turkey are signs that chemical changes are happening!

Fresh bread, roast turkey and simmering soup
From left to right: People smelling fresh bread, roast turkey and simmering soup (Sources: gpointstudio via iStockphoto, FG Trade via iStockphoto and eclipse_images via iStockphoto).

 

The third clue is a change in colour. Think of the perfect golden brown you can get when you toast marshmallows over a fire. Or that dark brown colour when you accidentally burn it. Whoops! 

When tomatoes change colour from green to red, that means they are ready to eat. When bananas change from yellow to black, that means they are not good to eat anymore.

Toasted marshmallows, burnt toast, bananas
Toasted marshmallows, burnt toast and overripe, ripe and under ripe bananas (Elenathewise via iStockphoto, ImageDB via iStockphoto and kolesnikovserg via iStockphoto).

 

Colour change can be a tricky clue. Sometimes it’s hard to tell if a change in colour is because of a physical change or a chemical change. 

Imagine that you add red food colouring to white icing. Now you have pink icing. Do you think this is a physical change or a chemical change? Remember stirring sugar in water? Food colouring works the same way. The colour just spreads through the icing. It doesn’t change the icing itself. This is a physical change.

Physical and Chemical Changes (2018) by Chemistry for Kids FreeSchool (5:23 min)

 

So how can you tell if a change is physical or chemical? Ask yourself:

  • Is the substance the same thing now as it was before the change? If it’s the same, it is a physical change. If it isn’t, it is a chemical change.
  • Was there a change in colour or smell? Was a gas formed? If any of these things happened, it was a chemical change.
  • Can the change be reversed easily? If so, it was a physical change.

Your kitchen can be an amazing laboratory to explore the world of chemistry! 

Starting Points

Connecting and Relating
  • What foods do you like to make at home? 
  • Have you experienced a chemical change in food? Explain.
  • Which baking or cooking smells do you particularly like?
Connecting and Relating
  • What foods do you like to make at home? 
  • Have you experienced a chemical change in food? Explain.
  • Which baking or cooking smells do you particularly like?
Relating Science and Technology to Society and the Environment
  • What things do supermarkets have in place to limit the physical and chemical changes in food? 
  • How can the safety of food be affected by chemical or physical processes?
Relating Science and Technology to Society and the Environment
  • What things do supermarkets have in place to limit the physical and chemical changes in food? 
  • How can the safety of food be affected by chemical or physical processes?
Exploring Concepts
  • What is a physical change in matter? Give three examples of physical changes.
  • What is a chemical change in matter? Give three examples of chemical changes.
  • If you are using a blender or a food processor, what type of changes are you making to food?
  • How can the process of cooking food in an oven or a microwave make changes in foods?
Exploring Concepts
  • What is a physical change in matter? Give three examples of physical changes.
  • What is a chemical change in matter? Give three examples of chemical changes.
  • If you are using a blender or a food processor, what type of changes are you making to food?
  • How can the process of cooking food in an oven or a microwave make changes in foods?
Nature of Science/Nature of Technology
  • Is cooking an art or a science? Explain.
  • Do some research to learn what the field of food science involves.
Nature of Science/Nature of Technology
  • Is cooking an art or a science? Explain.
  • Do some research to learn what the field of food science involves.
Media Literacy
  • Have you ever seen a baking competition “fail” on reality TV? What was the cause of the fail? Did the fail have to do with a physical change or a chemical change that went wrong? Explain. 
Media Literacy
  • Have you ever seen a baking competition “fail” on reality TV? What was the cause of the fail? Did the fail have to do with a physical change or a chemical change that went wrong? Explain. 
Teaching Suggestions
  • This article supports teaching and learning of chemistry related to change in matter. Concepts introduced include state of matter, chemical change, gas, bubbles, smell and colour.
  • After reading this article teachers could have students create an infographic that provides an overview of physical and chemical changes involved in preparing food. Refer to the Infographic Creator learning strategy.
  • To further consolidate and apply learning about this topic, teachers could have students complete a Pros & Cons Organizer learning strategy to consider the advantages and disadvantages of making physical changes in foods to help preserve them, give them a longer shelf life and make them more convenient (e.g., mixes, freezing, grating, pre-cutting, etc.). Ready-to-use reproducibles for this article that use this strategy are available in [Google doc] and [PDF] formats. 
Teaching Suggestions
  • This article supports teaching and learning of chemistry related to change in matter. Concepts introduced include state of matter, chemical change, gas, bubbles, smell and colour.
  • After reading this article teachers could have students create an infographic that provides an overview of physical and chemical changes involved in preparing food. Refer to the Infographic Creator learning strategy.
  • To further consolidate and apply learning about this topic, teachers could have students complete a Pros & Cons Organizer learning strategy to consider the advantages and disadvantages of making physical changes in foods to help preserve them, give them a longer shelf life and make them more convenient (e.g., mixes, freezing, grating, pre-cutting, etc.). Ready-to-use reproducibles for this article that use this strategy are available in [Google doc] and [PDF] formats. 

Learn more

Tammy Barrett: Manager, Research and Development

In this article for Let’s Talk Science, Tammy develops recipes for new ice cream flavours! She uses a lot of math, chemistry and creative skills to produce safe and delicious treats for people across Canada.

Claire Bartman: Quality Analyst

In this article for Let’s Talk Science, Claire puts her knowledge of chemistry to work every day, testing spices and other cooking ingredients for quality and safety.

Space Food

In this article from Let’s Talk Science, learn about the techniques used to prepare safe and high-quality food for use on the International Space Station. 

Chemical Changes (2015)

This video (3:50 min.) from Crash Course Kids, aimed at middle school students, explores the chemical changes involved in cooking some of our favourite foods.

The chemistry of cookies (2013)

This TED-Ed video (4:29 min.) by Stephanie Warren asks if you have ever wondered what’s really going on when your cookies are in the oven. Find out by watching this video.

References

Helmenstine, A. M. (2020, January 24). Examples of physical changes. ThoughtCo.

Levac, K. (2019, July 23). A merry molecular Christmas. Let's Talk Science.

Millam, S. (2016, November 14). Simple distillation.

Rader, A. (n.d.). Chemical changes versus physical changes. Chem4Kids.

Sohol, M. (2020). Chemical change vs. physical change. LibreTexts.