Molecules make up everything.
Sometimes when we make a mixture, new molecules are made. Sometimes the molecules stay the same but just rearrange.
Physical change: the molecules stay the same.
A physical change might make something a different size, shape or texture. The molecules might rearrange, but the molecules themselves stay the same.
Sometimes things change state of matter, but it is the same molecules inside. What molecule is in the snow we had last week, or the rain we are getting this week? [Water] Always water, but as a solid in snow or liquid in rain.
Chemical reaction: the molecules change.
Clues: new colour, makes heat or gets cold (temperature change), makes light, new state of matter. They are only clues to a chemical reaction occuring - you need to know what the molecules are doing to be sure.
Article with good graphic on the difference between physical and chemical changes: https://www.thoughtco.com/physical-and-chemical-changes-examples-608338
I often start with making mixtures with chosen solids and liquids (include vinegar to see a chemical reaction), then follow with another activity focused on physical change and/or chemical reacctions.
But any activities can be paired.
Run making mixtures as a play-debrief activity, as cited in the resource.
They will make many different mixtures. Allow students time to play, during which students will tend to mix everything all together. After a while, encourage them to simplify their ingredients to find out which minimal ingredients make one type of mixture.
For the debrief, write up all that they discover, and try to tease out the different kinds of mixtures and what minimal ingredients produced the different textures, colours (and chemical change).
Optionally discuss possible activities that students might follow up with, to explore some of these changes in more detail. Guide them through the scientific process of using controls and changing one variable at a time, before returning students to their experimenting.
Talk about mixtures around us:
Our world is made of things that have been mixed together to make new useful textures and shapes. e.g. concrete is made from sand, gravel and cement (a powder of rocks including chalk, clay and iron ore). When they are combined and dried they make hard concrete that we can build with. e.g. steel playground structures are made from iron (extracted from rocks) mixed with other chemicals.
Focus on one kind of mixture:
Physical changes
Focus on flour/cornstarch and water that makes variously goopy mixtures.
There is no chemical reaction, but there is a physical change as the molecules mix together to make new textures.
Students can continue making mixtures to make and test for the best glue..
Give students a recipe for individual or a larger batch of ooblek for students to play with.
Make an Epsom salt crystal painting as the water evaporates from a solution.
Chemical reactions
Focus on baking soda and vinegar chemical reaction that makes bubbles:
Tell students we will use the same solid, baking soda, to make a soda drink or rocket.
For older students, add in molecule modelling of the reaction.
A different chemical reaction: Elephant's toothpaste demonstration, a dramatic demonstration of a chemical reaction (oxygen bleach making oxygen gas) held in a foam mixture by the dish soap.
A candle burning is a chemical change. Students can use molecule models to see the chemical reaction
Red cabbage dye is a chemical change. The colour changes are a clue that it is a chemical reaction.
