Physical or Chemical?

Learning Objectives

In this activity, students will experience physical and chemical changes. Through their observations, students will note the differences between these processes and use their understanding of these differences to classify five different processes as chemical or physical.

Materials

• Old pennies
• Small dish
• Vinegar
• Glass cup
• Sugar
• Warm water
• Stirring stick/spoon
• 3 Deep set plates / pans (for the milk, vinegar, and soda water)
• Milk (preferably homogenous)
• Food colouring
• Dish detergent
• 2 medium sized pop bottles
• Baking soda
• Carbonated water (or soda pop…. But this is sticky)

Motivation

This activity is best done towards the end of your unit on physical and chemical changes. The reason for this is because in this activity, the students are asked to draw upon their knowledge of the changes, and to determine which of the processes are physical and which are chemical. While introducing the activity, it would be useful to remind students that chemical changes involve processes in which the substances present at the beginning of the change are not present at the end and new substances are formed. A chemical change can not be “undone”. Common indications of chemical changes are colour change, bubbles, the formation of a new substance or the emission of a gas. In a physical change, the material itself is the same before and after the change, although some extensive properties (like shape, phase, etc.) of the material changes. The change can be “undone.”

Part 1 is a chemical change. The rust on the penny reacts with the vinegar, which is why it gets removed from the penny. Part 2 is physical. Substances that are small and light enough can dissolve in water, and remain suspended between the water molecules, so that it seems they have disappeared. As the solution dries up, however, the sugar will reappear unchanged. Part 3 is a physical reaction as well, though it may not look like it. The soap breaks the surface tension of the milk, which causes currents. These milk currents carry the food colouring with them, which is why we witness the colours spreading out. Part 4 is a chemical reaction. Vinegar is acidic and baking soda is basic. When you mix the two, they react to form gaseous carbon dioxide. The bubbles observed are due to carbon dioxide escaping. Part 5 seems like it might be a chemical change as well, since bubbles are produced, but actually this is a physical change. Carbon dioxide is already dissolved in the carbonated water (hence the name) and shaking the closed bottle causes pressure to build up inside. When you open the bottle, that pressure is released, and so is the carbon dioxide.

In your discussion of this activity, you should try to clear up any troubles the students are having. For example, a student may wonder why the penny is a chemical reaction since it appears that we start and end with a penny and vinegar. However, the rust is actually a different substance than the penny, and it has reacted and mixed with the vinegar, which is why it is gone.

Procedure

Part 1

Shiny New Penny. Place an old rusted penny into a small dish of vinegar and wait for 30 seconds. Pull it out, and it should be shiny and looking just like new. Physical or chemical?

Part 2

Disappearing Sugar? Pour some sugar into a cup of warm water and start to stir. As the sugar dissolves into the water, it starts to disappear. Physical or chemical? Make note of reasoning on the worksheet.

Part 3

Milk Fireworks Put 2-3 drops of different coloured food colouring onto different spots on a plate with some milk in it. Then, drop some dish detergent in or around your drops and watch the colours spread! Physical or Chemical? Make note of reasoning on the worksheet.

Part 4

Bubbles! Fill a 700 ml pop bottle with 100 ml of vinegar. Make sure there is a decent sized plate or dish underneath the bottle to catch overflow! Now add a spoon of baking soda, close the cap, and give one little shake. Now open the pop bottle. Physical or chemical? Make note of
reasoning on the worksheet.

Part 5

Bubbles again? This time, fill a 700 ml pop bottle about half way with carbonated water. With the lid on, give it a quick shake, then open it up. Have a plate or dish ready as well. Physical or chemical? Make note of reasoning on the worksheet.

Investigating Questions

• Was Shiny New Penny a physical or chemical change? Why?
• Was Disappearing Sugar a physical or chemical change? Why?
• Was Milk Fireworks a physical or chemical change? Why?
• Was Bubbles! a physical or chemical change? Why?
• Was Bubbles again? a physical or chemical change? Why?
• What are the differences between physical and chemical changes?
• What signs can we look for to determine whether a change is physical or chemical?