Add bleach (about 10 mL) - chemical change. It takes a few minutes but the water will go back to clear.
Baking soda in water - physical change (solubility)
Baking soda in vinegar - chemical change.
If you have a flask or a bottle of water you can do this one and right after you add the baking soda put a balloon over the top and as it mixes the gasses will blow up the balloon. You can also do this with water and baking soda and they can see that nothing happens to the ballon.
1. Alka-Seltzer in a 35 mm film canister. Fill half-way with water and add a tablet. Put the lid on and set it down. You can even give it a little shake. I definitely make sure my kids wear goggles when we do this because some lids really take off.
2. Mentos and Coke: Buy a 2L of Coke and Dt.Coke (room temp.) and 2 rolls of Mentos (regular). I put the Mentos in a test tube and covered with my business card over top of the opened 2L. Remove the card and run because this is an instant reaction. This is a great activity to do OUTDOORS. This could be a fun, but expensive experiment to change a few variables around to see what would happen.
3. Another twist to making steel wool rust is to dip it into vinegar and let it sit on a paper towel. It will start to rust in about 5 min. Of course, if you leave it overnight, the rust is even better.
4. a burning candle or tea light
5. Curdle milk with vinegar (cider type). This looks like cottage cheese and really smells!!!
6. Dancing raisins: Mix a solution of water and vinegar and add baking soda. Gas bubbles visably form on the raisins which cause them to rise, then lower once the bubbles pop.
One thing that I find cool is that if you mix hydrogen peroxide and yeast together in a sealed plastic bag, it makes oxygen. To test that, break a Popsicle stick in half and light the end on fire. Blow out the flame but leave some embers. Quickly, open the bag and put the Popsicle stick in it. The popsicle stick will catch back on fire! This isn't a chem/phys change but it's a cool experiment. Give it a try!
Place an ice cube in a snack size zip lock baggie. Time how long it takes to melt. You might put one in the window sill and another somewhere else in the room.
When the cube is melted add a Alka-Seltzer tablet. Then zip the snack size baggie closed again. The tablet will dissolve in the water and the chemical reaction will produce gas and the snack size baggie will inflate well.