Fill up a bowl with water and put an egg carefully into the bowl. Watch the egg sink to the bottom. Remove the egg from the bowl and put it somewhere safe at room temperature. Leave it for a week, then put the egg back into a bowl of water. The egg sinks, but not so quickly. Repeat the process using the same egg a week later. The egg doesn't sink to the bottom. This is because as the egg gets older, gases build up inside the eggshell, because the egg is beginning to decompose. Gas is lighter than water, so it makes the egg start to float. The more the egg readily floats, the older it is, so if you want to be sure your egg for breakfast is fresh, put it in a bowl of water first.
Chop up a few purple cabbage leaves. Note the color of the leaves. Put the leaves in a pot of boiling water and let them cook for about 10 minutes. Drain the water from the pot. Check the color of the leaves. They have turned almost blue. Now add some more water into the pot and boil the water. Add a couple of tablespoons of white vinegar. Watch the color of the leaves change from blue to purple or dark red. Purple cabbage contains anthocyanins, which give the leaves the purple color, but they change color, depending on their pH level -- pH is a measurement between acidity and alkaline. Boiling the cabbage in water lowers the acid level and turns it alkaline. Adding vinegar increases the pH level making it acidic.
Some foods freeze well, while others don't, so there must be an underlying reason for this. Separate an equal number of strawberries into two portions. Have your fifth-graders put the strawberries into two plastic containers with lids. Put one in the refrigerator and the other into the freezer. Do exactly the same thing with peas; they must be fresh peas. Leave the strawberries and peas overnight so the items in the freezer have a chance to freeze fully. Remove the strawberries and peas from the freezer and let them defrost fully. Remove the other two items from the refrigerator. Compare the fresh and frozen fruit and vegetables. The frozen strawberries are mushy, but the fresh ones are fine. However, both the frozen and fresh peas are fine. This is because strawberries mainly consist of water, unlike peas. Water crystallizes when frozen and expands, which breaks down the molecular structure. When the strawberries defrost, they are mushy, as the internal molecular structure has broken down.
Have your fifth-graders fill one glass with tap water and another with fizzy water. Drop five or six raisins into each glass. Tell them to watch what happens. The raisins in the tap water sink to the bottom of the glass and remain there. However, the raisins in the fizzy water initially sink, but then rise to the surface, stay there for a while and then sink back to the bottom. This is because the fizzy water contains carbon dioxide, which is a gas, and once the fizzy water is put into a glass, the carbon dioxide is released and bubbles float to the surface. The bubbles attach themselves to the rough edges on the raisins and when enough bubbles are attached, the raisins are lifted to the surface. Once the raisins reach the surface, the gas bubbles burst so the raisins sink back to the bottom.