By: Aamarpali Puri
Experiment .1: Which grape is heavier?
(Cohesion and adhesion)
Material required: Fresh grapes, 7-Up Soda, Clear drinking glass
Method: Pour and fill a glass with 7-Up Soda. Peel one grape and take an unpeeled grape. The two grapes are dropped at the same time in glass. The unpeeled grape rises to the top of the cup while the peeled grape remains at the bottom.
- Why does the peeled grape sink to the bottom?
- Which of the two grapes is lighter in weight?
- What makes the unpeeled grape float?
- Would an unpeeled grape also float in water?
Explanation: This activity relates adhesive and cohesive forces (CO2 bubbles adhering to the grape skin) and the effects of hydrophobic and hydrophilic properties. This demonstration is suitable for studying bonding interactions, i.e. hydrogen bonds. The unpeeled grape has water-repelling–hydrophobic–properties and thus the CO2 bubbles from the 7-Up can adhere to this surface. The peeled grape does not have hydrophobic skin, thus it is hydrophilic–water-attracting. The bubbles of the 7-Up have no way of adhering to the grape. This causes the grape to stay at the bottom of the glass. The unpeeled grape becomes lighter in weight, because of the adhering bubbles, and rises to the surface of the liquid. There it loses some of the bubbles to become heavier again. It may sink for a while, picking up more bubbles, to bob up to the surface again.
Precautions: The soda used should be non flat.
Experiment .2: Stem end blackening in potato.
Material Required: Potato, water, burner, baking soda, tartar, pH paper
Method: A potato is cooked in boiling water for 40 minutes. After being removed from the water and allowed to cool, the end of the potato that has been connected to the plant will produce a dark gray color. This is called “Stem-end Blackening” region.
Why a particular portion of potatoes shows change in colour after boiling.
What is the effect of pH of water on stem end blackening?
Explanation: The reason for this color change is that iron ions in the potato are experiencing a “redox” reaction. During cooking ferric (Fe+3) ions are formed from ferrous (Fe+2) ions by the gaining of electrons from phenolic substances in the region where darkening has occurred.
The water in which we are boiling potatoes should be checked for pH because this “Stem-end Blackening” is pH sensitive. Neutral and basic cooking solutions produce a greater amount of blackening effect, and acidic water causes little to no color change.
Note: Baking soda will be used as a sweetening agent to make the basic cooking solutions, and cream of tartar will serve in making the acid solution. Universal indicator paper will be used to check that each solution is in the specified pH range.
Experiment .3 Negative inflation
Material Required: Baloon, Plastic bottle, hot water, cold water
Method: Fill a plastic (PET) bottle with hot water (NOT boiling water) and fill a bowl with cold water. Let them sit for one minute, then empty the bottle quickly. Stretch a balloon over the open end of the bottle and push the bottle down into the cold water. The balloon is pulled in and get inflated inside the bottle.
Question 1. : Why it happens that when you squeeze egg completely surrounding it with your hand it doesn’t break but it breaks if you are wearing ring?
Answer 1. Eggs are surprisingly strong and will stand up to forces applied evenly to them. They don’t stand up so well to uneven forces, which is why they crack easily on the side of a bowl or break when they hit the ground. Hold an egg, completely surrounding it with your hand. Squeeze it, making sure that the pressure is even all around. You should be able to squeeze it very hard without breaking it (do this over a sink or bowl just in case). The egg may break if it is already slightly cracked, or if you are wearing a ring.
Questions 2. : Why does the balloon inflate into the bottle?
Answer 2. The warm water heats the bottle which, in turn, heats the air inside the bottle after the water is poured out. When the bottle is placed in the cold water, the air inside cools and contracts, causing outside air to be drawn in, pulling the balloon in and inflating it inside the bottle.