Solution: The puffed puri would float on the water. This is because the puri is filled with air, which makes it less dense than water. The air trapped inside gives it buoyancy, allowing it to stay on the surface.

Solution: If a steel plate is placed carefully on the surface of the water, it would likely float. This is because its shape allows it to displace a volume of water that weighs more than the plate itself. However, a steel spoon, being smaller and denser, would displace less water relative to its weight and would therefore sink.

Solution: The cap of a plastic bottle would float on water. Plastic is generally less dense than water, and the air trapped inside the cap further reduces its overall density, enabling it to float.

Solution: Yes, I have observed that some objects float while others sink. This happens because of the difference in density between the object and the water. An object floats if its weight is less than the weight of the water it displaces (buoyancy). If the object's weight is greater than the weight of the water it displaces, it sinks.

Solution: This is an experimental activity. Here are the expected observations:

Things to be put in water | I guessed before I did it | I saw when I did it

Empty bowl (Katori) | $(\backslash checkmark)$ | $(\backslash checkmark)$

After putting in 6-7 small pebbles, one-by-one | (X) | (X)

Iron nail or pin | (X) | (X)

Matchstick | $(\backslash checkmark)$ | $(\backslash checkmark)$

Empty plastic bottle with its lid closed | $(\backslash checkmark)$ | $(\backslash checkmark)$

Bottle half-filled with water | $(\backslash checkmark)$ | $(\backslash checkmark)$

Bottle full of water | (X) | (X)

Aluminium foil

open and spread out | $(\backslash checkmark)$ | $(\backslash checkmark)$

pressed tightly into a ball | (X) | (X)

in a cup like shape | $(\backslash checkmark)$ | $(\backslash checkmark)$

Soap cake | (X) | (X)

Soap cake on a small plastic plate | $(\backslash checkmark)$ | $(\backslash checkmark)$

A piece of ice | $(\backslash checkmark)$ | $(\backslash checkmark)$

Solution: After discussing with other groups, we found that objects like an empty bowl (katori), matchstick, empty plastic bottle, half-filled bottle, spread-out aluminium foil, aluminium foil in a cup shape, and soap cake on a plate floated. Objects like pebbles, iron nails, full water bottle, tightly balled aluminium foil, and soap cake (without a plate) sank.

Solution:

a) The iron nail sank in water but the katori floated. I think this happened because the iron nail is denser than water, while the katori, being hollow and shaped like a bowl, displaces a larger volume of water, making its effective density less than water.

b) The empty plastic bottle floated on water. The bottle filled with water sank because the empty bottle contains air, making it light and buoyant. When filled with water, its weight increases significantly, making it denser than the water it displaces.

c) The aluminium foil floated when it was spread out. When pressed tightly into a ball it sank. This may have happened because when spread out, the foil could trap air and displace a larger volume of water, reducing its overall density. When balled up, it becomes more compact and denser, causing it to sink.

Solution: Yes, after adding salt to the water, the lemon, which initially sank, started to float. As more salt was added, the lemon rose higher in the water.

Solution: After adding salt, the water became denser. This increased density provided enough buoyant force to overcome the weight of the lemon, causing it to float. The salt dissolved and increased the mass of the water without significantly increasing its volume, thus increasing its density.

Solution: To dissolve sugar quickly, Hamid can try the following methods:

1. Stirring the mixture: Stirring helps to move the sugar crystals away from the undissolved lump and brings fresh water into contact with them, speeding up the dissolution process.
2. Warming the mixture: Heating the water increases the kinetic energy of the water molecules, causing them to collide more frequently and with more force with the sugar crystals, thus dissolving them faster.

Solution: Here are the observations from the experiment:

Things | Did it dissolve or not? | What happened after keeping for 2 minutes?

Salt | Yes | Salt and water got mixed together completely. Water remained transparent.

Soil | No | Soil settled down at the bottom. Water became muddy.

Chalk powder | No | Chalk powder settled down at the bottom. Water became whitish.

1 spoon milk | Yes | Milk and water got mixed together completely. Water became whitish.

Oil | No | Oil floats on the surface of water.

Solution: No, I could not see the salt after it dissolved in water. This is because salt is soluble in water, meaning it breaks down into tiny particles that are evenly distributed throughout the water, becoming invisible to the naked eye.