A boat full of iron scrap is floating on water in a lake . If all iron is dropped into water , level of water will fall dowm. Why ?

If something is submerged in water, it will displace an amount of water equal to its volume. If something is floating in water, it will displace an amount of water equal to its weight.

Initially, iron and boat is floating. So, water displaced is equal to weight of iron+weigt of boat.
Now, iron is dropped in water, but boat is still floating.
So, now,
Volume of water displaced= Weight of boat+volume of iron.
We know, Density of iron is greater than that of water..
Density= mass/ volume.
As density is higher for iron, its volume will be less than its mass/ weight. So, Volume of water displaced in second case when iron is dropped is less than when boat is filled with iron .
So, water will drop/ fall when you dump iron in water.

Therefore, if you have anything more dense than water (iron) floating in a boat and you throw it into the water, the water level will go down because the object displaces less water when submerged, than when it is floating.