(1) When an object is submerged in water, the volume of boiling water is equal to the volume of the object; When ρ=mV, the mass is equal, and the larger the volume, the smaller the density.

So, the answer is: the volume of boiling water; ρ= mV； Small;

(2) The principle of measuring solid density is ρ=mV, m is measured with a balance, and volume is measured with a measuring cylinder according to the drainage method.

So the answer is:

② Measure a proper volume of water with a measuring cylinder (V 1).

(3) Tie the metal block with a thin thread and sink it into the water of the measuring cylinder, and read out the total volume V2 of the water and the metal block.

④ρ=mV2？ V 1