Substance Z, with a density of 0.98 g/cm³, will float on the surface of water because its density is less than that of water, which typically has a density of around 1.0 g/cm³.
The question asks which substance listed in a table with given densities will float on the surface of water. The density of water is typically around 1.0 g/cm³. Substances with a density less than that of water will float on it, while those with a density greater than water will sink. Based on the densities provided for substances W, X, Y, and Z, we can determine that substance Z, with a density of 0.98 g/cm³, is less dense than water and therefore will float.
It's important to remember that the specific gravity of a substance indicates how it will behave in water. If a substance's specific gravity is less than one, it will float. Since the density of substance Z is less than the density of water, its specific gravity is also less than one, hence it will float on water. Substances W, X, and Y all have densities greater than that of water and will sink.