Different materials have different densities. Density is a characteristic property of a substance and can be used to identify the substance. Materials which are less dense than water float in water. Materials denser than water sink in water.
Strength
The strength of a substance is its load-carrying ability. that is, its ability to support a load without breaking or tearing. Gold is not a very strong substance and bonds easily on applying load. Pure gold cannot be used make jewellery. To make jewellery, its strength is increased by adding other metals to it. Similarly steel is stronger than iron and is used for construction. Steel consists of iron with carbon added to it.