Answer:
3. (a) more dense (b) sinks to the bottom
4. (a) less dense (b) rises above
Explanation:
To understand this, first you need to know what density is. Density is the quantity of mass per unit. If something is dense it has many molecules per unit of volume. If something is not so dense it has few molecules per unit of volume.
Cold water is close to becoming ice. In ice (solid), the molecules are compacted very tightly, therefore there are more molecules per unit of volume. This means that ice is very dense. In cold water, the molecules are close but not as close as in ice. This means that cold water is dense.
Hot water is close to becoming a water vapour. In water vapour (gas) the molecules are spaced very far apart and move around freely. They have lots of energy so they vibrate and move around a lot. There are less molecules per unit of volume in water vapour. This means that water vapour is not dense. In hot water, the molecules are not so far apart as the molecules in water vapour. This means that hot water is not so dense. But it is still considerably denser.
Salt water has two different types of molecules. The salt molecules like to cluster around the water molecules very close together. This makes salt water more dense than just plain water (freshwater).
