Water is the most common liquid we work with, and it has a known density:

• Density of water = 1,000 kg/m³ (or 1 g/cm³)

This value becomes our reference point for predicting whether an object will sink or float.

When an object is placed in water, we compare its density to the density of water:

• If the object's density is greater than 1,000 kg/m³ → the object sinks
• If the object's density is less than 1,000 kg/m³ → the object floats
• If the object's density is equal to 1,000 kg/m³ → the object stays suspended (neither sinks nor floats)

Why Does This Happen?

Think of water particles being packed tightly together. When an object denser than water is placed in it, the object is "heavier" for its size than the water it displaces, so it sinks. When an object is less dense, it is "lighter" for its size than the water it displaces, so it floats.

A simpler way to determine sinking and floating is to use relative density (also called specific gravity). This is the ratio of an object's density to the density of water:

$\text{Relative Density (R.D)} = \frac{\text{Density of object}}{\text{Density of water}}$

Since water has a density of 1 g/cm³ or 1,000 kg/m³, the relative density is simply the object's density expressed as a number without units.

The relationship becomes very simple:

• If R.D > 1 → the object sinks
• If R.D < 1 → the object floats
• If R.D = 1 → the object stays suspended

A piece of dry wood has a density of 700 kg/m³. Predict whether it will sink or float in water.

Solution:

Step 1: Compare the density of wood to water.

• Density of wood = 700 kg/m³
• Density of water = 1,000 kg/m³

Since 700 < 1,000, the wood is less dense than water.

Step 2: Use relative density:

$\text{R.D} = \frac{700}{1000} = 0.7$

Since R.D = 0.7, which is less than 1, the wood will float.

From the densities of common substances:

You can discover this relationship using a tank of water and a few objects:

1. Fill a clear container with water.
2. Gather objects like a small stone, dry wood, a metal key, and a plastic bottle cap.
3. Gently place each object in the water and observe what happens.
4. Feel the wet objects after removing them — the ones that sank feel heavier.
5. Now compare: objects that sank have higher density than water, while objects that floated have lower density than water.

This is exactly how scientists deduce the relationship between density, sinking and floating!

In Tanzania, this concept is used when fishing on Lake Victoria. Traditional fishing floats (made of lightweight wood or plastic) are designed to float because they have low density, while fishing net weights (made of stone or metal) sink because they are denser than water. Understanding density helps fishermen choose the right materials for their equipment to ensure nets stay upright in the water.