🔹 What is density?

Density tells us how much mass is packed into a certain volume.For water (or anything), the formula is:

Density=Mass / Volume

• If you have 1 liter of water and it weighs 1 kg, the density is 1 kg per liter (or 1000 kg/m³).

• The denser a material is, the more “stuff” is packed in a certain space.

  
  

Water’s density:

• At 4°C, pure water has a density of 1000 kg/m³ (1 g/cm³).

• When it freezes into ice, the density drops because the molecules spread apart. That’s why ice floats.

  
  

🔹 What is buoyancy?

Buoyancy is the upward force that a fluid (like water) applies on an object that is placed in it.It comes from the difference in pressure between the top and bottom of the object.

• The deeper you go into water, the higher the pressure.

• The bottom of an object feels more pressure than the top.

• This pressure difference creates an upward push: the buoyant force.

  
  

🔹 Archimedes’ Principle:

The buoyant force on an object is equal to the weight of the water displaced by that object.

• If you drop something in water, it pushes water out of the way.

• The water “wants” to come back, and the reaction is an upward force equal to the weight of the displaced water.

  
  

🔹 Connection between density and buoyancy:

• If the object’s density is less than water, it displaces enough water to balance its weight before it sinks completely → It floats.

• If the object’s density is more than water, it can’t displace enough water to balance its weight → It sinks.

  
  

Example:

• Wood floats because its density is lower than water.

• Steel sinks because its density is higher—unless it’s shaped like a boat so it displaces more water and lowers its overall density.

  
  

🔹 Why water is special:

1. High density compared to air: That’s why buoyancy in water is much stronger than in air.

2. Anomalous expansion: Water’s density is highest at 4°C; when it freezes, it expands, making ice float.

3. Salt and temperature: Adding salt or making water colder (above freezing) increases its density. This changes buoyancy:

   • In the Dead Sea (very salty), you float easily.

   • In warm freshwater, it’s harder to float because density is slightly lower.

     
     

🔹 Everyday examples:

• Ships: Heavy ships float because their shape makes them displace a lot of water.

• Fish: They control buoyancy using a swim bladder, adjusting their density.

• Submarines: They fill tanks with water to sink (increase density) and pump water out to rise (decrease density).

• Icebergs: Float with most of their mass underwater because ice is slightly less dense than water.

  
  

✅ In deep simple words:

• Density measures how tightly packed matter is in a space.

• Buoyancy is the upward push from water caused by pressure differences.

• The fight between the object’s weight and the buoyant force decides whether it sinks or floats.

• Water’s own unique density behavior (like ice floating) makes buoyancy in water different from most other fluids.