Physical Properties of Water — explained for a 13-year-old

Water is one of the most important substances on Earth. Below are its main physical properties, what they mean, why they matter, and easy experiments you can try.

1. States (solid, liquid, gas)

• Water can be a solid (ice), a liquid (water), or a gas (water vapor/steam).
• Which state you get depends on temperature and pressure. At normal air pressure (1 atmosphere):
• Freezes (solidifies) at 0°C (32°F).
• Boils (turns to vapor) at 100°C (212°F).

2. Density and why ice floats

• Density of liquid water is about 1.00 g/cm³ at 4°C. That means 1 cubic centimeter of water has a mass of about 1 gram.
• Water is unusual because it reaches its maximum density at 4°C. When it freezes into ice, it becomes less dense (ice ≈ 0.92 g/cm³) because the water molecules form a crystal structure with more space between them.
• Because ice is less dense than liquid water, ice floats. This is very important for life — ice on top of lakes insulates the liquid water below so fish can survive winter.

3. Specific heat (water holds heat well)

• Specific heat is the amount of energy needed to raise the temperature of 1 gram of a substance by 1°C. For water this is about 4.18 J/g·°C — one of the highest for common substances.
• This means water heats up and cools down slowly. That property helps stabilize climates near oceans and keeps organisms’ bodies from changing temperature quickly.

4. Latent heat (melting and boiling use a lot of energy)

• Energy required to melt ice (heat of fusion): about 334 J/g.
• Energy required to turn water into steam (heat of vaporization): about 2260 J/g.
• Because these are large, melting and evaporating water take a lot of energy—this is why sweating cools you (evaporation uses heat from your skin).

5. Surface tension, cohesion, and adhesion

• Surface tension is the "skin" on the surface of water caused by water molecules pulling on each other. For water at ~20°C it's about 0.073 N/m.
• Cohesion = attraction between water molecules (makes drops hold together). Adhesion = attraction between water and other materials (helps water cling to glass).
• Because of surface tension, small insects (like water striders) can walk on water, and droplets form near-spherical shapes.

6. Capillary action

• When adhesion to a surface (like glass or plant tissue) is strong, water can climb upward in narrow spaces against gravity. This is how water travels up plant stems and paper towels soak up liquid.

7. Viscosity and flow

• Viscosity is how "thick" or resistant to flow a liquid is. Water has a low viscosity (~0.001 Pa·s at 20°C), so it flows easily.

8. Transparency and color

• Water is clear in small amounts so light passes through, which allows sunlight to reach plants in lakes and oceans. In large amounts, pure water has a faint blue tint.

9. Thermal conductivity and electrical conductivity

• Thermal conductivity (around 0.6 W/m·K) means water conducts heat moderately well; this helps spread heat in oceans and in our bodies.
• Pure water is a poor electrical conductor, but normal water contains minerals and salts that let electricity pass through it easily.

10. Solvent behavior (why water dissolves many things)

• Water is called a "universal solvent" because it can dissolve many substances. This is because water molecules are polar and attract other charged or polar substances, helping them separate into solution.
• Although dissolving is often considered a chemical property, the way water physically surrounds and separates particles is a physical process you can observe (salt dissolving, sugar dissolving).

Simple experiments you can try at home or in class

1. Ice floats: Put an ice cube in a glass of water. Observe it floats. Explain: ice is less dense because of its crystal structure.
2. Surface tension test: Gently place a small paperclip or sewing needle on the surface of water in a bowl. It can float if placed carefully because of surface tension. Try adding a drop of soap — surface tension will break and the object sinks.
3. Capillary action: Put one end of a paper towel strip into colored water and watch the color move up the strip. This shows adhesion and capillary action.
4. Specific heat demo (safe): Heat equal volumes of water and vegetable oil on low heat (with adult supervision!) and measure temperatures with a thermometer over time. Water will warm more slowly than oil because of its higher specific heat.

Quick facts (numbers to remember)

• Freezing point: 0°C (32°F)
• Boiling point (1 atm): 100°C (212°F)
• Maximum density: at 4°C (~1.00 g/cm³)
• Ice density: ~0.92 g/cm³
• Specific heat: 4.18 J/g·°C
• Heat of fusion: ~334 J/g; heat of vaporization: ~2260 J/g
• Surface tension (20°C): ~0.073 N/m

Short summary

Water's physical properties—its three states, its unusual density behavior (ice floats), high specific heat, strong surface tension, and capillary action—make it essential for life and affect weather, climate, and how living things work. Simple experiments let you see these properties in action.

Quick questions to test yourself

1. Why does ice float on liquid water?
2. What does it mean that water has a high specific heat, and why is that important?
3. Give one example of how surface tension affects everyday life.

If you want, I can give answers to those questions or suggest a short experiment you can do and record results from. Which would you like?