Notes For All Chapters – Geography Class 9th
1. Introduction
What is Sea Water?
- Sea water is the water in oceans, the largest water storage on Earth.
- It tastes salty because it has dissolved salts.
What We’ll Learn:
- Main properties: Temperature, Salinity, and Density.
- How these properties change and affect the ocean.
2. Temperature of Sea Water
What is Temperature?
- Temperature tells us how hot or cold the sea water is.
- It changes depending on where you are and how deep you go.
Surface Temperature:
- Equatorial Areas (near equator): Around 25°C (warm due to strong sunrays).
- Mid-Latitudes (middle areas): Around 16°C.
- Polar Areas (near poles): Around 2°C (cold due to less sunlight).
Factors Affecting Temperature:
- Latitude: Temperature drops from equator to poles.
- Ocean Currents: Warm currents increase temperature, cold currents decrease it.
- Other Factors: Cyclones, rainfall, sea waves, seasons, and pollution.
Temperature with Depth:
- Sunrays heat the surface, but less heat reaches deeper water.
- Temperature decreases up to 2000 meters depth.
- After 2000 meters, it stays steady at 4°C everywhere (equator to poles).
- Water doesn’t freeze at great depths because it stays above 0°C.
Differences:
- Equatorial areas: Temperature drops fast with depth.
- Polar areas: Less change because surface is already cold.
- Landlocked Seas vs. Open Seas: Landlocked seas are warmer due to higher salinity.
3. Salinity of Sea Water
What is Salinity?
- Salinity is the amount of salt dissolved in sea water.
- Measured in parts per thousand (e.g., 40g salt in 1kg water = 40%).
Why is Sea Water Salty?
- Salts come from rocks on land, carried by rivers into the sea over millions of years.
- Evaporation removes water, leaving salt behind.
Uses of Salt:
- For cooking, making chemicals, medicines, preserving food, and in ice factories.
- We get salt from salt-pans near the sea.
Effects of Salinity:
- Increases buoyancy (helps ships float).
- Too much salinity can harm sea life.
Factors Affecting Salinity:
- Evaporation: More evaporation = higher salinity (e.g., tropical zones).
- Fresh Water Supply: Rivers, rainfall, and snow lower salinity (e.g., polar areas).
- Location: Landlocked seas (e.g., Dead Sea) have higher salinity than open seas (e.g., Atlantic Ocean).
Salinity Around the World:
- Equatorial Zone (0°-15°): 34% (high evaporation but lots of rain and rivers like Amazon).
- Tropical Zone (15°-35°): 37% (hot deserts, less rain, high evaporation).
- Temperate Zone (35°-65°): 33% (more river and snow water, less heat).
- Polar Zone (65°-90°): 31% (low evaporation, lots of snowmelt).
Special Seas:
- Mediterranean Sea: 39% (landlocked, high evaporation).
- Red Sea: 41% (hot, little fresh water).
- Bal Wtic Sea: 7% (lots of river water, low evaporation).
- Dead Sea: 332% (very high salinity, no life except tiny organisms, people float easily).
How to Measure Salinity:
- Use tools like hydrometer, refractometer, or salinometer.
- Example: 35g salt in 1kg water = 35% salinity.
4. Density of Sea Water
What is Density?
- Density is how heavy or packed the water is.
- Depends on temperature and salinity.
How Temperature Affects Density:
- Cold water is denser (sinks).
- Warm water is less dense (rises).
How Salinity Affects Density:
- More salt makes water denser.
- Temperature has a bigger effect than salinity.
Changes with Depth:
- Surface water (up to 500m): Affected by sun, currents, and wind; density changes.
- Deep water (below 1000m): Temperature, salinity, and density stay steady (around 4°C).
Why It Matters:
- Differences in density create ocean currents.
- Currents control Earth’s climate by moving heat around.
Leave a Reply