Evaporation happens at the liquid’s surface, while the volume of liquid increases with the amount. When you double a soup in the same pot, it boils across the same surface area, so it loses roughly the same amount of water in the same amount of time. The proportional loss is cut in half, and the soup stays thinner than the recipe’s intended consistency. That is why you should not simply multiply the water when scaling up a recipe.
Most ingredients increase linearly when you scale up a recipe; water does not, for a geometric reason. In a boiling pot, water evaporates from the surface where the liquid meets the air. The diameter of the pot determines the size of that surface, which stays the same as long as the pot does not change. By contrast, the amount of liquid added to the pot increases with volume. So when the liquid doubles, the amount that evaporates stays the same, and the proportional loss is cut in half. Even if the pot is doubled in size, its surface area does not double, because surface area grows with the square of the diameter.
The result is easy to see in the kitchen. A single serving of soup loses a considerable amount of water and thickens in forty minutes, while twice as much soup cooked in the same pot loses proportionally much less in the same time and stays thin. The recipe writer adjusted the consistency for their own pot and quantity; that consistency does not happen automatically in a pot with twice as much soup. That is why cooking and serving a larger batch of soup for the recipe’s stated time almost always produces a thinner result than expected. For the same reason, dishes cooked in large cauldrons are simmered longer than home-cooked versions.
The right approach is to add less water than the multiplied amount calls for. Multiply the solid ingredients by the scaling factor, but add less water than the recipe’s scaled amount—for example, just enough to cover the ingredients. Check the consistency as it cooks and add hot water if needed. You can always add more; you cannot take it back out. The same approach works for stews, sauces, and the liquid served over rice; rice is the exception, because its grain-to-water ratio must stay fixed, so if you need to adjust there, change the pot instead of reducing the water.
The second way to speed up thickening is to increase the surface area. If you transfer the same volume to a wider pot, the evaporating surface also grows, bringing the proportional loss closer to its previous level. That is why, for a larger batch of soup, choosing a wider rather than deeper pot keeps the consistency closer to the recipe. The lid is part of the same equation: with the lid on, steam condenses and drips back, and evaporation nearly stops. If you want the soup to thicken, leave the lid ajar or remove it completely near the end. Leaving the lid ajar is a middle ground that lets steam escape without stopping the boil.
Adjust salt separately in response to this change. When soup thickens less than expected, the salt is also less concentrated than expected; in other words, the soup may taste undersalted even if you multiplied the recipe’s salt amount. Conversely, when soup is simmered for a long time to thicken it, the salt becomes more concentrated as water evaporates, and an amount that was right at the start may end up being too much. That is why you adjust the salt after the consistency has set; any decision about salt made while the liquid amount is changing is always an early one.
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Suyu da ikiyle çarpıp çorbayı sulandırmak benim klasik hatamdı, buharlaşmanın yüzeyle ilgili olması akla yatkın.