Learn Spanish (Spain): El café solo está muy caliente.

In Spain, un café solo literally means a coffee alone. This is the standard way to order a black coffee because it is simply coffee with no milk added.

Why is está used here instead of es?

We use está (from the verb estar) because the temperature of the coffee is a temporary condition. Even though coffee is meant to be hot, its current temperature will change as it cools down. We use ser (es) for permanent characteristics.

Would caliente change its ending if the noun was feminine?

No, adjectives that end in -e stay exactly the same for both masculine and feminine nouns. So, whether it is el café (masculine) or la sopa (feminine, the soup), you still use caliente.

Why can't I use calor instead of caliente to mean hot?

Calor is a noun that means heat and is used to talk about the weather or a person feeling hot (like hace calor). To describe the physical temperature of an object like coffee, you must use the adjective caliente.