Hueco vs. vacío
The Spanish words hueco and vacío can both describe something that lacks contents, but they have distinct applications. The word hueco refers to a physical cavity, meaning something is structurally hollow or has a gap. Conversely, vacío translates directly to empty, indicating that a space, container, or even a person is completely devoid of contents or feelings.
Hueco
The word hueco describes an object that has an empty space or a cavity within its physical structure, translating best to hollow. It is used for solid things that have an internal void, physical holes in a surface, or open gaps in a schedule.
El árbol en el patio trasero es hueco por dentro.
(The tree in the backyard is hollow inside.)
Golpeó la pared para comprobar si el muro era hueco.
(He knocked on the wall to check if the structure was hollow.)
Compré un conejo de chocolate hueco para la celebración de Pascua.
(I bought a hollow chocolate bunny for the Easter celebration.)
Ten cuidado de no meter el pie en ese hueco de la calle.
(Be careful not to put your foot in that hole in the street.)
Tengo un hueco en mi horario a las tres de la tarde para atenderte.
(I have a gap in my schedule at three in the afternoon to help you.)
Vacío
The word vacío means empty and focuses on the complete absence of contents, people, or emotions within a given space. It describes a temporary or permanent lack of things inside a container, a vacant room, or a figurative emotional void.
El vaso de agua que dejé en la mesa está completamente vacío.
(The glass of water I left on the table is completely empty.)
La sala de cine estaba casi vacía cuando comenzó la película.
(The movie theater was almost empty when the film started.)
Después de escuchar las malas noticias sintió un gran vacío en su corazón.
(After hearing the bad news she felt a great emptiness in her heart.)
Por favor tira esta caja de cartón vacía en el contenedor de reciclaje.
(Please throw this empty cardboard box in the recycling bin.)
Sus promesas resultaron ser palabras vacías sin ninguna intención real.
(His promises turned out to be empty words with no real intention.)
Summary
In summary, you should use hueco when talking about a structural cavity, a gap, or a solid object that is physically hollow inside. On the other hand, you should use vacío to describe a container, physical space, or emotional state that is currently empty or devoid of contents.