固体（こたい、kotai）
- solid

The Japanese word 固体 (こたい, *kotai*) means "solid" or "solid matter." It refers to a state of matter where substances have a definite shape and volume, and the particles (atoms or molecules) are closely packed together, resulting in strong intermolecular forces. This state is characterized by rigidity and resistance to compression. 固体 is often used in scientific contexts, describing materials that are not liquids or gases.

⚫︎ 水は温度が０度以下になると固体になります。 Translation: Water becomes a solid when the temperature is below 0 degrees Celsius. ⚫︎ 鉄は丈夫な固体です。 Translation: Iron is a strong solid. ⚫︎ 科学の授業で、様々な種類の固体について学びました。 Translation: We learned about various kinds of solids in science class.

This word is used when discussing materials in a solid state. It's common in science, engineering, and everyday conversation when describing materials like rocks, metals, ice, and wood. You would use it to differentiate a substance from its liquid or gaseous forms. For instance, when describing the properties of a material or explaining a phase change.

⚫︎ The word is primarily written using the kanji 固体. ⚫︎ The reading is *kotai*. ⚫︎ There is no variation in the okurigana.

There aren't direct synonyms that fully replace 固体, but words that relate to the properties of 固体 could be considered similar in meaning depending on the context: ⚫︎ 固い (*katai*) - hard, firm (describes the physical property) ⚫︎ 物質 (*busshitsu*) - substance, matter (broader term that includes solids)

⚫︎ 液体 (*ekitai*) - liquid ⚫︎ 気体 (*kitai*) - gas

この金属は常温で固体です。 ・Translation: This metal is a solid at room temperature. ・Explanation: This sentence describes the state of the metal at room temperature.

氷は水の固体の形態です。 ・Translation: Ice is the solid form of water. ・Explanation: This sentence explains that ice is the solid state of water.

The word 固体 is a compound word: 固 (*ko*, meaning "solid," "firm," or "hard") and 体 (*tai*, meaning "body" or "form"). This combination of characters literally translates to "solid body" or "solid form," directly reflecting the concept of a substance in its solid state.