Substituting one rice for another usually produces a dish that is wrong in a specific way rather than merely different. The cause sits inside the grain, in the proportion of two starch molecules.
Two starches with opposite behaviour
Rice starch consists of amylose, which is long and straight, and amylopectin, which is branched. Every variety contains both in a different ratio.
Amylose molecules realign as the rice cools and set into a firm structure, which keeps grains separate and gives a drier result.
Amylopectin holds water and stays soft and sticky, so varieties high in it cling together and yield the texture that chopsticks and hand-eating require.
Cuisines chose the grain their dishes needed
Long-grain varieties with high amylose separate cleanly, which suits dishes where the rice must absorb sauce without turning into a mass.
Short-grain varieties with high amylopectin hold together, which is what makes formed rice possible and what allows a risotto to become creamy without cream.
Neither is a better rice. They are answers to different questions about how the finished plate is meant to be handled.
Age and processing change the same variety
Freshly harvested rice holds more moisture and cooks softer, while aged rice has dried and absorbs more water, expanding further and staying firmer.
This is why some cuisines specifically prize older rice for dishes that require distinct, elongated grains.
Milling matters too: removing the bran layer lets water reach the starch quickly, which is why brown rice of the same variety takes far longer and never softens the same way.
Rinsing and soaking are variety-specific instructions
Washing removes loose surface starch released during milling, which would otherwise thicken the cooking water and glue grains together.
Cuisines that want separation therefore wash thoroughly, while dishes that depend on that released starch for their texture skip it deliberately.
Soaking hydrates the grain before heat arrives, which shortens cooking and reduces the risk of a soft exterior around a hard core in long grains.
Substitution fails in predictable directions
Using a high-amylose rice where a sticky one belongs gives a dish that will not hold together and a mouthfeel that reads as dry.
Using a sticky rice where separation is needed produces clumping, and no amount of adjustment to water or heat recovers it.
The reliable approach is to match the starch behaviour first and treat grain length, aroma and origin as secondary preferences.