性質 and 性格: material properties and personal traits
Compare dissolving substances and quick-drying fabric with a cheerful younger sister and twins whose personalities differ, retaining the representative subjects.
性質 in substances and fabric
「水に溶けやすい物質の性質を調べた。」 (We examined the properties of substances that dissolve readily in water.) 性質 (せいしつ) means nature or property. The completed examination concerns properties of readily dissolving substances, rather than a person's personality.
「この布は乾きやすい性質を持っている。」 (This fabric has the property of drying quickly.) The fabric has a quick-drying property. 乾きやすい states that characteristic without specifying a fixed drying time or a numerical material measurement.
性格 in a younger sister and twins
「妹は明るい性格で、初対面の人ともすぐ話せる。」 (My younger sister is cheerful and easily strikes up conversations with people she has just met.) 性格 (せいかく) means personality or character. The younger sister's cheerful trait is linked to talking easily with people she has just met. It remains a description of this individual.
「双子でも二人の性格は驚くほど違う。」 (Although twins, their personalities are remarkably different.) These two twins have remarkably different personalities. 双子でも keeps their twin relationship alongside the stated difference rather than supplying a rule about all twins.
Read the representative subjects without an exclusive rule
In these sentences, 性質 describes properties of substances and fabric, while 性格 describes the sister's and twins' personalities. Identify what dissolves or dries and whose personal trait is being described.
The material/person comparison follows these representative examples. It does not restrict every possible use of the two words to mutually exclusive classes of objects and people.
What is described by the dissolving 性質 and cheerful 性格?
Show the answer and why
The first describes a material property, and the second the younger sister's personality. This identifies the subjects in these examples without imposing an exclusive objects-versus-people rule.