Before we discuss how substances change, let’s talk about how we identify them. Substances have physical and chemical properties, or traits. You can use those traits to identify a substance. A substance’s physical properties include its shape, volume and freezing point. You can measure and observe these properties without changing the composition of a substance. For example, you can freeze liquid water to observe its freezing point without changing what the water is made of. The liquid water would become ice, but each molecule of water still contains two atoms of hydrogen and one of oxygen. A substance’s chemical properties include its pH level, flammability and reactivity with oxygen. You can’t measure these properties unless you also change the way the atoms or molecules are arranged.

In the passage above, what does the “composition” of a substance mean