1. Variable Composition:
* Pure substances have a fixed and definite composition. This means the ratio of elements in the substance never changes.
* Not a property: Mixtures have variable compositions. For example, saltwater can have different amounts of salt dissolved in water.
2. Heterogeneous Appearance:
* Pure substances are homogeneous, meaning they have a uniform appearance and composition throughout.
* Not a property: Mixtures can be heterogeneous, meaning they have distinct parts with different properties. Think of a salad with different types of vegetables.
3. Separable by Physical Means:
* The components of a pure substance cannot be separated by simple physical methods like filtration, evaporation, or magnetism.
* Not a property: Mixtures can be separated by physical means. For instance, sand and water can be separated by decanting (pouring off the water).
4. Varying Chemical Properties:
* Pure substances have consistent chemical properties. They react in predictable ways under given conditions.
* Not a property: Mixtures can have varying chemical properties due to the presence of different components.
Examples:
* Pure substance: Water (H₂O) is always made up of two hydrogen atoms and one oxygen atom, regardless of its source.
* Mixture: Milk is a mixture of fats, proteins, and water. It can have varying compositions depending on the type of milk (skim, whole, etc.).
In essence, the defining feature of a pure substance is its constant and consistent composition. Anything that deviates from this is not a characteristic of a pure substance.
