Air is a mixture because:1) Its components vary from place to place2) Its components continue to vary3) Its components can be seperated by physical means
Water is a compound because:1) Its components are bound together chemically2) Its components always occur in a fixed proportion -- be it tap water, sea water, etc..

Reasons why air is a mixture:

1. Composition:
- Air is a mixture because it is composed of various gases such as nitrogen, oxygen, carbon dioxide, argon, and trace amounts of other gases. These gases are not chemically combined with each other but exist as individual entities in the air.
- The composition of air can vary depending on the location, altitude, and presence of pollutants. For example, in urban areas, air pollution can introduce additional gases and particles into the mixture, altering its composition.

2. Variable Proportions:
- Another reason why air is considered a mixture is that the proportions of its components can vary. The percentage of each gas in the air is not constant and can change due to natural processes, human activities, and geographical factors.
- For instance, the concentration of oxygen is higher at lower altitudes, while nitrogen is more abundant at higher altitudes. Similarly, the presence of pollutants can significantly alter the proportions of gases in the air, affecting its quality.

3. Physical Separation:
- The components of air can be physically separated from each other. This is demonstrated through processes such as fractional distillation, where air is cooled and compressed to separate its components based on their boiling points.
- For example, fractional distillation can be used to separate liquid nitrogen and liquid oxygen from air. This ability to separate the components of air indicates that it is a mixture rather than a compound.

Reasons why water is a compound:

1. Chemical Composition:
- Water is a compound because it is composed of two different elements, hydrogen and oxygen, chemically bonded together. The chemical formula for water is H2O, representing two hydrogen atoms bonded to one oxygen atom.
- This chemical bond between hydrogen and oxygen is a result of the sharing of electrons, forming a stable molecule with distinct properties. The composition of water is fixed, with each molecule always containing two hydrogen atoms and one oxygen atom.

2. Fixed Proportions:
- Unlike a mixture, the proportion of hydrogen to oxygen in water is fixed. For every molecule of water, there are always two hydrogen atoms and one oxygen atom. This fixed ratio is essential for the properties and behavior of water.
- Any deviation from the ratio of two hydrogen atoms to one oxygen atom would result in a different substance altogether. This consistent composition is a characteristic of compounds.

3. Unique Properties:
- Water exhibits unique physical and chemical properties that are distinct from its constituent elements. These properties, such as high boiling point, surface tension, and polarity, arise from the arrangement and interaction of water molecules.
- The hydrogen bonds formed between water molecules contribute to its cohesive and adhesive nature, allowing it to form droplets, capillary action, and surface tension. These properties are specific to water due to its compound nature.

In summary, air is a mixture because it is composed of various gases with variable proportions that can be physically separated. On the other hand, water is a compound because it consists of hydrogen and oxygen chemically bonded together in fixed proportions, exhibiting unique properties distinct from its constituent elements.