Mixtures behave in ways that are quite complex, especially when we compare them to pure substances. This complexity makes it tough to analyze them in thermodynamics.

1. Complex Interactions: Pure substances have clear phase diagrams, which show how they change states (like solid, liquid, and gas). But mixtures can have many different ingredients. This means that their interactions can be unpredictable, making it hard to use traditional rules, like Gibbs' phase rule.

2. Variable Composition: The way mixtures behave depends a lot on what they are made of. Even small changes in the amount of each ingredient can cause big shifts in the phase diagram. This can complicate how we predict phase changes.

3. Dew and Bubble Points: Figuring out the dew and bubble points (the temperatures and pressures where gas and liquid phases exist) in mixtures can be tricky. It often requires difficult calculations and experiments that might not be easy to access.

Solutions:

• Use of Advanced Models: We can use special formulas, called equations of state (EOS), like Peng-Robinson or Soave Redlich-Kwong, to better understand these interactions.
• Simulation Software: There are advanced computer tools and phase diagram software that can help us visualize and predict how mixtures behave under different conditions.

Even with these solutions, the complex nature of mixtures makes studying their phase behavior a challenging task in thermodynamics. It requires careful thought and detailed analysis.