Which of the following is not one of the three basic phases of matter?

• A. Solid
• B. Liquid
• C. Gas
• D. Plasma

Answer: (D)

Matter can exist in different forms called phases. The familiar trio describes how matter behaves under ordinary conditions: a solid holds its own shape and volume, a liquid keeps its volume but flows to fill the container, and a gas expands to fill the space available. Plasma is different because it occurs at much higher energy levels, where atoms are so energetic that electrons are stripped away, creating a hot, ionized mix of ions and free electrons. This state conducts electricity readily and responds to magnetic and electric fields, which sets it apart from the three basic phases. Plasma appears in stars, lightning, neon signs, and many high-energy processes. Because of these distinct properties, plasma is considered a fourth state of matter, not one of the three basic phases.