What causes a change of state in matter?

• A. Adding or removing heat energy.
• B. Changing the color of matter.
• C. Changing the pressure only.
• D. Mixing with water.

Answer: (A)

Change of state happens when energy is added to or removed from a substance. Heat transfer changes the energy and motion of the particles, and when enough energy is provided to overcome the forces holding a solid together, it melts into a liquid. With more energy, a liquid reaches its boiling point and becomes a gas. Removing heat reverses these steps: a gas can condense into a liquid, and a liquid can freeze into a solid. The exact temperatures at which these changes occur depend on the substance, and during a phase change the temperature stays the same until the change is complete because the added energy goes into breaking or forming intermolecular bonds rather than raising temperature. Pressure can shift the conditions a bit, but heat energy is the direct driver of state changes. Color change or mixing with water doesn’t inherently cause a change of state.