Understanding Matter Particles Through Observation

The question asks for the best evidence that matter is composed of tiny particles. This idea is fundamental to the particle theory of matter, which states that all matter is made up of constantly moving particles (atoms, molecules, or ions) separated by empty space.

Analyzing the Options

• Option 1: A drop of ink spreads uniformly in water
  This phenomenon, known as diffusion, directly demonstrates the particle nature of matter. The ink particles move randomly from the region of higher concentration (the drop) to the region of lower concentration (the water) until they are evenly distributed. This movement shows that both ink and water are made of particles that are in constant motion and have space between them.
• Option 2: Water boils at $100^\circ C$
  This describes a physical property, the boiling point. While boiling involves particles gaining enough energy to change state (from liquid to gas), it's a consequence of particle behavior rather than direct visual evidence of the particles themselves spreading out.
• Option 3: Ice melts to form water
  Similar to boiling, melting is a phase change. It shows how particles change their arrangement and movement with temperature, but it doesn't illustrate the constant random motion and spreading as clearly as diffusion does.
• Option 4: Metals conduct electricity
  Electrical conductivity in metals is due to the movement of free electrons. While electrons are particles, this property is specific to metals and relates more to electrical phenomena than the general principle that all matter is particulate.

Conclusion

The uniform spreading of ink in water (diffusion) is the strongest indicator among the choices that matter is made of particles, as it visually represents their inherent random motion and tendency to occupy available space.