Describe what happens to the particles in a chocolate bar as it melts.
a. as the chocolate bar changes from a solid to a liquid a chemical change happens.
b. as the chocolate bar changes from a solid to a liquid the particles slow down.
c. as the chocolate bar changes from a solid to a liquid, the particles move the same speed but they take on the shape of their container
d. as the chocolate bar changes from a solid to a liquid the particles move around more.

The correct answer is D. As the chocolate bar changes from a solid to a liquid the particles move around more.

Explanation:

Changes in the state of a substance (for example from solid to liquid) affect the arrangement and motion of the particles. In this way, in the solid-state particles are organized in a fixed structure and only vibrate; however, as the substance is heated and the state of the substance changes (for example, the chocolate bar is melted) particles gain energy, and this makes them move around more, which causes liquids to have an undefined shape. According to this, in the case of a chocolate bar "As the chocolate bar changes from a solid to a liquid, the particles move around more".

a

answer: semi-infinite solid initially at a uniform temperature of 400 ◦c is cooled at its surface by convection. the cooling fluid at a constant temperature of 200 ◦c has a convective coefficient (ha) of 250 w/m2.k. the physical properties of the solid: k = 20 w/m.k and α = 4 * 10-5  m2/s. using δx = 0.04 m and δt = 10 s. write the finite difference equations describing the transient heat transfer of the first five δx and the temperature profile after 30 s.