At 6pm, you see logs piled up for a campfire. later you see flames and feel heat from it. at 8pm, only smoldering ashes remain. what changed have taken place? how can you tell?

The process that took place is called oxidation or burning where wood was burnt into ashes.

Wood is an organic compound formed of different types of carbohydrates, proteins and lipids. These components do have carbon oxygen hydrogen as most abundant atoms and also has some amount of nitrogen, phosphorous, and other metal ions like magnesium, calcium etc.

When the logs were burnt, the chemical reaction took place between the atmospheric oxygen and the atoms present in the wood, forming different oxides. As carbon was the most abundant atom present, a huge smoke containing carbon dioxide and carbon monoxide evolved. The hydrogen became the water vapour, and other gases were also formed like nitrogen dioxide, sulphur dioxide etc. These gases went off into atmosphere. The rest that were left behind were mixture of unburnt part of wood as well as the solid metal oxides. There were unburnt particles of carbon mixed with them too.