The best way to explain is with an example.
Sodium reacts violently on contact with water, while chlorine is a highly toxic greenish-yellow gas.
However, sodium chloride, the compound which contains these two elements, is harmless common salt, showing the properties of an element are very different from the properties of that element’s compounds.
The black powder used to produce a photocopy is finely ground carbon in its elemental form.
The particles are extremely small and arranged at random. Any light which falls on them is absorbed and not re-emitted, so the powder looks black. The sandwich certainly contains carbon but not in its elemental form.
Here, it is combined with oxygen and hydrogen as carbohydrates.
These compounds have their own properties which are nothing like the properties of their constituent elements. The slices of bread emit light of many wavelengths reasonably well, so when we look at bread in daylight, it appears white.
Carbon is normally found as an amorphous solid, which means it lacks a definite crystalline structure. Because of this, and because of the position of certain electrons in the outer orbits of the carbon atom, light is absorbed and not re-emitted.
This means the carbon atoms in graphite, soot and carbon black appear black.
Diamond, which is also carbon, is normally clear, because its crystalline structure alters the electrons and their positions to create a colorless crystal. Diamonds can be colored if other atoms, usually metals, are present and alter the electron bonds to create blue, yellow, pink and green versions.
Carbon, as present in foodstuffs such as bread and potatoes, is in hydrated form, the carbon has been chemically bonded with water and so does not appear black.
To get the black carbon back you need to remove the water, usually by heating.
This is why burnt toast is black.
Sugar is also carbon and water. But add concentrated sulphuric acid and you’ll see black carbon appear as the acid efficiently sucks out the water.
