A student takes four samples of a silver, metallic solid and and subjects each piece to different treatment. Piece 1: The solid is hammered into a flat sheet. Piece 2: The solid is heated in a flame until it melts. Piece 3: The solid fizzes when acid is dropped on the surface. Piece 4: The solid is place in a beaker of water and it floats.

Explanation:

A chemical change  is only manifested when a chemical reaction occurs, which means that the original substance is transformed into one or more different substances. In a chemical change either the original substance decomposes (by action of the heat, light or other influence) or combines with other substances, becoming a new substance.

Piece 1: The solid is hammered into a flat sheet.

In this case the silver has not been transformed into a different substance; it has only changed its shape. Thus, this is just a physical change.

Piece 2: The solid is heated in a flame until it melts.

Again, the final substance is still silver. It has only suffered a change of state. Thus, this is a physical change too.

Piece 3: The solid fizzes when acid is dropped on the surface.

Fizzing is a sign that a new substance has formed. In this case, the acid reacted with the piece of silver, forming a compound of silver and a gas (likey hydrogen).

Typically metals react with acids forming a salt ans hydrogen gas.

Piece 4: The solid is place in a beaker of water and it floats.

The silver has not suffered any change.

pre-sliced bread is a relatively recent development in baking technology. previous to that, it was always sold in a bakery in daily-made contiguous loaves with no wrapping, and was moderately perishable, usually being used with a day or two (which is why every bakery had a discounted “day-old” bread table.) therefore, pre-sliced bread would grow stale quickly. the hard crust provided a protective layer, and once the exposed dry slice left from the previous cut was removed, the interior was still soft.

what keeps bread soft is the moisture retained in the crumb (the light spongy part that’s not crust). the reason the crust develops is the surface of the dough gets exposed to hot dry air during baking which browns the outer layer and seals it, preventing all the moisture from escaping the dough. longer exposure to air will also eventually cause your bread to lose flavor and become stale.

people were used to cutting slices as they needed them and storing bread in a specially-made wooden or metal breadbox or pail with a lid to preserve it longer, but the first automatically sliced commercial loaves were produced on july 6, 1928, in chillicothe, missouri, using a machine invented by otto rohwedder, an iowa-born, missouri-based jeweler.[1] pre-sliced bread would indeed dry out, were it not for another 20th century technology: the plastic bag. bread could now be pre-sliced, and if the enclosing bag remains sealed as much as possible, the bread will remain soft and fresh for longer than if sitting unwrapped on a shelf or enclosed in a bread box that is not air-tight. sealed packaging meant bread could also be sold on store shelves and eliminate frequent trips to a bakery.

sealing bread in a plastic bag had another effect: the moisture in the loaf will re-distribute into the crust and soften it, which is sometimes more pleasing in a sandwich - especially for children whose teeth might not yet be strong enough to tear through a hard crust. a homemade loaf of bread can be sealed in a bag overnight and the crust will become soft like store-bought sandwich bread.

these were novel changes in how bread was produced and sold, which led to the coining of the phrase “the greatest thing since sliced bread.”

"record the time to complete a chemical reaction" would most likely be part of a lab procedure.