for question 2c earlier - Would you describe the effect of heat on the white powders as a physical change or a chemical change? give one reason for your answer. - My sister told me the answer would be for whichever one is more difficult to reverse but I don't know which one that is. I think it might be a chemical change but if the actual substance didn't change than it should be easy to reverse right?

The original post was a day or so ago and I don't remember all of the question. Have you identified the two white powders? If so, post that information and I can respond.

Well I know the first two answers. 2a) was sublimination- turning directly from a solid to a gas and I worked out that 2b) was zinc oxide- it turned yellow on heating and white on cooling.

Changing a solid to a gas by sublimation is a physical change because it was the same material before and after; i.e., the composition has not changed. I also think the ZnO is a physical change since the composition did not change. (Some oxides decompose on heating to give the metal + oxygen but I don't think ZnO is one of those).

Determining whether the effect of heat on white powders is a physical change or a chemical change involves understanding the characteristics of each type of change.

A physical change refers to a change in the physical properties of a substance, such as its shape, size, or state of matter, without altering its chemical composition. In contrast, a chemical change involves a rearrangement of atoms or molecules, resulting in the formation of new substances.

In the case of the white powders, if heating causes them to change color, release gas, or form new substances, it would be indicative of a chemical change. On the other hand, if the powders only undergo a change in temperature or state of matter (such as melting or evaporating) without any significant alteration in their chemical composition, it would be considered a physical change.

To determine the difficulty of reversing a change, you can consider the ease of returning the substance back to its original state. Physical changes are generally easier to reverse since they do not involve the formation of new substances. For example, if a white powder melted due to heat, it could be easily cooled back down to revert to its solid state.

In the case of a chemical change, reversing the process is often more complex, if not impossible, as it involves breaking chemical bonds and reforming them in the original arrangement. If the white powders undergo a chemical change upon heating, it might be challenging or even impossible to reverse the process completely.

Considering these factors, if the white powders undergo a chemical reaction, it would be considered a chemical change. However, if the effect of heat only causes a change in the physical properties of the powders, such as melting or evaporating, it would be deemed a physical change.

It's important to note that actual experiments or observations with specific white powders would be necessary to provide a definitive answer.