Question

Why are chemical changes considered "unseen"?

a) "Because they always produce gas"
b) "Because they produce visible flames"
c) "Because they result in the formation of new substances"
d) "Because they occur very slowly"

Answer 1

Chemical changes are considered "unseen" because they involve the formation of new substances at the molecular level, which cannot be directly observed. The correct answer to the question is 'Because they result in the formation of new substances'.

The correct option is c.

Step-by-step explanation:

Chemical changes are considered "unseen" because they result in the formation of new substances. These are changes at the molecular level where old bonds break and new bonds form, creating different substances with distinct properties.

During a chemical reaction, we may observe signs like temperature changes, light production, unexpected color changes, or the formation of bubbles (gas), but we cannot actually see the molecules changing. Moreover, these changes can be either rapid or slow and may or may not require an input of energy (heat).

For instance, when a candle burns, the wax undergoes a chemical change to produce water and carbon dioxide. Similarly, the burning of natural gas in a furnace transforms methane and oxygen into water and carbon dioxide, which demonstrates a clear chemical change with the creation of substances that have different chemical properties than the original ones.

The correct option is c.

Answer 2

Chemical changes are considered "unseen" because they involve the formation of new substances with different properties, and the molecular changes are not visible to the bare eye. So, the correct option is C.

Step-by-step explanation:

Although some chemical reactions are evident, like the burning of gasoline where flames and heat are produced, we cannot directly observe the molecular changes happening at the atomic level.

Examples of chemical change include the burning of a candle where wax reacts with oxygen to form water and carbon dioxide or the reaction of methane in a furnace producing water and carbon dioxide. The original molecules in these reactions alter their structures, resulting in new molecules with distinct properties. Hence, even if some chemical changes have visible effects like color change or bubble formation, the actual chemical changes—molecules breaking and forming bonds—are happening on a microscopic level that we cannot see with the bare eye.

Therefore, C is the correct option.