Chapter Six Test: Compounds And Elements

The observation that baking soda loses mass when heated and gives off a gas indicates that it is not an element. Elements are pure substances that cannot be broken down into simpler substances by chemical reactions. When baking soda loses mass and releases a gas, it is undergoing a chemical reaction, suggesting that it is a compound rather than an element.

The correct answer is that the ratio of the volume of hydrogen produced to the volume of oxygen produced does not depend on any of the above because water is a compound. This is because water is made up of two hydrogen atoms and one oxygen atom, so the ratio of their volumes produced during electrolysis is always 2:1, regardless of factors such as the duration of electrolysis, temperature, or amount of water used.

When hydrogen and oxygen are mixed together and a spark is added, they react to form water through a chemical reaction called combustion. However, in this case, the amounts of hydrogen and oxygen are not equal. According to the balanced chemical equation for the reaction, 2 molecules of hydrogen combine with 1 molecule of oxygen to form 2 molecules of water. Therefore, if there is an excess of oxygen (10 cm3), it will be left over after the reaction is complete, while all the hydrogen will be used up to form water.

If getting the original substance back requires a chemical reaction, it implies that the original material underwent a chemical change. Compounds are substances that are formed when two or more elements chemically combine, resulting in a new substance with different properties. Therefore, if a chemical reaction is required to obtain the original substance, it suggests that the original material was most likely a compound.