C. The lowest mass will be the element in period 1, group 1 (this element is hydrogen)
Answer : The balanced reduction half-reaction is:

Explanation :
Redox reaction or Oxidation-reduction reaction : It is defined as the reaction in which the oxidation and reduction reaction takes place simultaneously.
Oxidation reaction : It is defined as the reaction in which a substance looses its electrons. In this, oxidation state of an element increases. Or we can say that in oxidation, the loss of electrons takes place.
Reduction reaction : It is defined as the reaction in which a substance gains electrons. In this, oxidation state of an element decreases. Or we can say that in reduction, the gain of electrons takes place.
The given balanced redox reaction is :

The half oxidation-reduction reactions are:
Oxidation reaction : 
Reduction reaction : 
In order to balance the electrons, we multiply the oxidation reaction by 2 and reduction reaction by 3 and then added both equation, we get the balanced redox reaction.
Oxidation reaction : 
Reduction reaction : 
The balanced redox reaction will be:

Thus, the balanced reduction half-reaction is:

Answer:
Chemical reaction.
Step-by-step explanation:
In the experiment, a catalyst is added to concentrated hydrogen peroxide.
The hydrogen peroxide <em>decomposes</em> rapidly, according to the equation
2H₂O₂(ℓ) ⟶ 2H₂O(ℓ) + O₂(g)
New substances are formed, and old ones disappear, so this is a chemical reaction.
The reaction also releases a <em>large amount of heat</em>.
The rapidly-expanding oxygen gas caries with it droplets of water and hydrogen peroxide (and probably some steam, as well).
The visual effect is like a genie escaping from its bottl<em>e</em>.
Answer : The half-life of the compound is, 145 years.
Explanation :
First we have to calculate the rate constant.
Expression for rate law for first order kinetics is given by:

where,
k = rate constant = ?
t = time passed by the sample = 60.0 min
a = let initial amount of the reactant = 100 g
a - x = amount left after decay process = 100 - 25 = 75 g
Now put all the given values in above equation, we get


Now we have to calculate the half-life of the compound.



Therefore, the half-life of the compound is, 145 years.