For statements (I) and (II), state for each if they are true or false. (I) Group one and group two metal oxides are basic anhydrides. (II) Group one and group two metal oxides can be formed by removing water from the corresponding metal hydroxide. If both are true, state if (II) is a correct explanation for (I).
Let’s begin with statement (I). Group one is on the far left of the periodic table and includes elements like lithium, sodium, and potassium. Group two is also on the left and includes elements like magnesium and calcium. We’re being asked about metal oxides. These include both metal atoms and oxygen atoms. We’re being asked if these metal oxides are basic anhydrides. Remember that basic means that it forms a basic solution, and anhydride means without water, “an” meaning without. So in this instance, we’re talking about a metal oxide which forms a basic solution when dissolved. And it’s formed by the elimination of water from something else.
To help answer this question, let’s look at some of the group one and group two metal oxides. Here we have the formula for sodium oxide, potassium oxide, and magnesium oxide. We can work out the formula for sodium oxide by remembering that oxygen forms a two minus ion. And sodium in group one tends to form a one plus cation. So to balance out the charge on the oxygen, we need two sodium ions. On the other hand, magnesium in group two forms a two plus cation, so we only need one magnesium to balance out the charge on one oxygen. To find out whether these metal oxides are basic, we need to dissolve them in water.
Let’s see what we form in each example. In the case of sodium oxide, when we dissolve it in water, we get sodium hydroxide, NaOH. The same is true for our other metal oxides. So do these form basic solutions? You should be familiar with hydroxide ions, OH−, and know that when we have hydroxide ions dissolved in water, it is indeed a basic solution. So we know that group one and group two metal oxides are basic. But are they anhydrides? When we look at the formula for each of our metal oxides, we can see that there is indeed no water in our formula. None of them have any hydrogen atoms, so we can assume that statement (I) is true.
Let’s look at statement (II). Statement (II) says that group one and group two metal oxides can be formed by removing water from the corresponding metal hydroxide. Really, we’ve already answered this. We’ve already seen that if we add water to our metal oxide, we form a metal hydroxide. So if we go the opposite way, we can remove water from our hydroxide to form our metal oxide again. This is simply the opposite way. So statement (II) is true. You can form group one and two metal oxides by removing water from the corresponding metal hydroxide.
Because both statements are true, we need to answer the last part of the question. Is statement (II) a correct explanation for statement (I)? Statement (II) says that metal oxides are formed by the removal of water, the elimination of water. And in statement (I), we see that metal oxides are anhydrides, meaning without water. So the reason that group one and two metal oxides are anhydrides is indeed because they can be formed by removal of water from the corresponding metal hydroxide. So the last part of the question is also true.