 Let's look at combination reactions. Now these are reactions in which two or more species combine to form a single product. So we can represent it in this form where let's say we have A and B combining to form this product AB and these can be free elements or compounds that lead to the formation of this product AB. So one very common example of combination reactions is when one of these reactants has oxygen. So a lot of the oxidation reactions are combination reactions. Like here for example we have carbon in graphite form reacting with oxygen to form carbon dioxide. Another example of a combination reaction is the reaction of sulphur trioxide with water giving this product which is aqueous H2SO4. And this reaction may look unfamiliar but you probably have heard about this reaction. So this reaction is what leads to the phenomena that is called acid rain. So where industry pollutants which contain this SO3 in the atmosphere react with the water in the rain resulting in acid rain but this is also an example of a combination reaction. Another example is this reaction of magnesium oxide with carbon dioxide to form magnesium carbonate. And again you can see how magnesium oxide and carbon dioxide combine to form this magnesium carbonate. One thing that is usually observed in combination reactions is that because reactants combine to form the product new bonds are formed between the reactants. And we know that when new bonds are formed energy is released. So usually combination reactions are exothermic in nature. And one more point here is that although we have only represented this reaction having two reactants, combination reactions are not restricted to only two reactants. There could be two or more reactants forming a single product. So let's take another example. So in this case we have nitrogen reacting with water and oxygen to give ammonium nitrate. So here you can see that there are three reactants combining to give one product. Now in all the examples that we saw before the oxidation numbers of the reactants usually changed. Like if we take this example if we look at the oxidation states of the reactants nitrogen and oxygen have an oxidation state of zero because they are free species. Hydrogen here in this case will have an oxidation number of plus one and oxygen will have an oxidation number of minus two. Now in the case of this ammonium nitrate this is a slightly tricky example and it's not very straightforward. But looking at this compound we know something about both of these ions that is the NH4 plus ion and the NO3 negative ion. So we know that the charge on the NH4 positive ion is plus one and on the NO3 negative ion is minus one and we know that this is a neutral compound. So the sum of oxidation numbers of these ions individually should be zero which we can see from here because if the charge here is plus one and it is minus one their sum is zero. So using this idea if we calculate the oxidation number of nitrogen here we know that the sum of the oxidation numbers should be plus one. So let's say the oxidation number of nitrogen is x plus four times the oxidation number of hydrogen which is usually plus one is equal to one. So if we solve this we get the oxidation number of this nitrogen to be minus three. Now in this case if we carry out a similar exercise let's say the oxidation number of nitrogen is x plus three times the oxidation number of oxygen which is usually minus two should be equal to minus one which is the charge on this NO3 negative. So if we solve for this we find out that x will be equal to plus five. So this nitrogen will have an oxidation number of minus three and this nitrogen will have an oxidation number of plus five. So in both of these cases if we look at the change in the oxidation numbers nitrogen in one case is going from zero to minus three and it is also going from zero to plus five. So because there is an oxidation and a reduction happening here this is a redox reaction. Then the question is can we say that all combination reactions are reduction reactions? Not necessarily because for a reduction reaction there must be change in these oxidation numbers. There is a possibility that a combination reaction happens without change in the oxidation numbers. One such example of that is this reaction where quick lime reacts with water to form calcium hydroxide. Now if we look at the oxidation numbers in this case we know that oxygen is usually minus two and since quick lime here is a neutral compound the oxidation state of calcium should be plus two here. Similarly for this hydrogen it will be plus one. This oxygen will be minus two and in this case here we know that hydroxide in an ion form has a charge of negative one. So if the hydroxide ion has a charge of negative one and there are two such ions so the net charge on this part will be minus two. So the corresponding oxidation number of calcium here will be plus two. So here if we look at the change in oxidation numbers for calcium there is no change in the oxidation number because it's plus two on the reactant and the product side and for oxygen and hydrogen also because the oxidation states are minus two and plus one again there is no change in the oxidation numbers for oxygen or hydrogen as well. So this is an example where the reaction is not a redox reaction but still it is a combination reaction.