What changes occur when adding differing amounts of acids & bases?


> 
> In an experiment, $\pu{5 cm^3}$ of $\pu{1 mol/dm^3}$ sodium
>  hydroxide are gradually added to $\pu{10 cm^3}$ of $\pu{1 mol/dm^3}$ hydrochloric acid containing methyl orange. What changes occur in the mixture?
> 
> 
> 1. The concentration of $\ce{H+}$ ions increase (*decreases*)
> 2. The methyl orange changes colour
> 3. More water molecules are formed (*water is formed, but use of the word 'more'??*)
> 4. A precipitate is formed (*no ppt formed, $\ce{NaCl}$ is soluble*)
> 
> 
> 


I thought the answer would be (2). However when thinking about the question the methyl orange would be red in the acid and given the proportions of the reactants and the 1:1 mole ratio, the $\ce{HCl}$ would be in excess, so would it still change colour?




It is perhaps difficult to comment on the intended semantics of the option 3, but I think that is the most appropriate answer.


Option 1 and 4 are wrong because of the reasons you cite. 


To comment upon option 2, we need to know the pH range over which this indicator changes colour. [According to Wikipedia](https://en.wikipedia.org/wiki/Methyl_orange), an unequivocal shift from red to yellow occurs from a pH of $3.1$ to a pH of $4.4$. For our case, we would end up with a $\pu{\frac 13 M}$ solution of $\ce{HCl}$, which, assuming complete dissociation and negligible contribution of $\text{H}^+$ from water, we end up with a pH of around $0.48$ which is well in the no-colour-change region of the pH values.


Regarding option 3, the 'more' just refers the water molecules formed as a result of neutralisation reaction, which are additional to the ones already present in either of the mixing solutions. Along with the formation of 'new' $\ce{NaCl}$ molecules (as in they did not exist bonded to one another before), water molecules are also formed. 


$\ce{NaOH + HCl -> NaCl + H2O}$, i.e. table salt and water


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