Pearson Edexcel Comb. Sciences – investigating pH on powdered calcium hydroxide to hydrochloric acid

This Pearson Edexcel (9-1) Combined Sciences Core Practical for Chemistry video with take you through an investigation of the change in pH on adding powdered calcium hydroxide to a fixed volume of hydrochloric acid. For more information about Pearson Edexcel GCSE (9-1) Sciences, please see here:

Hi i’m paul roberts i’m a chemistry teacher here at andover college and today i’m going to talk you through how to carry out core practical neutralizing an acid now although this is a very straightforward practical i’m going to give you some hints and tips of how you can get some really good results so first of all we’re going to take 50 centimeters cubed of

One molar hydrochloric acid and add it to a beaker good idea to use a 250 centimeter cube beaker for this as you’re going to be stirring it and gives the the solution room to move just with a glass stirring rod it doesn’t have to be glass but i’m just going to put a dot of the acid on a piece of universal indicator wait a few moments and see the color change okay

So that’s our initial ph and looking at my um swatch card here that’s a ph of round about one so we’re going to start the neutralization and as i mentioned before the neutralization is going to be using calcium hydroxide now for each time you perform the neutralization you’re going to add point three of a gram of calcium hydroxide i’ve measured some out already

On the balance here and i took time to tear the balance before weighing it and just add it into the pot using the glass rod now to stir and you’ll notice a couple of things happening that it will start off being quite cloudy and this is because the calcium hydroxide isn’t very soluble it takes time for it to dissolve but over time it will dissolve and once all

The calcium hydroxides dissolved into solution and then reacted with the acid the solution should become colorless again you might notice a very small amount of um fizzing taking place in the beaker and that’s because calcium hydroxide typically has a little bit of calcium carbonate impurities in it so don’t be alarmed if you see a few bubbles appearing okay

So now you can see the solution is colorless and i can again perform another ph check noticing that the ph hasn’t really changed it’s still around ph1 you could alternatively at this point be using a ph meter you’re going to get uh more more precise results but still we’d expect so still a low ph and now we’re going to continue adding portions of 0.3 grams of

Calcium hydroxide and stirring each time you stir it will go cloudy and then there’s the calcium hydroxide reacts with the acid the precipitation should disappear and it become colorless you can always gently tap on the precipitate in the bottom of the beaker and it should clear it up continue stirring until there’s no more noticeable solid and once we get to

That point you can then tap and measure the ph i’m going to continue again this is my my third addition of calcium hydroxide each time i’m adding 0.3 of a gram and once there’s no more precipitate i’ll check the ph’s solution and it still remains fairly acidic this is my fourth edition and after the fourth edition the calcium hydroxide again all reacts with the

Hydrochloric acid this means hydrochloric acid is essentially in excess it’s taking longer each time though once again i’m going to check the ph after my fourth edition and it’s still around about ph1 my fifth edition of calcium hydroxide and i go through the stirring process this time i may notice a slightly more neutral ph it’s all gone into solution that’s a

Good thing i’m gonna tap it there no we still have a very much a an acidic very strongly acidic ph okay that’s five editions this is my sixth edition when there’s calcium hydroxide is unreacted or the calcium hydroxide is in excess there will be a much more alkaline ph okay but stick with it keep an eye on the precipitate in the in the base of the beaker and give

It a stir see how much has gone after each stir it may not be the most exciting chemistry but if you’re patient you can get some very clean results it’s just about all gone into solution now and let’s have a look at the ph okay so the ph is looking slightly more orange there if you’re comparing the initial ph to the ph after six editions we’re definitely seeing

A closer neutralization so the next addition i’m expecting to see the final neutralization okay now i’m expecting there to be excess calcium hydroxide this time as a result if there’s excess calcium hydroxide no matter how much i stir it the calcium hydroxide won’t react and it will remain as a precipitate so i’m stirring it the solution isn’t clearing and this

Means that all the acid has reacted and now we have excess calcium hydroxide this isn’t dissolving i’m going to check the ph now and i’m expecting quite a alkali ph and there we have a ph this is looking a ph round about 10 10 to 12. it’s not particularly easy to see in the center it looks round about 12. so now we can see that neutralization has taken place

And we have excess unreacted calcium hydroxide i can add another proportion of 0.3 of a gram of calcium hydroxide just to confirm my results and this time when i add it and stir i know that the calcium hydroxide won’t dissolve and i’m just going to be adding to my excess i can now check my ph and i expecting a similar ph in fact it’s become ever so slightly more

Alkaline so what we have now are a range of colored ph paper and we can compare this to our swatch and determine the ph at each addition of calcium hydroxide a graph can now be plotted of ph versus mass of calcium hydroxide and determine the mass of calcium hydroxide required for neutralization to summarize the practical we’ve added calcium hydroxide base to

Hydrochloric acid and we’ve continued to add it until neutralization has taken place this has given us data to enable us to draw a graph of ph versus mass of calcium hydroxide added

Transcribed from video
Pearson Edexcel Comb. Sciences – investigating pH on powdered calcium hydroxide to hydrochloric acid By Pearson UK Educators