Video: Required practical on potable water

﻿Hello, and welcome to our next lesson in the using resources topic. Today's lesson is all about the required practical on potable water. Please make sure that you have a pen and a piece of paper ready, and that your phone is on silent and to one side so that you don't get distracted. So once you're ready, let's go. So in today's lesson, we're going to start off with a quick recap all about potable water. We will then move on and to start to look at the required practical. So we will start off by recapping how we test for pH and we will be looking at how we can test for dissolved solids in water. We will then do some maps where we're going to look at calculating the concentration of dissolved solids and then we will look at the second part of the required practical, which is where we look at distillation and how we can compare our samples. And to finish the lesson off, you will do the end of lesson quiz. So to start with our recap is going to be a series of quick fire questions. So I'm going to read the question and I would like you to select which answer is correct. Only pure water is safe to drink. Well done if you selected false, remember pure water is when it only contains water, whereas portable water is also safe to drink. And portable water contains some dissolved impurities such as mineral ions. Portable water contains dissolved mineral ions. Well done if you were listening after the first slide, you will know that that is true. What is the pH of pure water? Well done if you selected a pH of seven, remember a pH of seven is a neutral substance. Pure water has a pH of seven. Sea water contains high levels of Well done if you selected sodium chloride. Desalination is when salt is removed from sea water Well done if you selected true, remember sea water is not safe to drink due to the high levels of sodium chloride in the water. So, desalination is when salt is removed from the sea water. And there are two processes that we looked at last lesson. We looked at reverse osmosis where membranes are used to separate out the salt from the water and also distillation. So, we are moving on to the required practical now and essentially the required practical is split into two parts. So the first part of the required practical is testing our samples for pH and also testing to see how much dissolved solids are present in our water samples. The second part of the required practical is all about the purification of water where we use the method distillation. So we're going to look at it in two separate parts. So testing for pH, you would've have done this in Key Stage three, and you will have done this earlier on in the chemistry course. So testing for pH. So we're testing to see whether something is acidic, whether it is neutral, or whether it is alkaline. And there's essentially three different methods that we could use to test for pH. So we could use universal indicator paper, which is a small strip of paper, you add your sample to the universal indicator paper, and it will change colour depending on whether it's an acid and alkaline or whether it is neutral. We could use universal indicator solution. So this time, what we do is we add to the solution to our sample. So say we have a test tube with our sample in, we can add a few drops of universal indicated solution, we can give it a quick mix, and then it will change colour, again, depending on whether it's an acid and alkaline or whether it's neutral. Now the third way we can test for pH that potentially you haven't looked at yet, is when we use a pH metre. Now you will see in the video a little bit later in this video, a pH metre being used, and where is the universal indicator paper and the universal indicator solution just gives us a colour so say for example, the universal indicator solution goes green, we know that that's a neutral substance, so we know that has a pH of seven. But what the pH metre can do is it can gives us greater resolution so it can tell us to two decimal place is what the pH actually is. So if I had a neutral substance, my pH metre could tell me that the pH of it is 7.12. So it allows us to know with greater resolution, what that actual pH of something is. And this picture here is showing what a pH metre looks like. And as I said, when we watched the required practical video, you will see one of these in use. So three different ways of testing for pH. So what we're going to look at now is how we can test for the mass of dissolved solids in our water sample. So in the video, we will be looking at sea water and we will be looking at tap water. So first thing we're going to do is using a balance we're going to weigh the evaporating dish to two decimal places. And we're going to record this in our results table which is on the next slide. We will then use a measuring cylinder and we will measure out 10 centimetres cubed of water, we will add this 10 centimetres cubed volume of water to our evaporating dish. And we're going to place the evaporating dish on top of our tripod and gauze which sits above our Bunsen burner. And we are going to heat our water sample and we are going to do this until the water evaporates off. What we will then do is we will reweigh the evaporating dish. But what's really important to note is that the evaporating dish is going to be really really hot and therefore, we need to be careful that we don't burn ourselves if we touch it with our hands, we will burn ourselves. So it's really important we use the correct piece of equipment. So we use tongs to transfer the hot evaporating dish from the gauze to the balance. And then after we've reweighed the evaporating dish, we're going to be able to calculate the mass of dissolved solids. So let's have a look at what our results table would look like for this practical. So remember, we're recording the pH. And then we're also going to be recording the mass of solids dissolved in our volume of water are 10 centimetres cubed. It's worth noting for us to get that mass of solids dissolved, we're going to be looking at the mass of the evaporating dish before compared to the mass of the evaporating dish after. So we're looking at two samples we're looking at sea water and we're looking at tap water. So we're going to watch a video of this requires practical now. So a big thank you to Miss Wittek. One of my old academic colleagues who has filmed this required practical for us to watch. So what we've got here is some of the tap water to get started with have been poured into a boiling tube. This piece of equipment here is our pH probe. So the pH probe, remember will give us a value to two decimal places. What we've got now is a sample of the tap water being removed and added to universal indicated paper. So that's the second way we can measure pH. And remember the third is when we directly add universal indicator solution to the sample. And we can see there that our water sample has turned green indicating a neutral pH. So what we're doing now is, we're going to start looking at the dissolved mass of solids in our water sample. So we are going to take 10 centimetres cubed of our tap water sample. And we can see that it's really important we get that measurement correct. So by using a pipette, we can make sure that we have exactly that sample. We are putting now the evaporating dish onto our balance and we can record the mass of that to two decimal places. We then add our known volume of 10 centimetres cubed to our evaporating dish. And the equipment which we've got here is our Bunsen burner, a beaker which contains water and then our evaporating dish is going to sit on top of our beaker. And remember what is going to happen is the water is going to evaporate off and leave our dissolved solids. So next up, we're going to take our second sample. This time we are looking at sea water. And we are just going to take a unknown volume because all we're doing is testing for PH. So once again, we have got our pH metre enables us to record the pH to two decimal places. Then we're going to take a sample of our sea water and we're going to add it to our universal indicator paper. We would expect that universal indicator paper to go green in colour. And then once again if we add a sample of our universal indicator solution to our sea water sample and we mix it Again, we can see the colour change there. So we're going to do exactly the same, we need to record the mass of dissolved solids in our sea water. So we record the mass of the evaporating dish, and then we add to it 10 centimetres cubed of our water sample. So what we've got here is for our tap water sample, the liquid has evaporated off and we've just got our mass of dissolved solids. Note how we don't touch it with our hands because it is very, very hot. So therefore use a pair of tongs to remove it from the heat. Now obviously that just now there was still a bit of water left. So we just leave it to it is fully evaporated And then it's important that we dry off the bottom of the dish. So that doesn't record effect, sorry, our recording, and we can see that there's no water, it's just our dissolved solids. So what we do is we place the evaporating dish onto our balance, and we record the mass after. And then we repeat that for our second sample, which is sea water. So we're going through a series of quickfire questions now just to check what you have learnt so far. So which of the following cannot be used to measure pH? Well done if you selected litmus paper, we can only use a pH metre and then universal indicator paper or universal indicator solution in order to measure the pH how many decimal places should we record the mass of the evaporating dish to? Well done if you selected two decimal places, this is what most balances in the lab are able to record to. When the liquid evaporates, what is the state change? Well done if you selected from a liquid to a gas, remember this state change is called evaporation. So what we're going to do, is we're going to have a look at interpreting some results. So in this case, we have three different samples of water. We've got sea water, tap water, and river water. And you can see we've got that pH value to two decimal places. And then we've got the mass of solids dissolved in 10 centimetres cubed of that sample. So what I would like you to do is to pause the video, I would like you to answer these three questions. And then once you have finished answering the questions, press play so that we can go through the answers. So please pause the video now. Okay, so let's mark the work. So please pause the video now. It's worth noting question three, you need to try to use the values wherever possible try to use the data. So please compare my answer to what you have written. And then press play once you have finished marking and correcting your work. Okay, so what we're going to move on now and look at is calculating the concentration of dissolved solids in our water sample. So, earlier on in the course, you may have seen this equation. So concentration is equal to the amount of solute in grammes divided by the volume of solution in centimetres cubed times by 1000. The reason why we have two times by 1000 is because the volume of solution we have got in centimetres cubed, but the unit for concentration is grammes per decimeter cubed. So to convert from centimetres cubed to decimeters cubed, we have to times by 1000. The amount of solute is our dissolved solid. So here we have got our data. So we've got the mass of solids dissolved and 10 centimetres cubed in our three water samples. So I'm going to do one with you to show you how you would do it and then you're going to do the other two by yourself. So concentration of sea water, my amount of solute in grammes is naught point six grammes. So that's my value for the amount of solute. We then divide it by the volume of solution. So in this case, we've used 10 centimetres cubed, and then we have to times that by 1000. Remember to convert from centimetres cubed to decimeters cubed. So we do not point six divided by 10 times by 1000. And we get an answer of 60 grammes per decimeter cubed. So what I would like you to do now is you're going to calculate the concentration of solids dissolved in tap water and in river water. So I would like you to pause the video now, to answer those questions you are allowed to use a calculator and then press play once you have answered the questions so that we can mark them. Fantastic. Let's go through the answers then. So for tap water, you should have done naught point naught one, divided by 10, times by 1000. And you get a concentration of one gramme per decimeter cubed. For river water you should have done naught point four, divided by 10, times by 1000. And that gives you a concentration of 40 grammes per decimeter cubed. So we are going to move on now to the second part of the required practicals. So remember the first part is about looking at the pH on water samples and finding out the mass of the dissolved solids in our water samples. What we're going to look at now is how we can purify our water sample, so we end up with pure water. Now we did look at this method last lesson, we're going to look at distillation of water samples. So in this picture here, we have got a round bottomed flask and it is sitting on some form of heater. Now if you were to do this in the lab, what we would do is we would use a round bottom flask or a conical flask or even to be honest a boiling tube and we would heat it using a Bunsen burner. Well we then have the next piece of equipment you can see that it connects to a condenser and we can see that the condenser has a blue surrounding it. Okay, so remember from last lesson, the job of the condenser is that the steam will pass through the condenser and the condenser is surrounded by cold water. So the drop in temperature will cause the steam which is a gas to turn back into a liquid where it can then be collected. And in this picture on the slide it is being collected by a measuring cylinder. So, what we would do is we would take 100 centimetre cubed sample of water, we are going to heat it to boiling points and then we collect the distilled water in a cooled test tube. So, the test tube could be sat in a beaker and the beaker contains ice and very chilled water. And what we can do is we can test the pH of the sample before we heated it and then we can test the pH of our distilled water sample. And what we can also do is with our pure water which we have collected the distillate, we can leave a small sample to evaporate. What we're trying to see is, are there any dissolved solids, which when we leave it to evaporate, we would be left with dissolved solids. So how does the distilled water differ from the original sample? So what do you think? Okay, so the distilled water would have a more neutral pH, so the pH would be different compared to the original sample compared to the original sample as well. If you let the original sample evaporate, you would probably find some dissolved solids in there. If you were to do that to the distillate, and you were to leave it to evaporate, there would be no dissolved solids in our pure water. So how could we check that the sample which we've collected through this distillation method, how could we check that that's pure water? What do you think? Okay, so, earlier on in the chemistry course, you will have looked at pure substances, and you will have learned that pure substances have a different boiling point to impure substances. So, we could check that the distillate is pure water by boiling it, and if it boils at 100 degrees Celsius, we have a pure water sample. So, this is a second part of the required practical getting a sample of purified water. So, it's worth noting and we touched on this last lesson that distillation is a really expensive process due to the energy needed to essentially boil the water. So it's typically used by wealthy countries, so wealthy countries who don't have easily available supply of groundwater or freshwater that they can use. And distillation is also used in emergency situations. So if there's a natural disaster and the water supplies contaminated or it's completely cut off, distillation can be used to generate potable water for people to drink. But it's not used all the time, because there are huge energy costs associated with it. So what I would like you to do, you've got six questions. I would like you to pause the video now have a go answering those question and then press play once you have answered them all. Well done. That's fantastic. Let's mark your work. Please pause the video now to mark your work and then press play once you have finished Okay, so let's recap what we have learned. So portable water is water that is safe to drink. It contains dissolved impurities such as mineral ions. We can test pH by using universal indicator or a pH metre. We can test for dissolved solids in a sample by evaporation of the water sample. We can use distillation to get a sample of pure water and we can compare the pH to the original sample. And then there will be no dissolved solids in the distilled water. So well done on all your hard work this lesson. Please make sure that you now click Next to complete the End of lesson quiz.

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Completed: Required practical on potable water

In this lesson we will look at the required practical on potable water. We will recap how to test for pH and calculate the mass of dissolved substances in water from different sources. We will also look at distillation as a method for purifying water.

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Required practical on potable water

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