[Music] hi and welcome back to free science lessons. co.uk by the end of this video you should be able to describe what's meant by a strong and a weak acid giving examples you should then be able to describe the effect on pH of the strength of an acid and finally you should be able to describe what's meant by a dilute and a concentrated acid let's start by looking at strong acids in a previous video we said that all acids produce hydrogen ions H+ in aquous Solutions remember that an aquous solution means dissolved in water I'm showing you that here for hydrochloric acid as you can see the hydrochloric acid molecule is splitting and it's releasing the hydrogen ion H+ now scientists use a special word for when an acid molecule splits like this we say that the acid molecule has ionized so we can say that an aqua Solutions acid molecules ionize in other words they split and release the hydrogen ion H+ hydrochloric acid is an example of a strong acid and you need to know what that means strong acids fully ionize in aquous Solutions in other words every acid molecule ionizes like this we can tell that from this Arrow the arrow goes in One Direction only telling us that the acid has fully ionized then there are three examples of strong acids that you need to learn these are hydrochloric acid sulfuric acid and nitric acid and I'm showing you them here these are all strong acids because they all fully ionize an aquous solution so let's take a look now weak acids I'm showing you here the weak acid carbonic acid we can tell this is a weak acid from this Arrow this is a reversible Arrow this means that some of the acid molecules ion but not all of them scientists say that weak acids partially ionize in other words only a fraction of the acid molecules ionize there are three weak acids that you need to learn these are carbonic acid ethanolic acid and citric acid now we've already seen that the pH scale tells us the acidity or alkalinity of a solution so how can we use this with strong and weak acids we're going to look at that now the pH scale gives us an idea of the concentration of hydrogen ions produced by an acid strong acids have a lower pH than weak acids for a given concentration of the acid that's because strong acids fully ionize producing a greater concentration of hydrogen ions than weak acids which only partially ionize now this brings us to a really important point about the pH scale as the pH scale decreases by one unit the concentration of hydrogen ions increases by 10 10 times if we compare ph1 with PH2 we can see that ph1 has a 10 times greater concentration of hydrogen ions than PH2 scientists call a 10 times difference one order of magnitude if we compare ph1 with ph3 we can see that ph1 has a 100 times greater concentration of hydrogen ions than ph3 a 100 times difference is called two orders of magnitude so as we've seen the strength of an acid affect its pH the pH can also be affected by the concentration of the acid so we're going to look at that now the concentration of an acid tells us the amount of acid molecules in a given volume so a dilute acid will have fewer acid molecules in a given volume than a concentrated acid even if the strength of the acid is the same remember you'll find plenty of questions on the strength of acids and pH in my VIs workbook and you can get that by clicking on the link above okay so hopefully now you should be able to describe what's meant by a strong and a weak acid giving examples you should then be able to describe the effect on pH of the strength of an acid and finally you should be able to describe what's meant by a dilute and a concentrated [Music] acid