in today's video we're going to look at the ph scale and see what acids and alkalis are ph is really just a measure of how acidic or alkaline a solution is and it's measured on a scale from 0 to 14 with low numbers being most acidic and high numbers being most alkaline and if the substance is neutral like pure water then the ph would be seven which is neither acidic nor alkaline to give you some idea of what all of this means the acid in your stomach which helps you to cure bacteria has a ph of around two while acid rain is around ph4 then for alkalis washing up liquid has a ph of around nine and the bleach we use to clean bathrooms has a ph of around 12. be aware though you don't have to remember these specific examples there just to give you some context now we can measure ph in a couple of different ways one is to use some kind of indicator which are a group of chemical dyes that change color depending on the ph and different indicators will change color at different phs some indicators contain a mixture of these different dyes though so their color will slowly change across a wide range of ph's so we often refer to them as wide range indicators the most common example is universal indicator which gives the colors we've shown our ph scale here ranging from deep red at a very low and acidic ph to yellow green and then bluey purple as we increase the ph and it becomes more alkaline the other way to measure ph is to use a ph probe connected to a ph meter by dipping the probe into a solution we can electronically measure the ph and get a numerical reading on the meter the benefit of the probe compared to an indicator is that it can be much more accurate and precise because it doesn't involve humans guessing shades or particular colors and as a general rule measurements that remove the need for human judgment are going to be more reliable the next thing we need to look at is what exactly makes something an acid you can define an acid as any substance that forms aqueous solutions with a ph of less than seven and the reason for this is that acids release hydrogen ions in water which make the solution acidic on the other side of the scale we have bases which we define as any substance with a ph greater than seven meanwhile alkalis which we mentioned before are actually a subgroup of bases that are soluble in water so we would say that an alkali is a base that dissolves in water to form a solution with a ph greater than seven and these alkalis form o h minus ions in water which we call hydroxide ions if we react an acid and a base together then we get a neutralization reaction which will always produce a salt and water for example hydrochloric acid plus sodium hydroxide which are the most common acid and base will react to form sodium chloride which is a salt plus water another way to show these neutralization reactions is in terms of hydrogen ions and hydroxide ions where h plus from the acid and o h minus from the base combine to form h2o and because the acid and base have both been neutralized in these sorts of reactions the ph of the products should be seven because they're neutral before we finish i just want to point out some of the common acids and bases that you'll come across in the course the main acids are hydrochloric acid sulfuric acid and nitric acid while the common bases are generally hydroxides or carbonates like sodium hydroxide or calcium carbonate there are loads more but it's definitely worth learning these few as they crop up a lot anyway that's all for today so hope you enjoyed this video and we'll see you next time