Physical changes

0:00- [Lecturer] Look at this water that is freezing or consider the example of gold melting or dissolving salt in water. The question is, what exactly are these changes? Well, let's explore them in detail. Changes can be classified into two categories, we have physical changes and chemical changes.

0:20Physical changes are the one where the identity of the substance does not change. Chemical changes are the ones where the identity of substance does change. So in this video we'll focus mostly on the physical changes with lots and lots of examples. So let's start with a simple one.

0:35We have an iron nail. Maybe you bend it or maybe you cut it into tiny pieces. These are all changes that we have made, right? But what kind of a change is this? Well, notice that the identity of the substance has not changed. It is still iron. So this is an example of a physical change.

0:53Here's another example. Let's say you take some salt and you crush it. Well, you now get very tiny pieces of salt. Again, notice it's still salt. The identity of the substance does not change. So this is another example of a physical change. Another example is you take water and you can do a lot of things with it.

1:11You can pour it from one, you know, glass to another, you can splash it, you can break into tiny droplets, but again, whatever you do, the identity of the water has not changed. It's still water, so this is also a physical change. Okay, let's take some slightly more complicated example.

1:28Let's say you say you take some water in an ice tray and then you put it in your freezer and you say keep it overnight, for example, and you take it out the next day and you get ice. So consider this example where water turned into ice. Is this a physical change?

1:44Well, at first it looks like something new has formed. From water we have gotten something new, ice. but if you could zoom in all the way to the molecule level, we see that in both cases we have H2O molecules. The difference is that they're arranged differently, but the identity has stayed the same.

2:00It is H2O. So zooming out water, freezing to ice is a physical change. Similarly, if you consider melting of ice, or for that matter, melting of gold, the identity does not change. This will stay H2O and the gold atoms stay gold atoms, nothing has changed. And so melting is another example of physical change.

2:24And same would be the case with boiling. Again, the identity does not change, it's a physical change. So these things that we just saw, freezing, melting, boiling, they're collectively called phase changes. When a solid turns into liquid or liquid turns into gas, these are all phase changes.

2:41And phase changes are also physical changes because the identity does not change. This also means we now have some new physical properties added to our list. These are boiling point, the temperature at which a liquid boils, and melting point, a temperature at which a solid melts.

2:58These are physical properties because when we are measuring them, the identity does not change. Only the face changes, but the identity does not change. So these are physical properties, and by the way, these are intrinsic properties. Remember, intrinsic properties are the one that does not depend on the amount of substance.

3:12Boiling point of water, for example, is 100 degrees Celsius or 202 degrees Fahrenheit. It does not depend about how much water you take, it's the same boiling point. So it does not depend on the amount of substance. It's an intrinsic physical property. Similarly, melting point is also an intrinsic physical property.

3:29And this can be pretty useful to us. For example, let's say somebody tries to give you lead painted in gold. Can you make out whether it's lead or gold? Well, sure you can. These two have different melting points. Lead has a much smaller melting point compared to gold.

3:46And so you can just try to melt it. And if it melts much earlier, you know, at a much lower temperature than gold, then you know that that's not gold. So it's pretty, pretty useful, right? Okay, finally, let's consider the salt dissolving in water. Is this a physical change or not?

4:04This seems a little tricky, but what's really going on when you dissolve salt in water, when you dissolve salt in water, what's really going on is that salt is breaking into tiny, tiny pieces and getting distributed evenly, forming a homogenous solution. But we have not created something new.

4:21It's salt still stays salt, and water still stays water. So salt, water is just a mixture. It's a physical combination of substances, right? And therefore it is just a physical change. The identity has not changed. And by the way, if you just heat this solution, water will boil at 100 degree Celsius, but salt has a much higher boiling point, and so the salt will stay behind.

4:49This is how you can recover the salt. Anyways, putting it all together. Physical changes are where the changes in which the identity of substance does not change. Some are easy to spot, other cases like face changes, it feels like something new is something new is formed, but face changes are also just physical changes.

5:09And same as the case with dissolving, you get a solution, but it's just a physical mixture. So it's a physical change.