Matter and its Classification

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. The milk packet in your fridge says: pure milk. And your mummy's gold chain is twenty two carat gold. Everyone calls it pure gold, right? But ask a chemist, and she will say: neither of them is pure! So then, what does pure even mean? That is exactly what we will find out today. And remember, this is the very first concept of chemistry. In JEE, questions on this are quick marks, if you know the rules. So come on, let's begin.

0:35Matter and its three states

So, first question: what is matter? Matter is anything that has mass and occupies space. Your pen, the water in your bottle, even the air in this room: all of it is matter, okay? Matter exists in three states: solid, liquid and gas. Every solid has a definite shape and a definite volume. Now tell me. Pour water from a glass into a plate. What changes, the shape or the volume? Yes, exactly! Only the shape. So a liquid has a definite volume, but no definite shape. And a gas has neither: no definite shape, and no definite volume. Change the temperature and the pressure, and one state turns into another, like ice melting into water. Clear?

1:22Pure substances: elements and compounds

Now, let's sort matter, just like the chart on screen. Matter splits into two big families: pure substances, and mixtures. First, pure substances. In chemistry, pure means one thing only: a fixed composition, okay? Pure substances are of two kinds. First, elements, made of only one kind of atom. Like sodium, N a; oxygen, O two; and sulphur, S eight. But wait. O two has two atoms, and S eight has eight atoms. So they must be compounds, right? No, no, careful! This is a favourite trap. Count the kinds of atom, not the number of atoms. O two has only oxygen atoms, and S eight has only sulphur atoms. One kind each, so both are elements. No doubt about it. Now, compounds: two or more elements, joined in a fixed ratio by mass. On screen, two plus means two or more. Like water, H two O, and sodium chloride, N a C l, okay? Water from Delhi or from Chennai, the ratio by mass is the same. Fixed, got it?

2:36Mixtures: homogeneous and heterogeneous

Now the other family: mixtures. Here the composition is variable. Think about it: one spoon of sugar in a glass of water, or two spoons: both are sugar solution, right? Different composition, same mixture. Mixtures also come in two kinds. Homogeneous mixtures are uniform throughout, the same everywhere in the sample. Like air, sugar solution, and even brass, okay? Heterogeneous mixtures are not uniform; one part of the sample is different from another. Like sand in water, or smoke. Look at sand in water: is the top the same as the bottom? No, it isn't. So it is not uniform, it is heterogeneous. Simple.

3:22Mixture or compound: how do we separate?

Now, the most important difference: mixture versus compound. In a mixture, every component keeps its own properties. Sand in water is still sand, okay? So tell me, how would you get the sand back? Yes, just filter it! Filtration. And that is the whole point: mixtures can be separated by physical methods, like filtration, distillation, crystallisation, sublimation, and chromatography. Five names. Write them down, okay? But a compound is different. It has completely new properties: water does not behave like hydrogen, or like oxygen. And a compound can be broken down only by chemical methods. No filter, no distillation will ever split water into its elements. Got it?

4:10Physical change or chemical change?

Alright, from substances to changes. In a physical change, the form or the state changes, but the composition does not. Now tell me. Sugar dissolves in water, and you cannot see it any more. Something new has formed, so it is a chemical change, right? No, no! Taste the water. It is sweet, isn't it? The sugar is still sugar; only the mixing has changed. So dissolving is a physical change, just like melting, and just like the sublimation of iodine. In a chemical change, new substances are produced. Burning paper gives carbon dioxide, water and ash. Rusting iron gives hydrated iron oxide. And souring milk gives lactic acid. All three are chemical changes. All clear?

4:56Extensive and intensive properties

Now a new idea, and JEE loves this one. Take a glass of water, and pour half of it into another glass. The mass is now half. The volume is half too. But tell me, what stays the same? Yes! The temperature is the same, and the density is the same. So properties come in two types. Extensive properties depend on the amount of matter: mass, volume, enthalpy, internal energy and Gibbs energy. More matter, bigger value, okay? Intensive properties do not depend on the amount: temperature, pressure, density, boiling point, molar mass and molarity. Half a glass or a full bucket, they stay the same, right? And now, a topper's trick, so you don't have to mug up every item. Divide one extensive property by another, and you get an intensive one. That sign on screen means divided by. Density equals mass divided by volume. Double the sample: mass doubles, volume doubles, and the ratio does not change. So density is intensive. With me?

6:01Example 1: classify air, brass, dry ice, milk and gold

Now, remember the milk packet and the gold chain? Let's settle it now. Classify these six: air, brass, dry ice, milk, diamond, and twenty two carat gold. Element, compound, or which kind of mixture? Use the chart on screen. Pause the video, and try it first. Done? Okay, let's check. Air is uniform throughout, so it is a homogeneous mixture. Brass, an alloy of copper and zinc, is also uniform, so also homogeneous, okay? Dry ice: don't let the word ice fool you. It is carbon dioxide, C O two, so a compound. And diamond, written C for carbon, has only one kind of atom, so it is definitely an element. Now, milk. It looks the same everywhere, smooth and white. So, homogeneous mixture, right? No, no, careful! This is the trap. Milk is a colloid, and it goes in the heterogeneous box, just like smoke. So the packet says pure milk, but for a chemist, milk is a heterogeneous mixture! And the gold chain. Twenty two carat gold is gold mixed with copper or silver, uniform throughout. So it is a homogeneous mixture, not a pure substance. So there you go, puzzle solved: for a chemist, neither the milk nor the chain is pure.

7:26Example 2: is specific heat intensive or extensive

Now a harder one, JEE level. Classify five properties as intensive or extensive: heat capacity, specific heat capacity, molar heat capacity, molar volume, and number of moles. And explain the pattern. Pause and try. Hint: imagine you double the sample. Done? Okay, let's check. Double the sample. The heat capacity doubles, and the number of moles doubles. So both are extensive; the arrow on screen means therefore, okay? Now, specific heat capacity. It has heat capacity right there in its name, and heat capacity is extensive. So specific heat is also extensive, right? No, no, careful! Classic JEE trap. Specific heat is heat capacity per gram: heat capacity divided by mass, extensive divided by extensive. Double the sample, and the top and the bottom both double. The amount cancels. So, intensive! Same story for molar heat capacity, which is heat capacity per mole, and for molar volume, which is volume per mole. Both are intensive, clear? So, the pattern? Each of these three is a ratio of two extensive quantities, so the amount cancels, and the result is intensive. Final answer: two extensive, heat capacity and moles, and three intensive. See how fast that was?

8:58Quick revision: mixtures, changes, intensive properties

So, quick revision. Pure substances have a fixed composition; mixtures have a variable one, and physical methods can separate them. In a physical change, the composition stays the same; in a chemical change, new substances form. And extensive divided by extensive gives intensive. Now go and try the practice problems on this page. You will find them easy, I promise. All the best!