Mixtures, States of Matter & Dissolving ഏകാത്മക മിശ്രിതങ്ങൾ · ഭിന്നാത്മക മിശ്രിതം
A solution (ലായനി) is a special kind of mixture, so this chapter starts with mixtures. On this page you will learn to tell a pure substance from a mixture, and a homogeneous mixture from a heterogeneous one. You will picture solids, liquids and gases as particles and see why dissolving is not the same as melting. You will also recall from Chapter 3 why water is polar and kerosene is non-polar, and see how filtering and evaporating separate mixtures. Nothing is assumed.
- Only the idea that everything is made of tiny particles: atoms, molecules or ions (Class 8 and Chapter 3).
- If you can already say why sugar water is homogeneous and sand in water is not, and why kerosene does not mix with water, do the sorters and the quiz, then go to page 2.
1. Pure substances and mixtures
A pure substance is made of only one kind of particle. It may be an element (copper, oxygen, gold) or a compound (water, common salt, sugar). A pure substance has fixed properties: pure water always boils at 100 °C at sea level.
A mixture contains two or more substances that are mixed together without reacting. Each substance keeps its own properties. The amounts can vary: you can add a little sugar to your tea or a lot. A mixture can be separated again by physical methods such as filtering and evaporating (section 6).
2. Homogeneous and heterogeneous mixtures
Stir a spoon of sugar into a glass of water. Taste a drop from the top and a drop from the bottom: both are equally sweet. You cannot see the sugar anywhere. A mixture that has the same properties in all its parts is a homogeneous mixture (ഏകാത്മക മിശ്രിതങ്ങൾ). Its particles are spread perfectly evenly.
Now stir sand into water and leave it. The sand sinks and you can see two separate parts. A mixture whose parts are different, and can often be seen, is a heterogeneous mixture (ഭിന്നാത്മക മിശ്രിതം). Oil floating on water and muddy water are heterogeneous too.
Every solution is a homogeneous mixture. That is the first definition of the chapter (page 3). Pages 7 and 8 then meet mixtures that only look homogeneous (milk, mist) and mixtures that clearly are not (muddy water).
3. Solids, liquids and gases as particles
Matter exists in three states: solid (ഖരം), liquid (ദ്രാവകം) and gas (വാതകം). The difference is in how the particles are arranged and how they move.
- Solid: particles packed close in fixed positions. They only vibrate. A solid has a fixed shape and a fixed volume.
- Liquid: particles still close together, but they slide past each other. A liquid flows and takes the shape of its container, but its volume is fixed.
- Gas: particles far apart, moving fast in all directions. A gas has no fixed shape or volume; it fills any container.
At normal air pressure, water freezes at 0 °C and boils at 100 °C. The molecules are drawn much bigger and far fewer than they really are.
4. Dissolving is not melting
People often say “the sugar melted in the tea”. A chemist would say the sugar dissolved. The two words mean different things:
- Melting: one substance changes from solid to liquid because it is heated to its melting point. Ice melts at 0 °C; sugar melts only at about 186 °C, in a hot pan.
- Dissolving: a substance (the solute, ലീനം) spreads out as single particles among the particles of another substance (the solvent, ലായകം). No heating is needed: sugar dissolves in water at room temperature. Together they form a solution (ലായനി).
5. Polar and non-polar molecules (a recap of Chapter 3)
In Chapter 3 you learnt that atoms join by sharing electrons (covalent bonds) or by transferring them (ionic bonds). In a water molecule the oxygen atom pulls the shared electrons more strongly than the hydrogen atoms do. So the oxygen end becomes slightly negative (δ−, “delta minus”) and the hydrogen ends slightly positive (δ+). A molecule with slightly charged ends like this is polar (പോളാർ).
In kerosene, petrol, wax and oils the molecules are long chains of carbon and hydrogen (hydrocarbons). Carbon and hydrogen share their electrons almost equally, so these molecules have no charged ends. They are non-polar (നോൺപോളാർ). Common salt is ionic: it is made of ions that carry full charges, Na⁺ and Cl⁻.
The book says covalent compounds generally do not dissolve in water. That is true for non-polar ones such as wax and kerosene. A few covalent compounds are polar, because they contain O–H groups like water does. Sugar, glucose and alcohol are examples, and they dissolve in water very well. Page 3 turns this into a rule: like dissolves like.
6. Separating mixtures: filtering and evaporating
Filtering: pour the mixture through filter paper in a funnel. Undissolved solid bits are too big to pass through the tiny holes and stay on the paper as the residue. The liquid that comes through is the filtrate. Evaporating: heat the liquid gently in a dish until the water has gone, and any solid that was dissolved is left behind.
Filter paper cannot separate salt from salt water, because the dissolved particles are far smaller than the holes in the paper. Particle size decides whether a mixture can be filtered. Page 7 uses this to tell true solutions, colloids and suspensions apart.
- A pure substance has one kind of particle; a mixture has two or more substances that do not react.
- Homogeneous mixture (ഏകാത്മക മിശ്രിതങ്ങൾ): the same in all parts (sugar water, air, brass). Heterogeneous (ഭിന്നാത്മക മിശ്രിതം): parts differ (sand in water, oil and water).
- Solid: fixed positions, vibrate. Liquid: close, slide past. Gas: far apart, fast.
- Melting = one substance, heated. Dissolving = a solute spreading into a solvent.
- Water is polar; salt is ionic; kerosene, petrol, turpentine, oils and wax are non-polar.
- Filtering removes undissolved solids; evaporating recovers a dissolved solid.
- “Sugar melts in tea” ✗ – it dissolves. Melting needs heat and only one substance.
- “Salt can be filtered out of salt water” ✗ – dissolved salt passes through filter paper; evaporate instead.
- “A mixture must look mixed” ✗ – homogeneous mixtures such as air and sugar water look like one substance.