Physical & Chemical Changes, Equations Recap ഭൗതികമാറ്റം · രാസമാറ്റം
This chapter is about chemical reactions (രാസപ്രവർത്തനം): how to recognise them, how to sort them into kinds, and what makes them fast or slow. Before we start, this page builds the tools you need. You will learn to tell a physical change from a chemical change and to name the reactants and the products. You will also count atoms, check whether an equation is balanced (from Chapter 4), and read the symbols ↑ ↓ (s) (l) (g) (aq) that appear in almost every equation of this chapter. Nothing is assumed.
- Only the idea that substances are made of atoms and molecules, and that each element has a symbol (H, O, Na, Cl …).
- If you can already explain why burning wood is a chemical change and balance N₂ + H₂ → NH₃, do the tables and the quiz, then go to page 2.
1. Physical change or chemical change?
When ice melts, it is still water. Put it back in the freezer and you get ice again. Only the state or the shape has changed and no new substance is formed. That is a physical change (ഭൗതികമാറ്റം). When milk turns into curd, the curd has a new taste and a new texture, and it never turns back into milk. One or more new substances have formed. That is a chemical change (രാസമാറ്റം), also called a chemical reaction (രാസപ്രവർത്തനം). A chemical change usually comes with an energy change too: heat or light is given out or taken in.
- Is a new substance formed? This is the real test. Yes → chemical change.
- Can it easily be undone? Melting, freezing, boiling and dissolving can usually be undone (evaporate the water and the sugar is back). Most chemical changes cannot easily be undone.
- Are there signs such as a new colour, a gas, a new smell, a precipitate, heat or light? These are clues to a chemical change (page 3).
2. Reactants and products
The substances that take part in a reaction are the reactants (അഭികാരകങ്ങൾ). The new substances formed are the products (ഉൽപന്നങ്ങൾ). In a chemical equation the reactants are written on the left of the arrow and the products on the right.
3. Counting atoms and balancing (a recap of Chapter 4)
A chemical equation (രാസസമവാക്യം) writes a reaction with symbols and formulae. In 2H₂O the big 2 in front is a coefficient: two molecules. The small ₂ is a subscript: two hydrogen atoms in each molecule. So 2H₂O contains 4 H atoms and 2 O atoms. A subscript after a bracket multiplies everything inside: Ca(OH)₂ has 1 Ca, 2 O and 2 H.
In a reaction no atom is created or destroyed; the atoms only change partners (the law of conservation of mass, Chapter 4). So a correct equation must have the same number of each kind of atom on both sides: it must be balanced. To balance, change only the coefficients, never the subscripts – changing a subscript changes the substance.
All these equations appear in this chapter. Start from all coefficients 1, then fix one element at a time. Leave oxygen and hydrogen until last if they appear in several formulae.
4. Symbols you will meet in this chapter
| Mark | Meaning | Example from this chapter |
|---|---|---|
| → | “react to form”, “gives” | CaO + H₂O → Ca(OH)₂ |
| + | on the left: “reacts with”; on the right: “and also” | Zn + 2HCl → ZnCl₂ + H₂ |
| ↑ | a gas (വാതകം) is given off | CaCO₃ → CaO + CO₂↑ |
| ↓ | a precipitate (അവക്ഷിപ്തം) forms – an insoluble solid | NaCl + AgNO₃ → AgCl↓ + NaNO₃ |
| (s) (l) (g) (aq) | physical state (ഭൗതികാവസ്ഥ): solid, liquid, gas, aqueous (dissolved in water) | N₂(g) + 3H₂(g) ⇌ 2NH₃(g) |
| a word or formula over the arrow | a condition (heat) or a catalyst (ഉൽപ്രേരകം) | 2H₂O₂ →MnO₂ 2H₂O + O₂↑ |
| + heat | heat is given out: an exothermic reaction (താപമോചക പ്രവർത്തനം) | CaO + H₂O → Ca(OH)₂ + heat |
| ⇌ | a reversible reaction (ഉഭയദിശാ പ്രവർത്തനങ്ങൾ): it can go both ways (page 8) | 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) |
On page 105 the book writes the marble reaction as CaCO₃(s) + 2HCl(l) → CaCl₂(s) + H₂O(l) + CO₂(g). Learn it as printed for the exam. A chemist would add one detail: hydrochloric acid is a solution of HCl in water, and the calcium chloride formed stays dissolved, so both are really (aq): CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g).
- Physical change (ഭൗതികമാറ്റം): no new substance (melting, boiling, dissolving). Chemical change (രാസമാറ്റം): one or more new substances form, usually with an energy change (burning, curdling, rusting).
- Reactants (left of →) change into products (right of →).
- A balanced equation has the same number of each kind of atom on both sides; balance by changing coefficients only: N₂ + 3H₂ → 2NH₃.
- ↑ gas · ↓ precipitate · (s) (l) (g) (aq) states · catalyst over the arrow · ⇌ reversible.