Chemical Reactions & How Equations Are Written രാസസമവാക്യം
This whole chapter is about chemical reactions (രാസപ്രവർത്തനം): how to weigh them, how to write them correctly, and how electrons move in them. Before we start, this page builds the tools you need. You will learn to tell a chemical change from a physical change, to spot the signs that a reaction is happening, and to name the reactants and the products. You will also learn to read every part of a chemical equation, to count the atoms in a formula such as 2NaOH, and to add up the masses of atoms in u. 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 read 2H₂ + O₂ → 2H₂O and say how many atoms it contains, do the tables and the quiz, then go to page 2.
1. Physical change or chemical change?
When ice melts, it is still water; it can freeze back. That is a physical change: no new substance is formed. When an iron gate rusts in the salty air of a coastal town, the shiny iron turns into a brown, crumbly new substance, rust. That is a chemical change (രാസമാറ്റം) – a chemical reaction (രാസപ്രവർത്തനം). In a chemical change new substances are formed, and it usually cannot be undone easily.
2. Signs that a reaction is happening
We cannot see atoms rearranging, but we can see clues. A chemical reaction usually shows one or more of these signs:
- a gas is given off – bubbles, fizzing (zinc in dilute acid);
- a change of colour (an iron nail in blue copper sulphate solution);
- a precipitate (അവക്ഷിപ്തം) – an insoluble solid appears when two solutions are mixed;
- heat is given out or taken in, and sometimes light – an energy change (ഊർജമാറ്റം).
Sodium in water and burning magnesium are dangerous: in school they are teacher demonstrations only (safety goggles, tiny amounts, never look straight at burning magnesium). Never try any of these at home.
3. 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.
4. Reading a chemical equation
A chemical equation (രാസസമവാക്യം) is a short, scientific way of writing a reaction with symbols and formulae. Every mark in it has a meaning. Step through the equation for sodium and water, which you will meet on page 3:
| Mark | Meaning | Example |
|---|---|---|
| → | “react to form”, “gives” | C + O₂ → CO₂ |
| + | “and”: on the left, “reacts with”; on the right, “is also formed” | Zn + 2HCl → ZnCl₂ + H₂ |
| ↑ | a gas is given off | Zn + 2HCl → ZnCl₂ + H₂↑ |
| ↓ | a precipitate forms | BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl |
| (s) (l) (g) (aq) | physical state (ഭൗതികാവസ്ഥ): solid, liquid, gas, aqueous solution (dissolved in water) | 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) |
| a name over the arrow | a catalyst (ഉൽപ്രേരകം) or a condition (heat, light) | N₂(g) + 3H₂(g) →Fe 2NH₃(g) + heat |
| + heat | heat is given out (on the right) or needed (on the left) – an energy change | C + O₂ → CO₂ + heat |
In 2H₂O, the big 2 in front is a coefficient: it means two molecules of water. The small ₂ low on the right of H is a subscript: it means each molecule has two hydrogen atoms. Changing the coefficient changes how many molecules; changing the subscript changes which substance it is: H₂O is water, but H₂O₂ is hydrogen peroxide, a bleach! When you balance equations (page 4) you may change only the coefficients.
5. Counting atoms in a formula
Rules: each symbol is one atom; a subscript counts the atoms of the symbol just before it; a subscript after a bracket multiplies everything inside the bracket; a coefficient in front multiplies the whole formula. So 2NaOH = 2 × (1 Na + 1 O + 1 H) = 2 Na, 2 O, 2 H = 6 atoms.
Type ordinary digits (5H2O). Capital letters matter: Co is cobalt, but CO is carbon + oxygen.
6. Masses of atoms in u
Atoms are far too light to weigh in grams one by one, so chemists use a tiny unit: the unified atomic mass unit, u (യൂണിഫൈഡ് അറ്റോമിക് മാസ് യൂണിറ്റ്). A hydrogen atom has a mass of about 1 u. These are the values used in this chapter:
| Element | H | C | N | O | Na | Mg | Cl (go further) |
|---|---|---|---|---|---|---|---|
| Atomic mass | 1 u | 12 u | 14 u | 16 u | 23 u | 24 u | 35.5 u |
The mass of a molecule is the sum of the masses of its atoms: H₂O = 2 × 1 + 16 = 18 u. For several molecules, multiply: 2H₂O = 2 × 18 = 36 u.
- A chemical change (രാസമാറ്റം) makes new substances; signs: gas, colour change, precipitate (അവക്ഷിപ്തം), heat or light.
- Reactants (അഭികാരകങ്ങൾ) → products (ഉൽപന്നങ്ങൾ); ↑ = gas given off, ↓ = precipitate; (s) (l) (g) (aq) = states; a catalyst is written over the arrow.
- A coefficient counts molecules; a subscript counts atoms in one molecule. 5H₂O = 5 molecules, 15 atoms.
- Atomic masses in u: H 1, C 12, N 14, O 16. A molecule's mass is the sum of its atoms' masses: CO₂ = 44 u.