Recognising a chemical reaction and writing its equation
Leave a bowl of warm milk with a spoonful of curd in it overnight and by morning it has set.
Stir it as much as you like: it will not turn back into milk.
Something new has been made.
Bacteria have changed the milk sugar into lactic acid, and the acid has made the milk proteins clump together.
A change like this, where new substances with new properties are formed, is a chemical change, and the process is a chemical reaction.
Compare that with melting ghee on a cold winter morning.
The solid ghee becomes liquid, but it is still ghee, and it sets again in the fridge.
No new substance has formed.
That is a physical change.
Chemistry is the study of changes like the first one.
In this section you will learn how to tell from the outside that a reaction has happened, and how chemists write a reaction down in a short, exact form.
How you know a reaction has happened
You cannot see atoms rearranging, so you look for signs.
You need to know four of them:
Sign
What you notice
An everyday or laboratory case
Change in state
a solid appears from two liquids, or a solid turns into a gas
two clear solutions mixed in a test tube give a cloudy solid
Change in colour
the colour of a substance or solution changes
a cut apple or brinjal turns brown in air; a blue solution fades
Evolution of a gas
bubbles or fizzing, sometimes a smell
an antacid tablet fizzing in a glass of water
Change in temperature
the container becomes warm or cold
water poured on quicklime makes the bucket hot
A fifth sign that examiners also credit is the formation of a precipitate, an insoluble solid that appears when two solutions are mixed.
It is really a special case of a change in state.
Why these signs work. A new substance has new properties: a different colour, a different state at room temperature, or a different solubility.
Breaking old bonds and making new ones also takes in or gives out energy, which you feel as a change in temperature.
So each sign is a clue that the particles have been rearranged into something new.
Be careful with "change in state" on its own. Ice melting and water boiling change state but make nothing new.
The sign counts only when a new substance is formed in the new state.
In the same way, a colour change when you dilute a coloured solution is not a reaction.
The safest answers name a change that cannot be undone simply by cooling, heating or adding water.
Burning magnesium: all the signs at once
A classic laboratory reaction shows the signs clearly.
A strip of magnesium ribbon is first rubbed with sandpaper to remove the dull layer of oxide on its surface, so that the clean metal can react.
It is held with tongs in a burner flame.
It catches fire and burns with a dazzling white flame, and a white powder, magnesium oxide, is left behind.
The shiny metal has become a white solid, and light and heat are given out.
Safety matters here: the flame is bright enough to hurt the eyes, so hold the burning ribbon at arm's length, do not stare at it, and wear safety goggles.
The photograph below shows the moment the ribbon catches fire, with the white powder it leaves behind.
Writing a reaction down
A sentence such as "when magnesium burns in the oxygen of the air, it turns into magnesium oxide" is clear but long.
Chemists shorten it in two stages.
1. The word equation. The starting substances, the reactants, go on the left.
The substances made, the products, go on the right.
A plus sign joins two reactants or two products, and an arrow points from reactants to products.
The arrow means "react to give", and its head shows which way the reaction goes.
magnesium + oxygen → magnesium oxide
2. The skeletal chemical equation. Replace each name with its chemical formula:
Mg + O₂ → MgO
This is shorter and tells you which atoms are involved.
Count the oxygen atoms, though: two on the left, one on the right.
An equation like this, with the correct formulae but unequal numbers of atoms on the two sides, is called a skeletal equation.
It is the skeleton that the next section teaches you to balance.
Why we need equations at all
An equation is a summary that any chemist in any language can read.
From Mg + O₂ → MgO you can see straight away that a metal has joined with oxygen.
Once the equation is balanced (next section), it also tells you how many particles of each substance react, which is what lets a factory work out how much raw material to buy.
In this unit you will meet equations for every type of reaction, so writing them quickly and correctly is the most useful skill in the chapter.
- Chemical reaction: a change in which one or more substances are changed into new substances with different properties.
- Reactants: the substances that take part in a reaction, written on the left of the arrow.
- Products: the new substances formed in a reaction, written on the right of the arrow.
- Word equation: a reaction written with the names of the reactants and products, joined by plus signs and an arrow.
- Skeletal chemical equation: a reaction written with the correct formulae of reactants and products, before the numbers of atoms on the two sides have been made equal.
Riya adds a spoonful of a fruit-salt antacid powder to a glass of water.
It fizzes strongly and the glass feels slightly cooler.
Later she stirs sugar into another glass of water and it disappears.
Which of the two is a chemical reaction?
Give two observations that support your answer. [2]
1. Look for new substances. In the antacid glass a gas is given off (the fizzing), so something new has been formed.
2. The temperature also changes, which is a second sign.
3. Sugar dissolving gives no gas, no colour change and no new substance: evaporate the water and the sugar is left. It is a physical change.
Answer: The antacid in water is a chemical reaction (½ for naming it).
Evidence: evolution of a gas, seen as fizzing (½ + ½); change in temperature, the glass becomes cooler (½).
Writing only "it fizzes" without saying a gas is evolved can lose the ½.
In a gas stove fed by piped natural gas, methane (CH₄) burns in oxygen to give carbon dioxide and water.
Write (a) the word equation and (b) the skeletal chemical equation. [2]
1. Reactants on the left: methane and oxygen. Products on the right: carbon dioxide and water.
2. Word equation: methane + oxygen → carbon dioxide + water (1).
3. Replace each name with its formula. Oxygen is a molecule of two atoms, O₂; carbon dioxide is CO₂; water is H₂O.
4. Skeletal equation: CH₄ + O₂ → CO₂ + H₂O (1).
Answer: (a) methane + oxygen → carbon dioxide + water; (b) CH₄ + O₂ → CO₂ + H₂O. It is still skeletal: there are four hydrogen atoms on the left and only two on the right.
You will balance it in the next section.
Give reasons why (a) setting curd from milk is a chemical change, (b) melting of wax is not, and (c) the browning of a cut potato is a chemical change. [3]
- (a) New substances (lactic acid and set milk proteins) form, with a sour taste and a solid texture, and the change cannot be reversed (1).
- (b) Molten wax is still wax and sets again on cooling; no new substance forms, only the state changes (1).
- (c) A new brown substance forms when the cut surface reacts with oxygen in the air: a change in colour (1).
Answer: Each reason names the new substance or says that none forms.
"It changes" earns nothing; the mark is for what changes and whether a new substance is formed.
In the board paper, questions on this chapter sit in Section B (Chemistry).
Divide your answer book into three sections, Biology, Chemistry and Physics, as the instructions ask, and write each answer in its own section.
When you list signs of a reaction, write the observation in full ("a gas is evolved, seen as bubbles"; "the solution changes from blue to pale green") and name both colours whenever there is a colour change.
- Calling every change of state a reaction. Melting and boiling make nothing new. A change in state is a sign only when a new substance appears in the new state.
- Writing only "bubbles" or "it gets hot". Say "evolution of a gas" or "change in temperature (the tube becomes warm)". Examiners credit the named sign.
- Putting products on the left, or using "=" for the arrow. Reactants always go on the left and the arrow points to the products. An equals sign suggests the two sides are the same substance.
- Mixing names and formulae ("magnesium + O₂ → MgO"). Write a word equation in words only, and a chemical equation in formulae only.
- Writing O for oxygen gas. Oxygen, hydrogen, nitrogen and chlorine exist as two-atom molecules: O₂, H₂, N₂, Cl₂.
Check your understanding
1. Consider the following changes:
- (i) a silver anklet slowly turning black
- (ii) ice cream melting on a hot day in Chennai
- (iii) milk turning sour in summer
- (iv) camphor disappearing from an open dish
Which of these are chemical changes?
- (A) (i) and (ii)
- (B) (i) and (iii)
- (C) (ii) and (iv)
- (D) (iii) and (iv) [1]
2. Which of the following correctly shows the skeletal equation for "zinc reacts with copper sulphate solution to give zinc sulphate solution and copper"?
- (A) Zn + CuSO₄ → ZnSO₄ + Cu
- (B) ZnSO₄ + Cu → Zn + CuSO₄
- (C) zinc + CuSO₄ → ZnSO₄ + copper
- (D) Zn + CuSO₄ = ZnSO₄ + Cu [1]
3. Assertion (A): A change in temperature when two substances are mixed is a sign that a chemical reaction may have taken place.
Reason (R): Every change in state, such as melting or boiling, produces a new substance.
- (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
- (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
- (C) Assertion (A) is true, but Reason (R) is false.
- (D) Assertion (A) is false, but Reason (R) is true. [1]
4. Tenzing heats a little sugar in a dry test tube over a burner. It melts, turns brown and then black, and drops of a colourless liquid collect near the cool mouth of the tube. Identify two observations that show a chemical reaction has taken place. Give a reason why the melting alone does not prove it. [3]
5. Write the word equation and the skeletal chemical equation for each of the following. (a) Hydrogen gas burns in oxygen to form water. (b) Copper is heated in air and forms copper oxide. [2]
6. A teacher in a Guwahati school mixes two colourless solutions in a test tube. A bright yellow solid appears at once and slowly settles to the bottom. Name the sign of a chemical reaction seen here, and the term for the solid. [2]
Answers
1. (B) / (i) and (iii) (1). Tarnishing of silver and souring of milk make new substances; melting and the vaporising of camphor are physical changes.
2. (A) / Zn + CuSO₄ → ZnSO₄ + Cu (1). (B) is the wrong way round, (C) mixes names and formulae, and (D) uses an equals sign.
3. (C) / Assertion (A) is true, but Reason (R) is false (1). Melting and boiling are physical changes and make no new substance.
4. Any two (1 each): the colour changes from white to brown to black (a new black substance, carbon, forms); a new colourless liquid (water) is formed and collects at the mouth; the change cannot be reversed on cooling. Melting alone is a change in state of the same substance; melted sugar would set again as sugar on cooling, so melting makes no new substance (1).
5. (a) hydrogen + oxygen → water; H₂ + O₂ → H₂O (½ + ½). (b) copper + oxygen → copper oxide; Cu + O₂ → CuO (½ + ½). Both skeletal equations are unbalanced, which is correct for this question.
6. Formation of a solid from two solutions / change in state, together with a change in colour (1). The insoluble solid is a precipitate (1).