Swapping partners: double replacement and neutralisation
Science Prep 3 Term 1, Unit 1: pp. 9–14
GoalExplain double replacement as swapping partners between ionic compounds, and write neutralisation as acid + alkali → salt + water.
New words
ionic compound · a compound made of positive ions (cations) and negative ions (anions), such as sodium chloride, Na⁺ and Cl⁻double replacement · a reaction in which the positive and negative ions of two compounds swap partners: AB + CD → AD + CBneutralisation · the reaction of an acid with an alkali that gives a salt and water
Explanation
Double replacement happens between two ionic compounds, usually dissolved in water. An ionic compound has positive ions, the cations, and negative ions, the anions: sodium chloride is Na⁺ and Cl⁻. In water the ions move freely, and they swap partners: AB + CD → AD + CB. Neutralisation is a double replacement: acid + alkali → salt + water. HCl + NaOH → NaCl + H₂O. A salt is any ionic compound made from an acid and a base, not only table salt. A universal indicator shows how acidic a liquid is: violet in a strong alkali, green when neutral, then yellow, orange and red as the liquid becomes more acidic. Careful: neutral does not mean that nothing happened. At the neutral point the acid and the alkali have just used each other up. The book’s experiment uses laboratory acids and alkalis, so a kitchen stand-in is safer.
AB + CD→AD + CB
the ions swap partners
Worked examples
Salma’s model: red clips are cations, blue clips are anions. Two pairs, red1-blue1 and red2-blue2, are separated and rejoined as red1-blue2 and red2-blue1. The clips were swapped, none were lost: this is double replacement.
HCl + NaOH→NaCl + H2O
Write HCl + NaOH → NaCl + H₂O as ions: H⁺ and Cl⁻ meet Na⁺ and OH⁻. The partners swap: Na⁺ joins Cl⁻ to make the salt NaCl, and H⁺ joins OH⁻ to make water.
Colour
violet
green
yellow–red
liquid
alkali
neutral
acid
An indicator is violet in the liquid. Drops of acid are added, and the colour changes to blue-green, then green, then yellow. So the liquid was first alkaline, became neutral at green, and is now acidic.
Class activity
Red cabbage indicator, with a grown-up. Soak shredded red cabbage in warm tap water for ten minutes and strain the purple liquid into three cups. Add a little lemon juice to one, a little baking soda solution to another, and keep the third as it is. Describe the three colours (acid usually pink-red, baking soda blue-green, water stays purple). Then add vinegar to the blue-green cup drop by drop and describe the colour sequence. Never taste anything.
Practice
1
In a double replacement, what do the ions do?
The cations and anions of the two compounds swap partners.
2
Name the salt made when sodium hydroxide neutralises hydrochloric acid, and the other product.
Sodium chloride and water.
3
The indicator is green. Is the liquid acidic, alkaline or neutral?
Neutral.
4
A friend says “neutral means no reaction took place”. What is wrong?
Neutral is the result of a reaction: the acid and the alkali have used each other up.
