Salts and When Double Replacement Happens盐与复分解反应发生的条件

Salt isn’t just table salt

In everyday speech, “salt” means table salt. In chemistry, a salt盐 is any compound made of metal ions (or ammonium ions) and the negative ions from an acid. Baking soda, washing soda, chalk, and marble are all salts.

Common salts

Sodium carbonate’s solution is basic, but sodium carbonate is a salt, not a base.

Sodium carbonate, sodium bicarbonate, and calcium carbonate all react with dilute hydrochloric acid to release carbon dioxide:

So to test for a carbonate, add dilute hydrochloric acid to a sample. If it gives off a gas that turns limewater cloudy, the sample contains carbonate (or bicarbonate) ions.

When does double replacement happen?

Lesson 1.6.4 defined a double replacement reaction as two compounds swapping partners to form two new compounds:

That lesson left a question open: when do two compounds mixed together actually react? Here’s the answer:

A double replacement reaction between acids, bases, and salts happens only if one of the products is a precipitate, a gas, or water.

Also, when a base reacts with a salt, or a salt with a salt, both reactants usually need to be soluble.

Solubility rules

To spot a precipitate, you need to know which compounds don’t dissolve in water. These rules cover everything in this course:

• All sodium, potassium, and ammonium salts and all nitrates are soluble.
• Chlorides are soluble, except silver chloride, $\ce{AgCl}$.
• Sulfates are soluble, except barium sulfate, $\ce{BaSO4}$; calcium sulfate and silver sulfate are only slightly soluble.
• Carbonates are insoluble (calcium carbonate, barium carbonate, and so on), except those of sodium, potassium, and ammonium.
• Most bases are insoluble, including blue copper(II) hydroxide and reddish-brown iron(III) hydroxide. Sodium, potassium, and barium hydroxides and aqueous ammonia are soluble; calcium hydroxide is slightly soluble.
• Common acids are soluble.

Common precipitates: white $\ce{AgCl}$, $\ce{BaSO4}$, $\ce{CaCO3}$, $\ce{BaCO3}$, and $\ce{Mg(OH)2}$; blue $\ce{Cu(OH)2}$; and reddish-brown $\ce{Fe(OH)3}$.

Pick any two solutions and predict whether they react before checking. Watch out for sodium hydroxide and hydrochloric acid, which react with no visible change:

Example 1: Will they react?

Predict whether each pair undergoes double replacement, and write an equation if it does: (1) barium chloride and sodium sulfate; (2) potassium nitrate and sodium chloride; (3) sodium carbonate and limewater.

Example 2: Has the sodium hydroxide gone bad?

A bottle of sodium hydroxide solution has been left open. How can you check whether it has changed?

Common mistakes

Quick check

Summary

• A salt is made of metal (or ammonium) ions and the negative ions of an acid; table salt, washing soda, baking soda, and limestone are all salts. Carbonates fizz with acid, releasing a gas that turns limewater cloudy.
• Double replacement happens only if a precipitate, a gas, or water forms; base–salt and salt–salt reactions need both reactants dissolved.
• Sodium, potassium, ammonium, and nitrate compounds are soluble; $\ce{AgCl}$ and $\ce{BaSO4}$ are insoluble; most carbonates and hydroxides are insoluble, though $\ce{NaOH}$, $\ce{KOH}$, and $\ce{Ba(OH)2}$ dissolve and $\ce{Ca(OH)2}$ is slightly soluble.
• To predict double replacement: swap partners, then look for a precipitate, gas, or water among the products.