Salt is far more than the white powder on your plate. In chemistry, a salt is what you get when an acid and a base react. NDA Chemistry loves this topic because it links acids, bases and daily life into simple one-line questions. The Cavalier breaks down common salts, their formulas and uses in plain language so you bank these marks with confidence.
Why Salts Matter in NDA
Open any NDA General Ability Test chemistry section and you will find at least one question on common salts — the chemical name of baking soda, the formula of bleaching powder, or the use of plaster of Paris. These are pure-recall, no-calculation marks.
What makes salts a scoring topic is that the same handful of compounds repeat year after year: common salt, washing soda, baking soda, bleaching powder, gypsum and plaster of Paris. Learn this short list well and you collect easy marks while others guess.
The topic also ties neatly into acids and bases, which the GAT loves to test together. Once you understand that a salt is simply the leftover product when an acid neutralises a base, half the chapter falls into place. There is no formula to derive and no long calculation — just clear, organised recall. That is exactly the kind of question you want to get right quickly so you can save time for the harder maths and reasoning sections.
For every common salt, memorise three things together: its chemical name, its formula, and one main use. Questions almost always ask for one of these three.
What Exactly Is a Salt?
A salt is an ionic compound formed when the hydrogen ion (H+) of an acid is replaced by a metal ion or an ammonium ion. In short, a salt is the product of a reaction between an acid and a base.
Acid + Base → Salt + Water
This reaction is called neutralisation. Example: HCl + NaOH → NaCl + H2O.
Every salt has two parts — a positive ion (cation) coming from the base, and a negative ion (anion) coming from the acid. In common salt, NaCl, the Na+ comes from the base NaOH and the Cl− comes from the acid HCl.
Salts are usually solid, crystalline and ionic. Because the ions are held in a rigid lattice, most salts have high melting points and conduct electricity when melted or dissolved in water but not in the solid state. Many salts dissolve readily in water, breaking into their free ions — which is why salt solutions are good conductors. These properties separate salts from molecular compounds like sugar, and the exam sometimes tests this difference directly.
The name of a salt comes from its parent acid: chlorides from hydrochloric acid, sulphates from sulphuric acid, nitrates from nitric acid, and carbonates from carbonic acid.
Types of Salts
Salts are grouped by the strength of the acid and base that made them, and this decides whether the salt solution is acidic, basic or neutral.
Four common categories
- Normal salt: all replaceable H+ of the acid is replaced. Example: NaCl, Na2SO4.
- Acid salt: only part of the acidic hydrogen is replaced. Example: NaHCO3 (sodium bicarbonate), NaHSO4.
- Basic salt: contains a hydroxide (OH) along with the salt part. Example: basic copper carbonate.
- Double salt: a combination of two simple salts, like potash alum.
Salt of strong acid + strong base → neutral (NaCl, pH 7).
Salt of strong acid + weak base → acidic (NH4Cl, pH < 7).
Salt of weak acid + strong base → basic (Na2CO3, pH > 7).
This idea is called hydrolysis of salts and NDA often tests it with a one-line question asking which salt solution is acidic or basic. The logic is simple: the stronger partner wins. If the base is stronger than the acid, the leftover ions make the solution basic; if the acid is stronger, the solution turns acidic; and if both are equally strong, they cancel out and the salt is neutral.
One more useful idea is water of crystallisation — fixed water molecules built into a salt crystal. Such salts are called hydrated (for example, washing soda and blue copper sulphate). A salt with no such water is called anhydrous. Heating a hydrated salt drives off this water, often changing its colour, and the dry form can then absorb moisture again.
Common Salt (Sodium Chloride, NaCl)
Sodium chloride, the salt on your dining table, is the most important salt of all. It is obtained from sea water and from underground rock deposits called rock salt.
Why it is so useful
- Essential in our diet and used to preserve food like pickles and fish.
- It is the raw material for making sodium hydroxide, washing soda, baking soda and chlorine gas.
- Spread on icy roads in cold countries to lower the freezing point of water.
When electricity is passed through brine (concentrated NaCl solution), it gives NaOH, Cl2 and H2. This is the chlor-alkali process — a favourite NDA fact.
Common salt is neutral, not acidic. It comes from the strong acid HCl and the strong base NaOH, so its water solution has pH 7.
Common salt is also the starting point for a whole family of useful chemicals. From NaCl we make sodium hydroxide (caustic soda), sodium carbonate (washing soda), sodium hydrogen carbonate (baking soda), chlorine and hydrogen. Because so many industries depend on it, common salt is often called the backbone of the chemical industry — a phrase worth remembering for the exam.
Sodium Hydroxide and Bleaching Powder
The chlor-alkali process gives us two more exam-important products from common salt.
Sodium hydroxide (NaOH)
Also called caustic soda, it is a strong base used in making soap, paper, and to clean drains. It is highly corrosive, so it is handled with care.
Bleaching powder (CaOCl2)
Calcium oxychloride, made by passing chlorine over dry slaked lime [Ca(OH)2].
- Used for bleaching cotton, linen and paper.
- Used to disinfect drinking water as it releases chlorine.
- Used as an oxidising agent in many industries.
Bleaching powder reaction: Ca(OH)2 + Cl2 → CaOCl2 + H2O. The active part is chlorine, which does the bleaching.
Baking Soda (Sodium Bicarbonate, NaHCO3)
Sodium hydrogen carbonate, or sodium bicarbonate, is a mild, non-corrosive basic salt made from common salt.
Where you meet it
- In the kitchen, it makes cakes and bread soft and fluffy by releasing carbon dioxide gas on heating.
- It is a key part of baking powder (baking soda + a mild edible acid like tartaric acid).
- Used as an antacid to neutralise excess acid in the stomach.
- Used in soda-acid fire extinguishers.
On heating: 2NaHCO3 → Na2CO3 + H2O + CO2. The CO2 released is what makes dough rise.
Tartaric acid is added to baking powder so the cake does not taste bitter. Without it, the leftover sodium carbonate makes the food bitter — a common NDA one-liner.
Washing Soda (Sodium Carbonate, Na2CO3·10H2O)
Washing soda is sodium carbonate with ten water molecules attached (hydrated). It is made by heating baking soda and then recrystallising the sodium carbonate formed.
Main uses
- Used in washing clothes and in laundries as a cleaning agent.
- Removes the permanent hardness of water.
- Used in the manufacture of glass, soap and paper.
- Used in the textile and chemical industries.
The 10 water molecules in Na2CO3·10H2O are called water of crystallisation. They are part of the crystal even though the salt looks dry.
Washing soda removes hardness by reacting with the calcium and magnesium salts dissolved in hard water. It converts them into insoluble carbonates that settle out, leaving the water soft enough for soap to lather properly. This is why washing soda is added to detergents and used in laundries. Its water solution is basic, which also helps loosen grease and dirt from clothes.
Do not mix up baking soda (NaHCO3) and washing soda (Na2CO3·10H2O). Baking soda is used in cooking and as an antacid; washing soda is used for cleaning and softening water.
Gypsum and Plaster of Paris
This pair of salts based on calcium sulphate appears in the exam almost every year.
Gypsum (CaSO4·2H2O)
A soft mineral, calcium sulphate dihydrate. It is added to cement to slow down its setting and used to make plaster of Paris.
Plaster of Paris (CaSO4·½H2O)
Made by heating gypsum to about 100°C (373 K). It is a white powder that sets into a hard solid when mixed with water.
- Doctors use it to support fractured bones in plaster casts.
- Used for making toys, statues and decorative materials.
- Used as a smooth surface coating on walls.
Setting reaction: CaSO4·½H2O + 1½H2O → CaSO4·2H2O. Plaster of Paris takes up water and turns back into gypsum, which is why it hardens.
Other Salts Worth Knowing
A few more salts make regular short appearances in NDA papers.
- Potassium nitrate (KNO3): used in gunpowder and as a fertiliser. Also called saltpetre.
- Ammonium chloride (NH4Cl): used in dry cells and as a soldering flux; its solution is acidic.
- Copper sulphate (CuSO4·5H2O): blue vitriol, used as a fungicide and in electroplating.
- Silver chloride (AgCl): used in photographic films; it turns grey in sunlight.
- Potash alum: a double salt used to purify water and stop bleeding from small cuts.
Many salts contain water of crystallisation: blue copper sulphate (5H2O), green ferrous sulphate (7H2O), washing soda (10H2O). Heating drives this water off and changes their colour.
Worked Example
Let us see how these facts turn into an exam answer.
Identify the salt and predict whether its water solution is acidic, basic or neutral: a white salt used in baking, made from a strong base and a weak acid.
Notice how a single salt is described in five linked facts. NDA can ask for any one of them, so learning them as a group pays off. The same five-step approach works for any salt in the syllabus: name it, write its formula, identify the parent acid and base, judge their strengths, and then state the nature of its solution. With practice, this becomes an instant mental check rather than a guess.
Previous-Year Style Question
Try this NDA-style question before reading the answer.
Q. Plaster of Paris is obtained by heating gypsum at a temperature of about:
Answer: About 100°C (373 K). Heating gypsum (CaSO4·2H2O) to roughly this temperature drives off most of the water of crystallisation, leaving plaster of Paris (CaSO4·½H2O). Heating it much higher gives dead-burnt plaster, which will not set with water.
If a question mentions a salt setting hard with water and used for bone casts, the answer is almost certainly plaster of Paris.
Quick Revision
- A salt is formed when an acid reacts with a base (neutralisation).
- Strong acid + strong base → neutral salt; weak acid + strong base → basic; strong acid + weak base → acidic.
- NaCl — common salt, raw material for NaOH, washing soda and baking soda.
- NaHCO3 — baking soda; antacid, cooking, fire extinguishers.
- Na2CO3·10H2O — washing soda; cleaning and softening hard water.
- CaOCl2 — bleaching powder; bleaching and disinfecting water.
- CaSO4·½H2O — plaster of Paris; bone casts and statues.
Run through this list the night before the exam and you will recognise nearly every salt question on sight.
Frequently asked questions
What is the difference between baking soda and washing soda?
Baking soda is sodium hydrogen carbonate (NaHCO3) used in cooking and as an antacid, while washing soda is sodium carbonate decahydrate (Na2CO3.10H2O) used for cleaning clothes and softening hard water.
Why is plaster of Paris stored in airtight containers?
Because it readily absorbs moisture from the air. If exposed, the water makes it set into hard gypsum, so it must be kept sealed and dry until use.
Is common salt (NaCl) acidic, basic or neutral?
It is neutral, with a pH of 7. NaCl is formed from the strong acid HCl and the strong base NaOH, so neither ion changes the pH of water.
What is the chemical name and use of bleaching powder?
Its chemical name is calcium oxychloride (CaOCl2). It is used to bleach cotton, linen and paper, and to disinfect drinking water by releasing chlorine.
Why is tartaric acid added to baking powder?
Tartaric acid neutralises the sodium carbonate left after baking soda decomposes. Without it, the leftover carbonate would make the food taste bitter.
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