Salts: Definition and Formation
Definition of Salts: Salts are special types of chemical compounds that are created when hydrogen ions (H⁺) from an acid are replaced by metal ions or ammonium ions from a base. This swapping happens during a neutralisation reaction, which is when an acid and a base combine and cancel each other’s effects, making the solution less acidic or basic.
Composition of Salts: Every salt is made up of two important parts: positively charged ions called cations, which usually come from the base, and negatively charged ions called anions, which come from the acid. Together, these ions balance out to form a neutral compound.
Formation During Neutralisation: In a neutralisation reaction, the hydrogen ions (H⁺) from the acid react with hydroxide ions (OH⁻) from the base to create water. The leftover ions that do not react to make water combine with each other to form a salt.
Types of Salts
Soluble Salts: Some salts, such as sodium chloride (common table salt), potassium nitrate, and sodium sulfate, can dissolve easily in water. When they dissolve, they create clear and watery solutions that look just like plain water.
Insoluble Salts: Other salts, like silver chloride, lead(II) sulfate, and calcium carbonate, do not dissolve well in water. Instead, they form solid particles that settle down at the bottom of the container, and these solids are called precipitates.
Crystalline Structure of Salts
Crystalline Form: Most salts form crystals, which are solid materials with regular, neat shapes. These shapes happen because the ions in the salt arrange themselves in a very tidy and repeating pattern, known as a lattice.
Crystal Shapes: Salt crystals can take on different shapes depending on how their ions line up. Some common shapes are cubes (like sodium chloride crystals), cuboids (box-like shapes), rhombuses (diamond shapes), prisms (like 3D rectangles), or even pyramid-like forms.
Ionic Arrangement: The specific shape of a crystal depends on how the positive and negative ions attract each other through electrostatic forces. These attractions cause the ions to line up in a certain way, creating a particular crystal shape.
Surface Properties: Crystals often have smooth, flat surfaces, straight edges, and sharp corners. Depending on how the ions are arranged and packed together, the crystals can be transparent (you can see through them), translucent (you can see light through them but not clearly), or opaque (you can’t see through them at all).
Uses of Salts
In Agriculture: In farming, salts such as ammonium nitrate and calcium sulfate are very important. They are used as fertilizers to help crops grow faster and healthier, and they can also help adjust the pH level of soil to make it more suitable for plants.
In the Food Industry: In food production, salts play many roles. Sodium chloride is sprinkled on food for seasoning. Sodium bicarbonate, also known as baking soda, helps cakes and breads rise and become fluffy. Sodium nitrite is used to preserve meats so they stay fresh longer.
In Medicine: In hospitals and healthcare, salts are used in many ways. Plaster of Paris is used to make hard casts for broken bones. Epsom salts are used as a laxative to help with constipation. Potassium permanganate is used as a disinfectant to clean wounds and prevent infection.
In Industry: In factories, salts are also very useful. Calcium sulfate is used to make plaster and paper. Silver bromide is important for making photographic films used in cameras.
Specific Salt Examples
Sodium Chloride (NaCl): This salt is formed when hydrochloric acid reacts with sodium hydroxide. It is the everyday table salt we use for cooking and seasoning food.
Calcium Sulfate (CaSO₄): This salt is found in gypsum and is used to make plaster of Paris, which is important in construction work and in making medical casts.
Potassium Nitrate (KNO₃): Potassium nitrate is an important ingredient in fertilizers that help plants grow better and is also a key material in making gunpowder.
Copper(II) Sulfate (CuSO₄): This salt is used by farmers to kill fungi on plants, and in a special technique called electroplating, where a thin layer of metal is coated onto another metal.
Sodium Carbonate (Na₂CO₃): Factories use sodium carbonate to make things like glass, soaps, and many cleaning products.
Solubility of Salts in Detail
General Solubility Trends: As a general rule, most nitrate salts, as well as all sodium, potassium, and ammonium salts, dissolve easily in water. On the other hand, many carbonates and hydroxides do not dissolve very well.
Exceptions in Solubility: Even though there are general rules about which salts dissolve, there are exceptions. For example, silver chloride and barium sulfate are salts that do not dissolve easily in water, even though other similar salts do.
Methods of Preparing Salts
Neutralisation Method: One common way to make a salt is to mix an acid with a base. They react together to make a salt and water.
Reaction with Reactive Metals: Some salts can be made by reacting an acid with a reactive metal. When they react, the products are a salt and hydrogen gas.
Reaction with Metal Oxides: Another way to prepare salts is by reacting acids with metal oxides. This reaction forms a salt and water.
Reaction with Metal Carbonates: When acids react with metal carbonates, they form three things: a salt, carbon dioxide gas, and water.
Precipitation Method: This method involves mixing two solutions, each containing a soluble salt. When mixed, they can form an insoluble salt, which sinks to the bottom as a solid.
Crystallisation
Process of Crystallisation: Crystals can be formed by first dissolving a salt in a solvent like water. Then, by slowly evaporating the water, the salt comes out of the solution and forms solid crystals.
Recrystallisation for Purification: To purify a salt and remove impurities, the salt can be dissolved again in water and then recrystallised carefully. This way, the pure salt forms crystals, while impurities stay in the solution.
Qualitative Analysis of Salts
Physical Properties Observation: Scientists can find clues about a salt’s identity by observing its colour, how well it dissolves in water, and whether it is a solid, liquid, or gas.
Heating Tests: When salts are heated, they might release gases like carbon dioxide. By observing these gases, scientists can figure out what kind of salt it is.
Flame Tests: In a flame test, different metal ions make flames turn different colours. These colour changes help scientists identify which metal is in the salt.
Reaction with Acids: Some negative ions in salts react with acids to produce gases, like carbon dioxide, which can help scientists identify the salt.
Reaction with Alkalis: Some metal ions in salts react with alkalis to form solid metal hydroxides. These solids can give clues about which metal is present in the salt.
Reactions of Salts
Thermal Decomposition: When some salts, especially metal carbonates, are heated strongly, they break apart into metal oxides and release carbon dioxide gas.
Reactions with Acids: Some salts can react with acids to produce a new salt and release gases like carbon dioxide.
Reactions with Alkalis: Certain salts react with alkalis to form new metal hydroxides, which appear as solid precipitates, and also form new salts.