MYP Integrated Sciences Formation and Uses of Salts Notes

What is a Salt?

In chemistry, a salt is an ionic compound made of:

1. Positive ions (cations) – often from a metal or the ammonium ion (NH₄⁺)
2. Negative ions (anions) – usually from an acid

These ions are held together by electrostatic attraction (ionic bonding) in a regular giant lattice.

Salts in Solution – Neutral, Acidic or Basic?

1. When a salt dissolves in water, its ions interact with water.
2. Depending on the strength of the acid and base it came from, the solution can be:
   1. Neutral – salt of strong acid + strong base (e.g. NaCl, KNO₃).
   2. Basic – salt of weak acid + strong base (e.g. Na₂CO₃, CH₃COONa).
   3. Acidic – salt of strong acid + weak base (e.g. NH₄Cl).
3. For MYP level, remember:
   1. Salts from strong acid + strong base → usually neutral solutions.
   2. Salts from weak acid + strong base → basic solutions.
   3. Salts from strong acid + weak base → acidic solutions.

How are Salts Formed?

Acid-Base Neutralization

1. A neutralization reaction between an acid and a soluble base (alkali) produces salt and water.
2. General word equation: $$\text{Acid} + \text{Base} \to \text{Salt} + \text{Water}$$

Metal and Acid Reaction

1. Acids can react with reactive metals to form salt and hydrogen gas.
2. General word equation: $$\text{Metal} + \text{Acid} \rightarrow \text{Salt} + \text{Hydrogen gas}$$

Acid + Insoluble Base (Metal Oxide or Metal Carbonate)

1. Some bases are insoluble in water (e.g. CuO, CaCO₃), but they still neutralize acids.
2. General word equation: $$\text{Metal oxide} + \text{Acid} \rightarrow \text{Salt} + \text{Water}$$

Ammonia and Acid Reactions

1. Ammonia solution (NH₃(aq)) is a weak base.
2. It reacts with acids to form ammonium salts.

Precipitation Reaction: Insoluble Salts

If the salt you want is insoluble in water, you can make it by mixing two soluble salts so that an insoluble product (precipitate) forms.

Soluble salt A + soluble salt B → insoluble salt C (precipitate) + soluble salt D

Choosing the Best Method to Prepare a Salt

To decide which method to use, ask:

1. Is the salt soluble or insoluble in water?
   1. Soluble salt → use neutralisation (acid + soluble base) or metal/insoluble base + acid, then crystallisation.
   2. Insoluble salt → use precipitation (two soluble salts).
2. Is the base soluble?
   1. Soluble base (alkali) (e.g. NaOH, KOH) → use titration for accurate neutralisation.
   2. Insoluble base (e.g. CuO, CaCO₃) → add in excess, then filter.
3. Is the metal reactive with acid?
   1. If yes (e.g. Mg, Zn, Fe) → can use metal + acid.
   2. If not (e.g. Cu) → use oxide or carbonate + acid instead.

Properties of Salts and Their Uses

Salts are not just abstract particles – their physical properties determine where they are used in real life.

Crystal Structure and Shape

1. Most salts are crystalline solids with a regular lattice structure.
   1. NaCl forms cubic crystals.
   2. CuSO₄·5H₂O forms bright blue crystals.
   3. Some salts are hydrated (contain water molecules in their crystal structure), others are anhydrous.
2. Uses linked to crystals:
   1. Copper(II) sulfate crystals – used in education to show crystallisation and as a fungicide.
   2. Gypsum (CaSO₄·2H₂O) – used in plaster and construction because it sets in a controlled way.

Solubility

1. Many salts (e.g. NaCl, KNO₃, Na₂SO₄) are highly soluble in water.
2. Some are sparingly soluble or insoluble, like BaSO₄ or AgCl.

Uses:

1. Soluble salts as fertilisers:
   1. KNO₃, NH₄NO₃, (NH₄)₂SO₄ dissolve easily in soil water and supply plants with nitrate and potassium.
2. Insoluble salts in medicine:
   1. Barium sulfate (BaSO₄) is used as a contrast agent in X-ray imaging of the digestive system.
   2. It is opaque to X-rays and insoluble, so it passes through the body without dissolving.
3. De-icing salts:
   1. Salts like NaCl or CaCl₂ are spread on roads in winter because they are soluble and lower the freezing point of water.

Electrical Conductivity

1. In the solid state, salts do not conduct electricity because the ions are fixed in the lattice.
2. When molten or dissolved in water, the ions are free to move, so they conduct electricity.

Uses:

1. Electrolytes in the human body (e.g. Na⁺, K⁺, Cl⁻) allow nerve impulses and muscle function.
2. Salts in electrolysis, e.g. molten NaCl used to produce sodium metal and chlorine gas.

Thermal Stability

1. Some salts decompose on heating:
   1. Many carbonates (except Group 1) decompose to oxides and CO₂.
   2. Ammonium salts (e.g. NH₄Cl) decompose on strong heating to give gases (e.g. NH₃ and HCl).
2. This affects:
   1. How they are prepared (e.g. ammonium salts crystallised by gentle evaporation, not strong heating).
   2. Their storage and industrial use.

How Do Properties of Salts Make Them Useful or Hazardous?

Salts are not all the same. Their properties determine whether they are helpful, harmful, or both:

1. Solubility – how well they dissolve in water.
2. Hygroscopicity – ability to absorb water from the air.
3. Toxicity – how harmful they are to living organisms.

Solubility

Highly Soluble Salts

1. Example: Sodium chloride, NaCl
   1. Dissolves easily in water.
   2. Forms ions that move freely → important for electrolyte solutions and de-icing roads.
2. Advantages:
   1. Easy to dissolve and spread.
   2. Cheap and widely available.
3. Risks:
   1. Runoff can increase soil and water salinity, harming plants and aquatic life.
   2. Corrodes metal structures (cars, bridges).

Low-Solubility Salts

1. Example: Barium sulfate, BaSO₄
   1. Very poorly soluble in water.
   2. Because it does not dissolve, it is not absorbed by the body.
2. Advantages:
   1. Good contrast agent for imaging.
   2. Passes through the body without entering the bloodstream.
3. Risks:
   1. If a soluble barium salt were used by mistake, it would be highly toxic.
   2. Must be used only under medical supervision.

Ability to Absorb Water (Hygroscopicity)

Some salts can absorb water from the air; some even form hydrated crystals or turn into a solution.

Desiccants (Drying Agents)

1. Examples: Calcium chloride (CaCl₂), magnesium sulfate (MgSO₄)
   1. Used to keep things dry by absorbing moisture from their surroundings.
   2. Often found in small sachets in electronics, shoe boxes, medicine bottles.
2. Advantages:
   1. Prevents corrosion, mould and spoilage.
   2. Useful in packaging, laboratories, and industry.
3. Risks:
   1. Can cause corrosion if they release absorbed water in the wrong place.
   2. Some can cause skin and eye irritation – handle with care and avoid ingestion.

Toxicity

Not all salts are safe. Some are essential nutrients; others are highly toxic.

Beneficial Salts

Examples:

1. Potassium chloride (KCl) – used in fertilisers to provide potassium, an essential nutrient for plants.
2. Sodium chloride (NaCl) – needed in small amounts in our diet to regulate fluid balance and nerve function.

Toxic or Hazardous Salts

1. Examples:
   1. Mercury(II) chloride, HgCl₂ – highly toxic; can damage the nervous system and organs.
   2. Lead(II) nitrate, Pb(NO₃)₂ – toxic; exposure can cause serious health problems, especially in children (brain and developmental issues).
2. Risks:
   1. Can contaminate water and soil.
   2. Can enter the food chain, accumulating in organisms.

How Are Salts Used in Food, Agriculture and Medicine?

Because of their varied properties, salts are used in many areas of everyday life.

Food Preservation

1. Main salt used: Sodium chloride (NaCl)
   1. Used to preserve food by drawing water out of microbial cells by osmosis.
   2. Less water → bacteria and fungi cannot grow easily.
2. Examples:
   1. Salting meat or fish to prevent spoilage.
   2. Pickling vegetables in brine.
3. Benefits:
   1. Extends shelf life.
   2. Reduces food waste.
   3. Traditional and low-cost method.
4. Health considerations:
   1. Our bodies need some salt.
   2. Too much dietary NaCl can contribute to high blood pressure (hypertension) and cardiovascular disease.

Agriculture

Fertilisers

1. Examples:
   1. Potassium chloride (KCl) – supplies potassium for plant growth.
   2. Ammonium nitrate (NH₄NO₃) – supplies nitrogen, important for leaf and stem growth.
2. Benefits:
   1. Boosts crop yield.
   2. Helps feed a growing global population.
3. Risks:
   1. Overuse can lead to soil salinity and nutrient imbalance.
   2. Nitrate can leach into water, contributing to eutrophication (algal blooms) in lakes and rivers.

Soil Conditioning

1. Example: Gypsum (CaSO₄·2H₂O)
   1. Improves soil structure by helping clay particles clump together.
   2. Helps reduce soil salinity and improve drainage.
2. Benefits:
   1. Better root growth.
   2. Improved crop health.

Medicine

Electrolyte Solutions

Examples: NaCl, KCl, other balanced salt solutions

1. Used in intravenous (IV) fluids to restore or maintain electrolyte balance.
2. Important during dehydration, surgery, or illness.

Epsom Salt (Magnesium sulfate, MgSO₄)

1. Used in baths to relieve muscle pain and inflammation.
2. Used medicinally as a laxative in controlled doses.
3. Benefits:
   1. Supports essential body functions (nerve impulses, muscle contraction).
   2. Can aid recovery and comfort.
4. Risks:
   1. Incorrect concentration or uncontrolled self-medication can be harmful.
   2. IV solutions must be carefully formulated and administered by professionals.

How Do We Assess the Benefits and Risks of Using Salts?

To evaluate a salt’s use, we need to consider several perspectives:

1. Environmental impact
2. Health risks
3. Economic cost
4. Ethical and regulatory factors

Environmental Impact

Soil Salinity

1. Excessive use of salts (NaCl, KCl) can build up in soil.
2. High salt levels make it harder for plants to take up water → reduced growth and yield.

Water Pollution

1. Salts from de-icing and fertilisers can wash into rivers and lakes.
2. Increased salinity can stress or kill freshwater organisms (fish, amphibians, invertebrates).

Health Risks

Toxic Salts

1. Salts of heavy metals (e.g. lead, mercury, cadmium) can cause serious health problems:
   1. Nervous system damage
   2. Kidney/liver damage
   3. Developmental issues in children
2. These must be:
   1. Carefully controlled in industry.
   2. Stored, handled and disposed of safely.

Dietary Salt

1. NaCl is essential in small amounts.
2. Too much in the diet is linked to hypertension and heart disease.

Economic Considerations

1. NaCl is cheap and widely available → attractive for de-icing and food preservation.
2. More environmentally friendly alternatives (like CMA) are more expensive.
3. Trade-off:
   1. Cheaper salts may have higher environmental costs.
   2. Greener alternatives may be less affordable for large-scale use.

Ethical and Regulatory Considerations

1. Governments may ban or restrict toxic salts (e.g. lead-based salts) to protect public health.
2. Industry has an ethical responsibility to:
   1. Use salts in ways that minimise harm.
   2. Comply with environmental regulations.
   3. Invest in safer alternatives and proper waste management.