Study Notes: Common Acids, Alkalis, and Neutralisation
Hello there! Ever wondered what makes lemons taste sour, or why soap feels a bit slippery? It's all down to a fascinating part of science involving two special groups of chemicals: acids and alkalis.
In these notes, we're going to explore what acids and alkalis are, how we can identify them, and what happens when they meet in a chemical reaction called neutralisation. This is super important because these reactions happen everywhere – from inside our own stomachs to the soil in a farmer's field! Let's get started.
1. Getting to Know Acids and Alkalis
Acids and alkalis are like two different teams in the world of chemistry. They have opposite properties.
What is an Acid?
Acids are substances that are very common in our daily lives, especially in food!
Properties of Acids:
- They usually have a sour taste. (Think of a lemon!)
- They can be corrosive, which means they can damage or "eat away" at other materials like metal or stone.
- They turn a special chemical called litmus indicator from blue to red. (We'll learn more about indicators soon!)
Common Examples:
- In the kitchen: Vinegar, lemon juice, orange juice.
- In the lab: Hydrochloric acid, sulphuric acid.
What is an Alkali?
Alkalis are the chemical opposites of acids. An alkali is a type of substance called a 'base' that can dissolve in water.
Properties of Alkalis:
- They often have a bitter taste. (But please don't taste chemicals to check!)
- They feel soapy or slippery to the touch. (Think about soap!)
- Like strong acids, strong alkalis can also be very corrosive.
- They turn litmus indicator from red to blue.
Common Examples:
- At home: Baking soda, soap, toothpaste, window cleaner.
- In the lab: Sodium hydroxide, calcium hydroxide (limewater).
Key Takeaway: Acids are typically sour and corrosive, while alkalis are typically bitter, feel soapy, and can also be corrosive. They are chemical opposites!
2. The pH Scale: A Ruler for Acidity
So, how do we know exactly how acidic or alkaline something is? We can't just taste it (that's very dangerous!). Instead, scientists use something called the pH scale.
What are Indicators?
An indicator is a special dye that changes colour depending on whether it is in an acid or an alkali. Think of it as a chemical detective that reveals the identity of a solution.
Did you know? You can make your own simple indicator at home! The juice from boiled red cabbage changes colour when you add acidic or alkaline things to it. This is an example of a natural indicator.
The pH Scale Explained
The pH scale is a range of numbers from 0 to 14 that tells us the strength of an acid or an alkali. It’s like a ruler for measuring acidity!
- A substance with a pH less than 7 is an acid. (The lower the number, the stronger the acid. pH 1 is a very strong acid!)
- A substance with a pH of exactly 7 is neutral. (Pure water is the best example!)
- A substance with a pH greater than 7 is an alkali. (The higher the number, the stronger the alkali. pH 14 is a very strong alkali!)
How to Measure pH
There are a few common ways to measure pH:
1. Universal Indicator or pH Paper: This is a mixture of different indicators that produces a whole rainbow of colours depending on the pH. You dip the paper in the solution (or add a few drops of the liquid indicator) and match the colour to a chart to find the pH.
Advantages: Cheap, quick, and easy to use.
Disadvantages: Not very precise; you only get a rough idea of the pH.
2. Electronic pH Meter: This is a device with a probe that you place in the solution. It gives you a precise digital reading of the pH value.
Advantages: Very accurate and reliable.
Disadvantages: More expensive and needs to be calibrated (set up correctly) before use.
Key Takeaway: The pH scale (0-14) measures acidity and alkalinity. We use indicators like pH paper or electronic meters to find the pH of a substance.
3. Neutralisation: When Acids and Alkalis Meet
What happens when you mix an acid with an alkali? They don't fight – they react to cancel each other out! This process is called neutralisation.
Analogy: Think of an acid as "hot" and an alkali as "cold". When you mix them in the right amounts, you get something "lukewarm" or neutral.
The Products: Salt and Water
When an acid and an alkali neutralise each other, they always produce two things: a salt and water.
This gives us a golden rule in chemistry!
The general word equation is:
$$ \textbf{Acid} + \textbf{Alkali} \rightarrow \textbf{Salt} + \textbf{Water} $$
For Extension: Writing Word Equations
Let's look at a real example. When hydrochloric acid mixes with the alkali sodium hydroxide:
$$ \text{Hydrochloric Acid} + \text{Sodium Hydroxide} \rightarrow \text{Sodium Chloride} + \text{Water} $$
(Sodium chloride is just the chemical name for common table salt!)
How to name the salt:
- Hydrochloric Acid makes salts called chlorides.
- Sulphuric Acid makes salts called sulphates.
- Nitric Acid makes salts called nitrates.
Following the pH Change
If we slowly add an alkali to an acid and measure the pH as we go, we see a cool pattern. This can be shown on a pH curve graph.
Step 1: We start with just the acid. The pH is very low (e.g., pH 1).
Step 2: As we add the alkali, the acid starts getting neutralised, and the pH rises slowly.
Step 3: Suddenly, right around the neutralisation point, the pH shoots up very quickly, passing through pH 7 (neutral).
Step 4: Once all the acid is gone, any more alkali we add just makes the solution more and more alkaline, and the pH becomes high (e.g., pH 13).
Conservation of Mass
Here's a super important rule in science: in any chemical reaction, matter is never created or destroyed. This is called the Law of Conservation of Mass.
In a neutralisation reaction, this means that the total mass of the acid and alkali you start with is exactly equal to the total mass of the salt and water you end up with. Nothing is lost!
Applications of Neutralisation in Real Life
This isn't just for the lab! We use neutralisation all the time:
- Indigestion Relief: Our stomach contains hydrochloric acid. If it produces too much, you get indigestion. Antacid tablets contain a weak alkali (like magnesium hydroxide) that neutralises the extra acid and makes you feel better.
- Farming: If soil becomes too acidic from acid rain, crops won't grow well. Farmers spread a powdered alkali like lime (calcium hydroxide) on their fields to neutralise the soil.
- Treating Stings: A bee sting is acidic, so it can be soothed with a weak alkali like baking soda. A wasp sting is alkaline, so it can be soothed with a weak acid like vinegar.
Key Takeaway: Neutralisation is the reaction between an acid and an alkali. It always produces salt and water, and the final solution has a pH around 7.
4. Hazards, Safety, and Acid Rain
The Corrosive Nature of Acids
Many acids are corrosive. This is why we must always be careful in the lab and wear safety goggles!
- Acids and Metals: When a dilute acid reacts with certain metals (like magnesium or iron), it produces a salt and hydrogen gas.
- Acids and Carbonates: When an acid reacts with a carbonate (like limestone or marble), it produces a salt, water, and carbon dioxide gas. This is why acid rain can damage buildings made of these materials!
Acid Rain
Acid rain is a major environmental problem. Here's how it happens:
1. Cause: Factories, power plants, and cars burn fossil fuels. This releases harmful gases like sulphur dioxide and nitrogen oxides into the air.
2. Reaction: These gases mix with water droplets in the clouds, forming weak solutions of sulphuric acid and nitric acid.
3. Effect: This falls to the Earth as acid rain. It can damage forests, harm fish and other life in lakes, and slowly dissolve stone buildings and statues.
Safety with Acids and Alkalis
Even household products can be dangerous. Strong drain cleaners are often powerful alkalis, and some toilet cleaners are strong acids.
- NEVER mix different cleaning products together. Mixing them can create dangerous, poisonous gases.
- Emergency Treatment: If you accidentally spill an acid or alkali on your skin, the most important thing to do is wash it off immediately with lots of running water. This will dilute it and wash it away.
Key Takeaway: Always handle acids and alkalis with care. Understand their potential hazards, like acid rain and the dangers of mixing chemicals, and know the basic first aid for spills.
Chapter Summary
Wow, we've covered a lot! You've done a great job. Let's quickly recap the main points.
- Acids (pH < 7) and Alkalis (pH > 7) are chemical opposites with distinct properties.
- We use indicators and the pH scale to measure how acidic or alkaline a solution is.
- Neutralisation is the reaction where an acid and an alkali cancel each other out.
- The magic formula is: Acid + Alkali → Salt + Water.
- Neutralisation has many important uses, from treating indigestion to farming.
- Always be safe! Strong acids and alkalis are corrosive, and acid rain is a serious environmental issue.
Don't worry if some of this seems tricky at first. The more you see these ideas in class and in the world around you, the more they will make sense. Keep up the great work!