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Acids and Bases

 
1. What is an acid? – the difference between strong and weak acids?
2. Give examples of usually used acids and what they are used for?
3. Mention two weak acids, chemical name, properties?
4. Name three strong acids, chemical name, properties?
5. The AIW-rule
6. What is a base?
7. Mention a weak base, chemical name, properties?
8. Mention two strong bases, chemical name, properties?
9. What is neutralization, also give an example?
10. What is BTB (bromothymol blue)? What color has it in acidic, neutral and alkaline solution?
11. How does the pH-scale work?
12. What does it mean if a solution is: dilute, concentrated or saturated?
13. Questions


What is an acid? – the difference between strong and weak acids?


An acid is a substance that leaves off or can leave off a hydrogen ion (H+). This means that there are free hydrogen ions in solution. The solution thus becomes acidic. A strong acid always leaves off a hydrogen ion and a weak acid leaves sometimes off a hydrogen ion. This means that even a weak acid has an acid solution, since there are free hydrogen ions in the solution.


Acids can cause nasty burns and also dissolve metals. Therefore, bottles of acid often have the warning symbol "warning corrosive".

Give examples of usually used acids and what they are used for?

Weak acids:
Acetic acid: used for preservation of e.g. cucumber, or as part of window cleaner.
Citric acid: used in cooking for acidification, or as part of a preservative.

Strong acids:
Sulfuric Acid: Used as a liquid (electrolyte) in car batteries.

Mention two weak acids, chemical name, properties?

Meta acid (formic acid): HCOOH
Use: silage crops for preservation for the winter. In this way crops as food for animals are saved for the winter months. Lactic acid bacteria acidify crops and preserve them.

Formic acid is clearly noticeable when stirring around in an anthill. The ants produce this acid as part of their defense system. If you have a wound on your hand and stirring the anthill the wound will sting thoroughly. (Note: be careful and considerate towards ants - they cooperate better than most humans - watch and learn).

Ethanoic acid (acetic acid): CH3COOH
Pure acetic acid solidifies already at a temperature of 17 oC. The acid can be produced in various reactions, e.g. through dry distillation of wood. Transforming action of certain bacteria in alcohol (ethanol) can also produce acetic acid.

Name three strong acids, chemical name, properties?

Hydrochloric acid: (HCl)
HCl is actually a gas. It dissolves in water and hydrochloric acid is formed. Hydrochloric acid often fumes as a result of the acid leaving the liquid as gas. The gas can once again form small droplets of hydrochloric acid. Hydrochloric acid has a pungent characteristic odor. The hydrochloric acid reacts with water as follows:

HCl    H+  +  Cl-

The liquid contains free hydrogen ions and free chloride ions.

Sulfuric acid: (H2SO4)
Sulfuric acid is an odorless acid with a fairly high density. It may seem slightly viscous. A noteworthy feature is that the acid adds water. Therefore, it has been used for adding moisture between glasses in windows. The container with acid has to be periodically emptied, since the amount of fluid is constantly increasing, and could otherwise overflow the container.

Nitric acid: (HNO3)
Nitric acid is stored in the dark, as it would otherwise decompose by sunlight. The acid has the ability to dye proteins yellow. This is the case if placing a droplet of nitric acid on a nail. After a while, the area is dyed yellow. When the acid reacts with base metals gases called nitrogen oxides are emitted. These are reddish brown and very toxic.

The AIW-rule

When you mix acid and water you should always pour the acid into the water. The reason is that heat develops when mixing these two substances. If you pour water into acid, a water droplet entering the acid will immediately get very hot and it can turn into a gas. Should this happen there is the risk of the droplet splashing up bringing some acid. This splashing can surprise the person who is mixing (bumping). If instead pouring acid into water the risk is avoided.

What is a base?

A base is a substance that can add a hydrogen ion. An example of a base is ammonia (NH3). When ammonia is poured into water, ammonia reacts with water adding a hydrogen ion taken from a water molecule. Left of the broken water molecule are one oxygen atom and one hydrogen atom joined as a hydroxide ion (OH-). In a solution of a base there are always hydroxide ions.


Bases dissolve fat and have therefore the safety alert "warning corrosive".

Mention a weak base, chemical name, properties?

Ammonia is a weak base since only part of the ammonia molecules reacts with water molecules and adding (absorbing) hydrogen ions, leaving behind hydroxide ions. Below is an example of an ammonia molecule reacting with a water molecule:

NH3  +  H2  NH4+  +  OH-

The bidirectional arrow indicates that not all ammonia molecules react with water molecules. The reaction gives rise to free hydroxide ions in solution, but there are also ammonia molecules that have not reacted with water molecules. The fact is that the reaction goes both directions.

Mention two strong bases, chemical name, properties?

Strong bases function in a slightly different way. Strong bases are mostly salts.


The above picture shows crystals of sodium hydroxide. The salt looks like small flakes.

A salt dissolves in water into positive and negative ions. The base sodium hydroxide (NaOH) will release sodium ions (Na+) and hydroxide ions (OH-). This happens according to the reaction formula:

NaOH    Na+  +  OH-

The hydroxide ion makes the solution alkaline. All hydroxide ions are released from the sodium ions. We say that the base is strong.

Another example of a strong base is potassium hydroxide. Potassium hydroxide is dissolved in water as:

KOH    K+  +  OH-

What is neutralization, also give an example?

A neutralization takes place if an acid reacts with a base.

Example:
HCl  +  NaOH    Na+  +  Cl-  +  H2O

Sodium ions and chloride ions are free in solution. Ordinary saline water remains. Hydrogen ions and hydroxide ions react as follows:

H+  +  OH-    H2O

Neutralization has occurred. The acid has neutralized the base or vice versa, i.e. the base has neutralized the acid.

What is BTB (bromothymol blue)? What color has it in acidic, neutral and alkaline solution?

An acid-base-indicator indicates whether a solution is acidic or alkaline. BTB (bromothymol blue) is a common indicator.


How does the pH-scale work?

pH < 7 : acid solution. Here are hydrogen ions (H+)
pH = 7 : neutral solution
pH > 7 : alkaline solution. Here are hydroxide ions (OH-)


What does it mean if a solution is: dilute, concentrated or saturated?


A dilute solution has just a small amount of salt dissolved. In a concentrated solution there is so much salt dissolved, that a little bit of extra salt would not be able to dissolve, but instead form an insoluble heap on the bottom. If that happens, the solution is called saturated.

Questions

1. What is an acid?
2. Which ion does an acid give off?
3. Mention a weak acid?
4. Mention a strong acid?
5. What is the difference between a weak and a strong acid?
6. What is the chemical symbol of methane acid?
7. What is formic acid used for?
8. What is the chemical name of ethanoic acid?
9. What is the popular name of ethanoic acid?
10. What is ethanoic acid used for?
11. Give the chemical symbol of ethanoic acid?
12. Give the chemical symbol of sulfuric acid?
13. Give the chemical symbol of hydrochloric acid?
14. Give the chemical symbol of nitric acid?
15. Show how hydrochloric acid dissolves in water - with chemical symbols?
16. What do the letters in the AIW-rule stand for?
17. What is the AIW-rule - what does it mean?
18. Define a base – what is a base?
19. What ion is present in an aqueous solution of a base?
20. Mention a weak base with its name and chemical name?
21. Show the chemical reaction formula of this weak base dissolves in aqueous solution?
22. Mention a strong base with name and chemical name?
23. Show how this strong base dissolves in water?
24. What is the difference between a weak and a strong base?
25. What is neutralization?
26. Show with chemical symbols how hydrochloric acid neutralizes sodium hydroxide?
27. What is bromthymol blue (BTB)?
28. What color has BTB in acidic, neutral and alkaline solution?
29. Tell about the pH-scale?
30. What is the meaning of a low pH?
31. What is the meaning of a high pH?
32. What is a dilute solution?
33. What is a concentrated solution?
34. What is a saturated solution?

 

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  • NGU Science
  • JAN 1, 2012
  • FEB 2, 2012
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These basic courses are compiled by Lars Helge Swahn. They are designed for 15-16 year old students in order to reach basic skills in science. The courses do also provide an easier way to understand the NGU (AIC) gnostic teachings.

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