Acids, Alkalis and Neutral Substances

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Acids, Alkalis and Neutral Substances

1. They are liquids.

2. They are solutions of compounds in water.

3. If concentrated they can be corrosive.

4. Acids taste sour (for example, vinegar).

5. Turn blue litmus paper red - this is an easy test for an acid!

6. Usually react with metals to form salts.

7. Acids contain hydrogen ions.

8. Turn Universal Indicator from green to red, and have a pH less than 7.

Examples of acids: are vinegar (ethanoic acid) and lemon juice (citric acid)

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magnesium + hydrochloric acid -> magnesium chloride + hydrogen gas

Some common acids used in your laboratories at school will be:

1. Hydrochloric acid, HCl(aq)

2. Nitric acid, HNO3(aq)

3. Sulphuric acid, H2SO4(aq)

1. They feel soapy to touch.

2. They are soluble bases.

3. Like acids, they can burn the skin.

4. They turn red litmus blue - this is how you test for an alkali!

5. Alkalis contain hydroxide ions (OH-).

6. They taste bitter.

7. Turns Universal Indicator from green to blue or purple.

Some common alkalis used in your laboratories at school will be:

1. Sodium hydroxide, NaOH(aq)

2. Ammonia, NH3NH4OH(aq)

3. Calcium hydroxide, Ca(OH)2(aq)

1. Litmus paper is not affected by neutral paper.

2. Tend to be harmless

3. Universal Indicator stays green.

Common examples of neutral substances:

1. Water

2. Sodium chloride solution, NaCl(aq)(common salt)

3. Sugar solution C6H12O6(aq)

The Strength of an Acid

Acids and alkalis can be strong or weak!

So how can we measure their strength?

The strength of an acid or alkali is shown using a scale of numbers called the pH scale. The numbers go from 0-14.

PH Table

On the scale it follows that:

An acidic solution has a pH number less than 7

An alkaline solution has a pH number greater than 7

A neutral solution has a pH number of exactly 7.

You can find the pH of any solution using universal indicator. Universal indicator is a mixture of dyes. It comes as a solution or in paper.

Universal indicator will change from green to a different colour depending on the pH of the solution you place it in.

universal indicator

Note:

In a strong acid, nearly all the acid molecules form ions.

In a weak acid, only some of the acid molecules form ions.

The more OH- ions (hydroxide ions), the more alkaline an alkali will be.

In other words, the more OH- ions there are the higher the pH number.

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