Acids, bases and alkalis

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Lesson notes

Acids and Their Properties

Acids are a group of chemicals with a sharp taste, found in foods like lemons and vinegar.
Our stomach contains hydrochloric acid to help digest food.
Strong acids can be hazardous and labelled with symbols: irritant, toxic, or corrosive.
Common lab acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃).

The pH Scale

The pH scale ranges from 0 to 14 and measures how acidic or alkaline a substance is.
Acids have a pH less than 7; lower pH means stronger acid.
A substance with pH 7 is neutral (e.g., pure water).
Alkalis have a pH greater than 7; higher pH means stronger alkali.

Bases and Alkalis

A base is a substance that neutralises an acid.
If a base is soluble in water, it is called an alkali.
Common alkalis include soap and bleach.
Lab alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂).

Neutralisation

Neutralisation is a chemical reaction where an acid and a base react to produce a neutral solution (pH 7).
It raises the pH of an acid or lowers the pH of an alkali to 7.

Hazard Symbols

Irritant – causes skin/eye irritation.
Toxic – harmful to health.
Corrosive – damages materials it contacts.

The pH scale

Bases and alkalis

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Practice questions

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1.Which of the following is true about acids?
Easy
- ASome foods are acidic.
- BAcids have a pH greater than 7.
- CAcids are always safe to handle.
- DAcids have a pH of exactly 7.
2.What is the pH of a neutral substance?
Easy
- A0
- B7
- C14
- D1
3.What does a base do to an acid?
Easy
- ANeutralises it
- BMakes it stronger
- CIncreases its pH above 7
- DTurns it into an alkali
4.Which of the following is an example of an alkali?
Easy
- AHydrochloric acid
- BPure water
- CSodium hydroxide
- DLemon juice
5.What is the pH range for acids?
Easy
- A0 to 6
- B7 to 14
- C0 to 14
- D8 to 14
6.Which of the following is a hazard symbol associated with strong acids?
Easy
- AFlammable
- BCorrosive
- CRadioactive
- DOxidising
7.A student tests a substance with pH paper and gets a pH of 9. What is the substance?
Medium
- AA strong acid
- BA weak acid
- CA weak alkali
- DA strong alkali
8.What is the difference between a base and an alkali?
Medium
- AAn alkali is a soluble base.
- BA base is a soluble alkali.
- CThey are the same thing.
- DBases have pH less than 7, alkalis have pH greater than 7.

More questions

9.Which of the following is a strong acid?
Easy
- ASoap
- BVinegar
- CHydrochloric acid
- DPure water
10.What is the pH of pure water?
Easy
- A0
- B7
- C14
- D1
11.A student adds an acid to an alkali. What happens to the pH?
Medium
- AIt increases towards 7.
- BIt decreases towards 7.
- CIt stays the same.
- DIt becomes exactly 7 immediately.
12.Which of the following is an example of a base that is not an alkali?
Medium
- ASodium hydroxide solution
- BCalcium hydroxide powder
- CPotassium hydroxide solution
- DHydrochloric acid
13.A student tests a sample of rainwater with pH paper and obtains a pH of 5. What does this indicate about the rainwater?
Medium
- AIt is a strong acid.
- BIt is a weak acid.
- CIt is neutral.
- DIt is an alkali.
14.A solution has a pH of 2. How many times more acidic is it than a solution with pH 5?
Hard
- A2 times
- B3 times
- C100 times
- D1000 times
15.Which of the following is a hazard symbol that might appear on a bottle of concentrated hydrochloric acid?
Easy
- AFlammable
- BCorrosive
- COxidising
- DExplosive
16.A student adds a few drops of an unknown solution to a beaker of hydrochloric acid. The mixture's pH rises from 1 to 7. What type of substance was added?
Medium
- AAn acid
- BA base
- CAn alkali
- DA neutral substance
17.A laboratory technician needs to neutralise a spill of sodium hydroxide solution (pH 13) before disposal. Which of the following substances would be most suitable?
Medium
- APure water (pH 7)
- BDilute hydrochloric acid (pH 2)
- CSodium chloride solution (pH 7)
- DSodium hydroxide solution (pH 11)
18.Which of the following substances is an alkali?
Easy
- ALemon juice
- BVinegar
- CSoap
- DPure water
19.A student measures the pH of four household substances: A (pH 3), B (pH 7), C (pH 9), D (pH 12). Which substance is a weak alkali?
Medium
- AA
- BB
- CC
- DD
20.A student mixes 25 cm³ of 0.1 M hydrochloric acid with 25 cm³ of 0.1 M sodium hydroxide. What is the pH of the resulting mixture?
Hard
- ApH < 7
- BpH = 7
- CpH > 7
- DCannot be determined
21.Which of the following is a base that is NOT an alkali?
Medium
- ASodium hydroxide
- BPotassium hydroxide
- CCalcium hydroxide
- DCopper(II) oxide
22.A student adds a few drops of universal indicator to a solution. The colour changes to red. What is the approximate pH of the solution?
Easy
- ApH 1
- BpH 5
- CpH 7
- DpH 13
23.Which of the following is the chemical formula for a strong acid?
Medium
- ANaOH
- BHNO3
- CKOH
- DCa(OH)2
24.A student is given four unlabeled bottles: one contains hydrochloric acid, one contains sodium hydroxide solution, one contains pure water, and one contains a weak alkali such as soap solution. Using only universal indicator paper, which sequence of steps would correctly identify all four substances?
Hard
- ATest each with universal indicator paper; the one turning red is acid, purple is neutral, blue is weak alkali, and dark purple is strong alkali.
- BTest each with universal indicator paper; the one turning red is acid, green is neutral, blue is weak alkali, and purple is strong alkali.
- CTest each with universal indicator paper; the one turning red is strong acid, orange is weak acid, green is neutral, and blue is alkali.
- DTest each with universal indicator paper; the one turning red is acid, yellow is neutral, green is weak alkali, and blue is strong alkali.
25.A student adds 10 cm³ of 0.1 M hydrochloric acid to a beaker containing 10 cm³ of 0.1 M sodium hydroxide solution. The mixture is stirred and the pH is measured. What is the expected pH of the resulting solution?
Hard
- ApH 7
- BpH 1
- CpH 13
- DpH 2
26.Which of the following is a correct method to safely dilute a concentrated acid?
Medium
- AAdd water to the acid slowly while stirring.
- BAdd the acid to water slowly while stirring.
- CMix the acid and water in any order quickly.
- DPour the acid into a beaker of water without stirring.
27.A student has a solution that turns universal indicator blue. What is the approximate pH of this solution?
Easy
- ApH 10
- BpH 4
- CpH 7
- DpH 1
28.Which of the following hazard symbols is most likely to appear on a bottle of concentrated sodium hydroxide solution?
Easy
- ACorrosive
- BFlammable
- COxidising
- DExplosive
29.A student tests a sample of rainwater with universal indicator and obtains a colour corresponding to pH 5. A second sample of rainwater from a different location gives pH 4. How many times more acidic is the second sample compared to the first?
Hard
- A10 times more acidic
- B2 times more acidic
- C20 times more acidic
- D1.25 times more acidic
30.A student is asked to neutralise a spill of dilute hydrochloric acid. Which of the following substances would be most suitable to use?
Medium
- ASodium hydroxide solution
- BPure water
- CSodium chloride solution
- DMore hydrochloric acid
31.Which of the following is a weak alkali?
Easy
- ASoap solution
- BSodium hydroxide solution
- CPotassium hydroxide solution
- DCalcium hydroxide solution
32.A student has two unlabeled solutions: one is 0.1 M hydrochloric acid and the other is 0.1 M ethanoic acid (a weak acid). They measure the pH of each. Which of the following results would you expect?
Hard
- AHydrochloric acid has a lower pH than ethanoic acid.
- BEthanoic acid has a lower pH than hydrochloric acid.
- CBoth have the same pH because they are the same concentration.
- DBoth have pH 7 because they are acids.
33.Which of the following statements about bases and alkalis is correct?
Medium
- AAll bases are alkalis.
- BAll alkalis are bases.
- CBases are always insoluble in water.
- DAlkalis have a pH less than 7.
34.What is the chemical formula of hydrochloric acid?
Easy
- AHCl
- BH₂SO₄
- CHNO₃
- DNaOH
35.A student adds universal indicator to a solution and observes a green colour. What does this indicate about the solution?
Easy
- AIt is a strong acid.
- BIt is a weak acid.
- CIt is neutral.
- DIt is a strong alkali.
36.Which of the following statements is true?
Easy
- ASome foods are acidic.
- BAll acids have a pH greater than 7.
- CPure water is alkaline.
- DNeutral substances have a pH of 0.
37.What is the pH value of pure water?
Easy
- A0
- B7
- C14
- D1
38.Which of the following is a base?
Easy
- AA substance that neutralises an acid
- BA substance with pH less than 7
- CA substance that turns universal indicator red
- DA substance that is always soluble in water
39.Which of the following hazard symbols is most likely to appear on a bottle of concentrated sulfuric acid?
Easy
- AIrritant
- BToxic
- CCorrosive
- DFlammable
40.What is the pH range for weak alkalis?
Easy
- ApH 0–3
- BpH 7–9
- CpH 10–14
- DpH 4–6
41.Some foods are acidic.
Easy
True or false?
42.Acids have a pH of less than 8.
Easy
True or false?
43.Pure water is alkaline.
Easy
True or false?
44.Neutral substances have a pH of 7.
Easy
True or false?
45.A base is a substance that neutralises an acid.
Easy
True or false?
46.If a base is insoluble in water then we call it an alkali.
Easy
True or false?
47.Match each hazard symbol to its correct description.
Easy
- Irritant
- Toxic
- Corrosive
- Can cause skin irritation
- Can cause harmful health effects
- Can damage materials
48.Place the following substances in order of increasing pH (lowest pH first).
Medium
- Strong acid
- Weak acid
- Pure water
- Weak alkali
- Strong alkali
49.Which of the following are examples of alkalis? (Select all that apply)
Medium
- ASodium hydroxide
- BHydrochloric acid
- CSoap
- DBleach
- ESulfuric acid
50.If a base is insoluble in water, we call it an alkali.
Easy
True or false?
51.State three hazard symbols that a strong acid might be labelled with.
Easy
52.State what pH value a strong acid might have.
Easy
53.State the pH value of pure water.
Easy
54.State what pH value a weak alkali might have.
Easy
55.State two examples of common acids.
Easy
56.Complete the sentences about the pH scale.
Medium
The pH scale ranges from ____ to ____. Acids have a pH ____ than 7. Neutral substances have a pH of ____.
57.Describe what neutralisation is.
Medium
58.State what pH value a strong acid might have.
Easy
- A4
- B2
- C3
- D1
59.State the pH value of pure water.
Easy
- A6
- B7
- C8
- D9
60.State what pH value a weak alkali might have.
Easy
- A10
- B11
- C8
- D9

Revision notes

## Acids, Bases and Alkalis - **Acids**: pH < 7; strong acids have lower pH (e.g., pH < 3). Examples: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃). Found in lemons, vinegar, stomach acid, batteries. - **Hazard symbols**: strong acids may be **irritant**, **toxic**, or **corrosive**. - **Neutral**: pH = 7; pure water is neutral. - **Base**: substance that neutralises an acid (neutralisation reaction raises pH to 7). - **Alkali**: a base that is **soluble** in water; pH > 7. Strong alkalis have higher pH. Examples: sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂). Common alkalis: soap, bleach. Alkalis can also be irritant, toxic, corrosive. - **pH scale**: 0 (strong acid) to 14 (strong alkali); 7 is neutral. ## Key Points - Acids: pH < 7; alkalis: pH > 7. - Neutralisation: acid + base → neutral (pH 7). - Soluble base = alkali.

Acid: A substance with a pH less than 7. Strong acids have lower pH values.
Base: A substance that neutralises an acid.
Alkali: A base that is soluble in water; has a pH greater than 7.
pH scale: A scale from 0 to 14 used to show how acidic or alkaline a substance is.
Neutral: A substance with a pH of 7, e.g., pure water.
Neutralisation: A chemical reaction where an acid is neutralised by a base, raising the pH to 7.
Irritant: A hazard label meaning can cause irritation to skin and eyes.
Toxic: A hazard label meaning can cause harmful health effects.
Corrosive: A hazard label meaning can damage substances it contacts.

Flashcards

What is the pH range of acids?

Acids have a pH of less than 7.

What is the pH of a neutral substance?

Neutral substances have a pH of 7.

What is the pH of pure water?

Pure water is neutral and has a pH of 7.

What is a base?

A base is a substance that neutralises an acid.

What is an alkali?

An alkali is a base that is soluble in water.

What is the pH range of alkalis?

Alkalis have a pH greater than 7.

What is neutralisation?

Neutralisation is a chemical reaction where an acid is neutralised by a base, raising the pH to 7.

Name three strong acids used in a lab.

Hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3).

Name three alkalis used in a lab.

Sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)2).

What hazard symbols might a strong acid have?

Irritant, toxic, and corrosive.

What pH value does a strong acid typically have?

A strong acid typically has a pH of less than 3.

Give two examples of common acids found in everyday life.

Lemons (citric acid) and vinegar (acetic acid).

Give two examples of common alkalis found in everyday life.

Soap and bleach.

What is the pH of a weak alkali compared to a strong alkali?

A weak alkali has a lower pH than a strong alkali, but still above 7.

What is the pH of a weak acid compared to a strong acid?

A weak acid has a higher pH than a strong acid, but still below 7.

True or false: Acids have a pH of less than 8.

False. Acids have a pH of less than 7.

True or false: If a base is insoluble in water, it is called an alkali.

False. An alkali is a base that is soluble in water.

What is the chemical formula for hydrochloric acid?

HCl

What is the chemical formula for sulfuric acid?

H2SO4

What is the chemical formula for nitric acid?

HNO3

What is the chemical formula for sodium hydroxide?

NaOH

What is the chemical formula for potassium hydroxide?

KOH

What is the chemical formula for calcium hydroxide?

Ca(OH)2

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