The smart way to improve grades

Comprehensive & curriculum aligned

Affordable pricing from £10/month

Understand Acids and Alkalis

Worksheet Overview

QUESTION 1 of 10

We use the pH scale to talk about how acidic or alkaline a solution is. What do the numbers mean?

All chemicals fit on a scale called the pH scale. This is a number scale going from 0 (very acidic) to 14 (very alkaline). 7 is in the middle, so that is the pH of pure water, called neutral

 

There are some patterns in this scale; acidic things (pH less than 7) often taste sour (think of lemon juice- citric acid, vinegar- acetic acid, or stomach acid- hydrochloric acid). Alkaline things are often used for cleaning (think of soap, or bleach). The more extreme the pH is, the more dangerous the chemical.

The rainbow pattern in the picture shows you one way of measuring the pH of a chemical, using universal indicator. We can have universal indicator as a liquid, or we can make paper with universal indicator soaked into it. If we add this to some chemical, the colour of the indicator will change to indicate the pH of the chemical. Universal indicator isn't very precise, but it gives us a rough idea of the pH of a solution. We can get much more accurate results using an electronic pH meter.

The thing that determines if a solution is acidic or alkaline is the presence of H+ ions or OH- ions. Acidic solutions have excess H+ ions dissolved in them. Alkaline solutions have excess OH- ions dissolved in them. The pH value is linked to the concentration of hydrogen ions in a solution; one step on the pH scale (so from 5 to 4, or 3 to 2) means the concentration of H+ ions increases by a factor of ten. This type of scale is called logarithmic.

Both H+ ions and OH- ions are very reactive, which is why both acids and alkalis are potentially harmful. However, if H+ and OH- combine, they make H2O, which is water. That's why we can use acid to neutralise alkali, and alkali to neutralise acid.

Ionic equations

When we dissolve acid in water, it releases H+ ions. For example, we make hydrochloric acid by dissolving hydrogen chloride in water: HCl (s) → H+ (aq) + Cl-( aq).

We make alkalis by dissolving sources of hydroxide in water. For example, we can dissolve sodium hydroxide in water: NaOH (s) → Na+ (aq) + OH- (aq).

Neutralisation involves the reaction H+ (aq) + OH- (aq) → H2O (l).

Strong and weak acids, dilute and concentrated acids

The concentration of H+ ions depends on two factors. One is the concentration of the solute in water. A solution with a large mass of solid acid and a small volume of water will increase the concentration of H+ ions, leading to a lower (more extreme) pH. A solution with a small mass of solid acid and a large volume of water is called a dilute solution. The more dilute the solution, the lower the concentration of H+ ions, leading to a pH nearer 7.

The other factor is the strength of the acid. A strong acid fully releases H+ ions into solution when dissolved. A weak acid only partially releases H+ ions into solution. Strong acids have lower (more extreme) pH than weak acids.

 

 

Use this diagram to help you:

Which of these statements describe lemon juice. Tick all the correct answers.

Lemon juice is acidic

Lemon juice is alkaline

Lemon juice turns orange when Universal Indicator is added

Lemon juice turns Universal Indicator orange

The pH of lemon juice is about 2

The concentration of H+ ions in lemon juice is about 2

Without looking it up (it isn't on the diagram in the introduction), which of these is the best estimate of the pH of apple juice?

1

4

8

11

Fill in the gaps in this paragraph about measuring pH. Use these words; you may need to use them more than once.

acidic

alkaline

pH

Indicators

1

4

8

11

What type of ion is found in alkaline solutions? Tick all the right answers.

hydrogen

OH-

hydroxide

H+

Which of these describe neutralisation? Tick all the correct answers. 

2 H+ + O 2- → H2O

pH changes to 7

acid + alklai → water

pH reduces

H+ + OH- → H2O

Which of these are advantages of using a digital pH meter? Tick all the correct answers.

Digital meters are more modern.

Digital meters can record changes quickly, using a datalogger

Digital meters give more precise data

Data from a digital meter are more reliable

Digital meters create less mess

What type of ion is found in acidic solutions? Tick all the right answers.

hydrogen

OH-

hydroxide

H+

How is an acid with pH 5 different to an acid with pH 6? Pick a half-sentence from each column to make a complete sentence.

hydrogen

OH-

hydroxide

H+

Which of these is the least dangerous?

A concentrated strong acid

A dilute strong acid

A concentrated weak acid

A dilute weak acid

Which of these has the highest pH?

A concentrated strong acid

A dilute strong acid

A concentrated weak acid

A dilute weak acid

  • Question 1

Use this diagram to help you:

Which of these statements describe lemon juice. Tick all the correct answers.

CORRECT ANSWER
Lemon juice is acidic
Lemon juice turns Universal Indicator orange
The pH of lemon juice is about 2
EDDIE SAYS
The colour of the indicator shows us the the pH of lemon juice is about 2. You don't need to memorise this, but you might need to interpret a table or diagram with data in it. This tells us that lemon juice is acidic. This tells us that the concentration of H+ ions is pretty high, but not that the concentration is 2.
  • Question 2

Without looking it up (it isn't on the diagram in the introduction), which of these is the best estimate of the pH of apple juice?

CORRECT ANSWER
4
EDDIE SAYS
We know that fruit juices are slightly acidic, so they have a pH of less than 7. They\'re not that acidic (or they\'d burn our throat when we drink them), so the pH can\'t be as low as 1. The only option left is 4.
  • Question 3

Fill in the gaps in this paragraph about measuring pH. Use these words; you may need to use them more than once.

acidic

alkaline

pH

Indicators

CORRECT ANSWER
EDDIE SAYS
Check the introduction if you're not sure of the details here. The thing to be careful of is that acid isn't red- acid turns Universal Indicator red.
  • Question 4

What type of ion is found in alkaline solutions? Tick all the right answers.

CORRECT ANSWER
OH-
hydroxide
EDDIE SAYS
The ion linked to alkalinity is OH-; a negative hydroxide ion.
  • Question 5

Which of these describe neutralisation? Tick all the correct answers. 

CORRECT ANSWER
pH changes to 7
acid + alklai → water
H+ + OH- → H2O
EDDIE SAYS
Neutralisation is when hydrogen (from acid) reacts with hydroxide (from alkali) to make water. The effect of this is to change the pH to 7, since there are no excess ions of either type.
  • Question 6

Which of these are advantages of using a digital pH meter? Tick all the correct answers.

CORRECT ANSWER
Digital meters can record changes quickly, using a datalogger
Digital meters give more precise data
Data from a digital meter are more reliable
EDDIE SAYS
Digital meters have lots of benefits- they give more precise data (usually one decimal place- Universal Indicator works to the nearest whole number), they don\'t depend on looking at a colour scale, and you can record changes very frequently. This is useful if you are tracking a chemical reaction. Universal Indicator can be messy, but not if you work carefully.
  • Question 7

What type of ion is found in acidic solutions? Tick all the right answers.

CORRECT ANSWER
hydrogen
H+
EDDIE SAYS
The ion linked to alkalinity is H+; a positive hydrogen ion.
  • Question 8

How is an acid with pH 5 different to an acid with pH 6? Pick a half-sentence from each column to make a complete sentence.

CORRECT ANSWER
EDDIE SAYS
Acidity is all about the concentration of H+ ions. Each step of one on the pH scale means the concentration has changed by a factor of ten.
  • Question 9

Which of these is the least dangerous?

CORRECT ANSWER
A dilute weak acid
EDDIE SAYS
A weak acid doesn\'t release all its H+ ions, and if it is dilute, they are spread across a large amount of water. As a result the solution will be low risk.
  • Question 10

Which of these has the highest pH?

CORRECT ANSWER
A dilute weak acid
EDDIE SAYS
This question's a bit sneaky! For acids, the highest that the pH can be is 7, which is when an acid becomes totally neutral. It's helpful to think about extreme pH values (very low for acid, very high for alkali) to get round this.
---- OR ----

Sign up for a £1 trial so you can track and measure your child's progress on this activity.

What is EdPlace?

We're your National Curriculum aligned online education content provider helping each child succeed in English, maths and science from year 1 to GCSE. With an EdPlace account you’ll be able to track and measure progress, helping each child achieve their best. We build confidence and attainment by personalising each child’s learning at a level that suits them.

Get started
laptop

Start your £1 trial today.
Subscribe from £10/month.