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Understand Acids and Alkali

Worksheet Overview

QUESTION 1 of 10

Acids and alkalis are both types of chemical you don't want to spill on your skin, but each of them is able to make the other one disappear. What's going on?

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.

For example, we could put a few drops of lemon juice onto universal indicator paper. The paper would turn orange where the drops were, which would show us that the pH of the juice was about 2. 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. Both these particles 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.

 

 

Use this diagram to help you:

Match up universal indicator colours with pH values

Column A

Column B

red
1
yellow
10
green
7
blue
5

Which of these Universal Indicator colours represent alkaline solutions?

orange

green

blue

purple

Use this diagram to help you:

What is the pH of lemon juice?

2.0

4.5

7.0

9.6

Use this diagram to help you:

What is the pH of rain water?

2.0

5.5

8.2

11.1

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

Match up these half-sentences on measuring pH.

Column A

Column B

Indicators are chemicals which
change colour according to pH.
Universal indicator solutions
measure pH and give a numerical answer.
Digital pH meters
turn red in acidic conditions and blue in alkaline...
Neutral solutions
turn Universal Indicator green.

What type of ion is found in acidic solutions?

H+

OH-

SO42-

What type of ion is found in alkaline solutions?

H+

OH-

SO42-

What is the name for the process where an acid and alkali react to cancel each other out?

What is the name of the chemical produced when hydrogen and hydroxide react?

  • Question 1

Use this diagram to help you:

Match up universal indicator colours with pH values

CORRECT ANSWER

Column A

Column B

red
1
yellow
5
green
7
blue
10
EDDIE SAYS
Don't worry too much about the exact numbers; remember that Universal Indicator colours go as a rainbow from red = acid, through green = neutral to blue = alkali.
  • Question 2

Which of these Universal Indicator colours represent alkaline solutions?

CORRECT ANSWER
blue
purple
EDDIE SAYS
Think about the rainbow- green is the midpoint, showing neutral pH. Blue and purple are alkaline, red and yellow are acidic.
  • Question 3

Use this diagram to help you:

What is the pH of lemon juice?

CORRECT ANSWER
2.0
EDDIE SAYS
Lemon juice is pretty acidic. You don't need to memorise the exact numbers, but you may be asked to look them up in a table of diagram.
  • Question 4

Use this diagram to help you:

What is the pH of rain water?

CORRECT ANSWER
5.5
EDDIE SAYS
Rain water is always slightly acidic (because of carbon dioxide dissolved in it). If pollution levels are high, rain water becomes more acidic, which damages the environment.
  • Question 5

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 6

Match up these half-sentences on measuring pH.

CORRECT ANSWER

Column A

Column B

Indicators are chemicals which
change colour according to pH.
Universal indicator solutions
turn red in acidic conditions and...
Digital pH meters
measure pH and give a numerical a...
Neutral solutions
turn Universal Indicator green.
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 7

What type of ion is found in acidic solutions?

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

What type of ion is found in alkaline solutions?

CORRECT ANSWER
OH-
EDDIE SAYS
The ion linked to alkalinity is OH-; a negative hydroxide ion.
  • Question 9

What is the name for the process where an acid and alkali react to cancel each other out?

CORRECT ANSWER
neutralisation
neutralization
EDDIE SAYS
Neutralisation (with an s) is British spelling, neutralization (with a z) is American spelling. Neutralisation is better, really, but you wouldn't lose marks for neutralization.
  • Question 10

What is the name of the chemical produced when hydrogen and hydroxide react?

CORRECT ANSWER
water
EDDIE SAYS
In words, hydrogen + hydroxide → water. In symbols, H+ + OH- → H2O.
---- OR ----

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