# Understand Acids and Alkalis

In this worksheet, students will learn about the quantitative description of acids and alkalis using the pH scale, and how changes in pH reflect changes in ionic concentrations.

Key stage:  KS 4

Difficulty level:

### QUESTION 1 of 10

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

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.

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 at pH 7, there are 10-7 moles of H+ ions per litre of water. That means there are 10-7 x 6 x 1023 H+ ions per litre, which is 6 x 1016 ions per litre. At pH 6, there are 10-6 moles of H+ ions per litre of water, so ten times more than at pH 7. At pH 5, there are 10-5 moles of H+ ions per litre of water and the pattern works for all pH values.

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.

Changing pH by dilution

One way of changing the pH of a solution is by adding water to reduce the concentration of the solution. For example, imagine having a 10 cm3 solution of an acid with pH 5.5. If we add water to top the solution up to 100 cm3, we will have reduced the concentration of the solution tenfold, which reduces the concentration of ions by the same factor. The pH of the dilute solution will now be 1 step lower, which is 6.5.

Which of these statements about acids is true? Tick all the right answers.

contain excess OH- ions

contain excess H+ ions

pH less than 7

higher pH values are more dangerous

pH more than 7

lower pH values are more dangerous

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

How is an acid with pH 3.5 different to an acid with pH 4.5? Pick a half-sentence from each column to make a complete sentence.

2 H+ + O 2- → H2O

pH changes to 7

acid + alklai → water

pH reduces

H+ + OH- → H2O

Which of these is the most 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

Which of these statements about alkalis is true? Tick all the right answers.

contain excess OH- ions

contain excess H+ ions

pH less than 7

higher pH values are more dangerous

pH more than 7

lower pH values are more dangerous

An acidic solution has pH 5.8. What does this mean?

There are 5.8 H+ ions per litre

There are 10-5.8 H+ ions per litre

There are 105.8 H+ moles of ions in total

There are 10-5.8 H+ moles of ions per litre

How many H+ ions are there in 0.05 litres of solution with pH 6?

Use the equation concentration (in mol / litre) = 10 - pH

6 x 1015

3 x 1016

6 x 1017

1.2 x 1019

If we start with 0.1 litres of an acid with pH 3.5, and dilute it with water until we have 1 litre, what will the pH of the new solution be?

Please don't try this for real!

Suppose you had 100 cm3 of an acid with pH 5.6. You boil it to evaporate water off, until only 1 cm3 water remains.

Please don't try this for real!

Before you can check the pH of the resulting solution, you are taken out of the chemistry lab for doing something dangerous. What would you expect the pH of the 1 cm3 solution to be?

• Question 1

Which of these statements about acids is true? Tick all the right answers.

contain excess H+ ions
pH less than 7
lower pH values are more dangerous
EDDIE SAYS
The starting point for thinking about acids is that pH 7 is neutral. As the concentration of H+ ions increases from there, the pH gets lower.
• Question 2

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

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 3

How is an acid with pH 3.5 different to an acid with pH 4.5? Pick a half-sentence from each column to make a complete sentence.

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. This doesn't have to be integer-to-integer.
• Question 4

Which of these is the most dangerous?

A concentrated strong acid
EDDIE SAYS
A strong acid fully ionises, releasing all its H+ ions. If it is also concentrated, they are spread across a small amount of water. As a result the solution will be high risk.
• Question 5

Which of these has the highest pH?

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.
• Question 6

Which of these statements about alkalis is true? Tick all the right answers.

contain excess OH- ions
higher pH values are more dangerous
pH more than 7
EDDIE SAYS
Alkalis are the converse of acids. The starting point for thinking about both types of chemical is that pH 7 is neutral. As the concentration of OH- ions increases from there, the pH gets higher.
• Question 7

An acidic solution has pH 5.8. What does this mean?

There are 10-5.8 H+ moles of ions per litre
EDDIE SAYS
The minus in the expression concentration = 10- pH is important, because lower pH is more acidic.
• Question 8

How many H+ ions are there in 0.05 litres of solution with pH 6?

Use the equation concentration (in mol / litre) = 10 - pH

3 x 1016
EDDIE SAYS
There's a lot going on here, but you will get tricky calculations in chemistry exams. First step is to work out the concentration of the ions; substituting 6 into the equation gives 10 -6 moles per litre. We multiply that by 6 x 1023 to get the concentration of ions per litre. Then we multiply by 0.05, since we only have 0.05 litres.
• Question 9

If we start with 0.1 litres of an acid with pH 3.5, and dilute it with water until we have 1 litre, what will the pH of the new solution be?

4.5
EDDIE SAYS
If we dilute a solution from 0.1 litres to 1 litre, we are reducing the concentration by a factor of ten. This changes the pH by one step. Since the dilute solution is more dilute, it will have a higher pH (less dangerous acid = higher pH).
• Question 10

Please don't try this for real!

Suppose you had 100 cm3 of an acid with pH 5.6. You boil it to evaporate water off, until only 1 cm3 water remains.

Please don't try this for real!

Before you can check the pH of the resulting solution, you are taken out of the chemistry lab for doing something dangerous. What would you expect the pH of the 1 cm3 solution to be?

EDDIE SAYS
By evaporating off so much water, you will have increased the concentration of the acid by a factor of 100, so the pH will get smaller. Since 100 = 10 x 10, that means that the pH will change by 2 steps, so from 5.6 to 3.6. But please- don\'t try this for real.
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