Link to the lesson

Electrical conductivity of aqueous solutions of hydroxides

Changing the colours of acid‑base indicators in aqueous solutions of hydroxides

Experiment 1

You will need

• foil sheet for documents,

• Pasteur pipette,

• water,

• red cabbage brew,

• phenolphthalein,

• universal indicator,

• aqueous solutions of hydroxide (e.g. sodium, potassium, calcium).

Instruction

1. Place a piece of paper with the table in the foil sheet.

R1CfqARXUhxlN

Tabela zawiera trzy kolumny. Pierwsza z nich ma tytuł wskaźnik, indicator. W trzech wierszach kolumny kolejno w dół zapisano wywar z czerwonej kapusty, decoction red cabbage, fenoloftaleina, phenolphthalein, wskaźnik uniwersalny, universal indicator. Druga kolumna tabeli jest podzielona na dwie mniejsze. Jedna z nich opisana jest jako woda destylowana, distilled water. Druga opisana jest jako wodny roztwór wodorotlenku, aqueous solution of hydroxide. Wiersze te oznaczono kolorami. Wywar z czerwonej kapusty kolor w wodzie destylowanej jest fioletowy, wodny roztwór wodorotlenku - kolor wskaźnika zielony. Fenoloftaleina w wodzie destylowanej jest bezbarwna, w wodnych roztworach wodorotlenku - różowy (malinowy). Wskaźnik uniwersalny w wodzie destylowanej jest żółty, w wodnych roztworach wodorotlenków - niebieski.

2. Apply a drop of water with Pasteur pipette in the second column and a drop of aqueous solution of hydroxide (e.g. sodium, potassium, calcium) in the third column.

3. In each row of the table, apply a drop of the appropriate acid‑base indicator directly on the drops of water and aqueous hydroxide solution placed there.

4. Observe the changes.

Summary

The acid‑base indicators used in the tested solutions changed the colour equally. It indicates that these contain a common element (hydroxide anions), which changes the colour of the indicators.

RGDlrXfxVhYbG

Zdjęcie przedstawia obwód elektryczny do badania przewodnictwa prądu przez roztwory wodorotlenków. dwie elektrody węglowe zanurzone w roztworze wodorotlenku w zlewce. Do elektrod przymocowane są kable, z których jeden połączony jest z żarówką a drugi ze źródłem prądu stałego (9V). Pręty połączone są z baterią stojącą na stole za pomocą kabli.

The flow of electric current through the liquid is possible only if there are free ions in it: positive (cations) and negative (anions), i.e. particles that have a deficiency or excess of electrons in comparison with electrically neutral particles. Well‑conducting liquids are aqueous solutions of bases, salts and acids, because in their case the phenomenon of electrolytic dissociation occurs very well.

Sodium hydroxide or porassium hydroxide are electrolytes. In solid form, it will not conduct electricity. When dissolved in water, however, it will. In order to conduct electricity, a substance has to be able to allow an electrical charge to move through it. In water, NaOH dissociates into sodium ions and hydroxide ions. These ions allow electric current to move through the solution.

Distilled water does not conduct electricity. The solutions of the bases conduct electricity - they contain ions that are carriers of electric charge.

Substances whose water solutions conduct electricity, we call electrolyteselectrolyteselectrolytes . Nonelectrolytes are substances whose aqueous solutions do not conduct electricity. Distilled water is nonelectrolyte, and sodium, potassium and calcium hydroxides are electrolytes.

Electrolytic dissociation of potassium and calcium hydroxides

Rcn5DiodTFod9

Grafika przedstawia proces dysocjacji elektrolitycznej wodorotlenku potasu. Wodorotlenek, hydroxide dysocjuje pod wpływem działania wody, it dissociates under the influence of water i daje w wyniku reakcji kation potasu, potassium cation oraz anion wodorotlenkowy, hydroxide anion. Równanie: ka o ha strzałka w prawo, nad strzałką zapisano ha dwa o, dalej ka plus dodać o ha minus.

RV31IYA4ohhOa

Grafika przedstawia proces dysocjacji elektrolitycznej wodorotlenku wapnia. Wodorotlenek wapnia, calcium hydroxide dysocjuje pod wpływem działania wody, it dissociates under the influence of water i daje w wyniku reakcji kation wapnia, calcium cation oraz dwa aniony wodorotlenkowe, hydroxide anions. Równanie: ca o ha dwa razy wzięte strzałka w prawo, nad strzałką zapis ha dwa o, dalej ca 2 plus dodać dwa o ha minus

Potassium hydroxide dissociateselectrolytic dissociationdissociates which means it disintegrates under the influence of water into a single‑positive potassium cation and a single‑negative hydroxide anion. Both the potassium cation and the hydroxide anion are surrounded by water molecules in the aqueous solution (hydrationhydrationhydration).

Calcium hydroxide dissociates into a double‑positive calcium cation and two single‑negative hydroxide anions. The amount of positive charges arising as a result of dissociation is equal to the amount of negative charges generated.

The theory of the disintegration of substances into ions was developed by Svante Arrhenius. According to it, bases are chemical compounds that dissociate into metal cations and hydroxide anions:

${\text{M(OH)}}_{\text{n}}\stackrel{{\text{H}}_{\text{2}}\text{O}}{\to }{\text{M}}^{\text{n+}}\text{+}{\text{nOH}}^{-}$

Ammonia fountain

Experiment 3

Research problem

Is pH of ammonia water (ammonia solution in water) alkaline?

Hypothesis

pH of aqueous solution of ammonia is alkaline.

You will need

• saturated aqueous solution of ammonia,

• distilled water,

• red cabbage brew,

• set for ammonia collection (test tube closed with a cork with lateral outlet, round bottom flask),

• cork with a thin glass tube embedded in it,

• crystallizer,

• burner.

Instruction

1. In a corked tube with a side outlet, heat the ammonia water. After a while move the bottom of a dry vessel (to which you can collect ammonia) closer to the side tube.

2. After a while, ammonia will fill the vessel. Then close the flask with a cork through with a thin glass tube embedded in it.

3. Immerse the tube outlet in a crystallizer with water, where you previously added a few cmIndeks górny 3³of red cabbage brew.

Summary

The water gradually fills the flask. A characteristic fountain called ammonia fountain is created. Red cabbage brew changes colour into green.

R6D1UGQvyxSQs

Zdjęcie przedstawia krystalizator wypełniony wywarem z czerwonej kapusty. Laborant trzyma zakorkowaną kolbę z amoniakiem. Z kolby wystaje szklana rurka, którą laborant umieszcza w roztworze wywaru z czerwonej kapusty. W kolbie widać początek powstawania fontanny amoniakalnej oraz zmianę barwy wskaźnika na zieloną.

Ammonia (${\text{NH}}_{\text{3}}$) is a gas with a characteristic odour, freely soluble in water. Aqueous solution of ammonia (ammonia water) ${\text{NH}}_{\text{3}}\text{·}{\text{H}}_{\text{2}}\text{O}$) is the base, as evidenced by a change in the colour of the indicator, e.g. the colour of a red cabbage brew turned green.

Ammonia water dissociates:

${\text{NH}}_{\text{3}}\text{·}{\text{H}}_{\text{2}}\text{O}\stackrel{{\text{H}}_{\text{2}}\text{O}}{\leftrightarrow }{\text{NH}}_{\text{4}}^{+}\text{+}{\text{OH}}^{-}$

$\text{Ammonia solution}\stackrel{\text{water}}{\leftrightarrow }\text{ammonia cation + hydroxide anion}$

Summary

• Water soluble hydroxides disintegrate in water into ions: metal cations and hydroxide anions.

• Hydroxide solutions conduct electricity - they are electrolytes.

• The ion charge is written down by noting the first digit standing after the symbol in the upper index, followed by the ion sign (number one is omitted), e.g. ${\text{OH}}^{-}$, ${\text{Mg}}^{\text{2+}}$. The number of the same ions is noted using the factor in front of the symbol, e.g. $2{\text{Mg}}^{\text{2+}}$.

• The hydroxide anion is always single‑negative.

Keywords

electrolytic dissociation, electrolytes, hydration, hydroxides

Glossary