Making soluble salts (Edexcel GCSE Chemistry Combined Science): Revision Notes
📚 Revision Notes
Making soluble salts
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Method 1: Using an Acid and an Insoluble Base
This method is used when you are reacting an acid with an insoluble base (e.g., a metal oxide or carbonate) to form a soluble salt.
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Steps:
- Heat the Acid:
- Gently heat the acid in a water bath to speed up the reaction. This should be done in a fume cupboard to avoid inhaling acid fumes.
- Add the Insoluble Base:
- Gradually add the insoluble base (e.g., a metal oxide like copper(II) oxide) to the acid. The base will react with the acid to form a soluble salt and water.
- Keep adding the base until it is in excess (i.e., when no more base reacts and the excess solid sinks to the bottom). This ensures all the acid has been neutralised and there is no leftover acid in the solution.
- Filter the Excess Base:
- Use philtre paper and a funnel to philtre out the excess solid base, leaving a solution that contains only the soluble salt and water.
- Evaporate the Water:
- Heat the salt solution gently using a Bunsen burner to evaporate some of the water.
- Leave the solution to cool and allow the salt to crystallise.
- Once crystals form, philtre the crystals from the solution and leave them to dry, either in a warm oven or by air drying.
Example:
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Method 2: Using an Acid and an Alkali (Titration)
When using an alkali (a soluble base, e.g., sodium hydroxide), you cannot simply add excess alkali to the acid because both the alkali and the salt produced are soluble, which could contaminate the salt. Instead, you need to use a process called titration to find the exact point where the acid is neutralised.
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Steps:
- Measure the Acid:
- Use a pipette to measure a set volume of the acid (e.g., hydrochloric acid) into a conical flask.
- Add a few drops of an indicator (e.g., phenolphthalein or methyl orange) to show when the reaction is complete.
- Add the Alkali:
- Fill a burette with the alkali (e.g., sodium hydroxide). Slowly add the alkali to the acid while swirling the conical flask.
- Keep adding the alkali until the indicator changes colour, showing that the acid has been neutralised. This point is called the end point.
- Repeat the Reaction Without the Indicator:
- To make a pure salt, repeat the experiment using the same volumes of acid and alkali, but this time do not use an indicator. This avoids contaminating the salt with the indicator.
- Evaporate the Water:
- Once the reaction is complete, you are left with a solution containing just the salt and water.
- Gently heat the solution to evaporate some of the water and then leave it to cool and allow the salt to crystallise.
- Philtre the crystals and leave them to dry.
Example:
Making sodium chloride (NaCl) from hydrochloric acid (HCl) and sodium hydroxide (NaOH):
Important Points:
- Do not use universal indicator for titrations because the colour change is too gradual. Use a more specific indicator like phenolphthalein (which turns pink in alkaline solutions and colourless when neutral) or methyl orange (which turns red in acidic solutions and yellow in neutral or alkaline solutions).
- Always ensure the acid and alkali volumes are measured precisely to avoid excess reactants that can contaminate the final salt product.