Testing for anions - 4. Identify the ions in an ionic compound - GCSE Chemistry (Single Science) Revision - CCEA - BBC Bitesize (2024)

FAQs

How to test for anions in GCSE? ›

How do you test for cations and anions dissolved in water? To test for aqueous cations, you use flame tests or the sodium hydroxide test. To test for aqueous anions, you use various reactions, such as the tests for carbonate, sulfate, and halide ions. This should help you identify an unknown ion dissolved in solution.

You can identify metal ions using flame tests or the sodium hydroxide test. Certain metal ions burn with different colours when held in a flame, whilst others form precipitates when mixed with sodium hydroxide. The visible results can be used to identify the unknown metal ion.

The most used test for sodium chloride is the chemical reaction with Silver Nitrate. The presence of sodium chloride yields a white precipitate upon drops of a silver nitrate solution.

We are able to detect anions first by using dilute hydrochloric acid, then by using concentrated sulfuric acid, and finally by using barium chloride solution if the anion has still not been identified. The order in which these reagents test the unknown anions has ramifications.

Add 1 or 2ccs of dilute H₂SO₄ to a small portion of the salt in a test tube and warm it gently. Brisk effervescence is obtained from colorless, odorless gas that turns lime water. Colorless gas with an odor of rotten eggs is obtained, turning lead acetate black paper. Sulphide anion is present.

If atoms lose electrons, they become positive ions or cations. If atoms gain electrons, they become negative ions or anions. Like fluoride, other negative ions usually have names ending in –ide. Positive ions, on the other hand, are just given the element name followed by the word ion.

Ions with a positive charge are called cations. Ions with a negative charge are called anions. Many normal substances exist in the body as ions. Common examples include sodium, potassium, calcium, chloride, and bicarbonate.

Tests for positive ions (cations)

Reacting ammonium ions with sodium hydroxide results in the formation of ammonia gas. Remember that ammonia gas can be detected by its pungent odour (it smells a lot like cat pee) and it will turn damp red litmus paper blue.

Remember, cations are positive ions—they are positively charged because they have lost one or more electrons and therefore have more protons than electrons. Anions are negative ions—they are negatively charged because they have gained one or more electrons and therefore have more electrons than protons.

How do you test for chloride anion? ›

Take a test tube filled with deionized or distilled water. To this add, 5% dilute nitric acid. Using a dropper add a few drops of silver nitrate. If a cloudy white precipitate is formed, it indicates the presence of Chloride ions in water.

When aqueous barium chloride is added to an acidified solution containing sulfate ions, an insoluble precipitate of BaSO4 results. This precipitate is white in color and confirms the presence of sulfate. The remaining anions at this point are phosphate, oxalate, and nitrate (refer to flow- chart above).

Ions of group 1 metals (Li⁺, Na⁺ and K⁺) form soluble hydroxides. Therefore, they are identified using flame tests and not by adding sodium hydroxide solution.

An anion has more electrons than protons, consequently giving it a net negative charge. For an anion to form, one or more electrons must be gained, typically pulled away from other atoms with a weaker affinity for them.

Flame tests

We place the acid-soaked wire into the centre of a hot Bunsen flame then place it into a sample of the metal we want to test. We then place it back into the non-luminous flame - it needs to be non-luminous (not very orange) so we can clearly see the colour. Lithium, Li ⁺ burns with a red flame.

To identify and quantify the anion composition in the sample traditional chemistry uses methods known as colorimetric methods. In modern chemistry, there is a method known as anion exchange chromatography or ion chromatography.